Adsorption of Cobalt and Strontium Ions on Plant-Derived Activated Carbons: The Suggested Mechanisms

: In this study, activated carbons derived from walnut shells (CA-N) and apple wood (CA-M) were used as adsorbents to remove cobalt(II) and strontium(II) ions from aqueous solutions. The novel materials were obtained using nitric acid (CA-Mox) and nitric acid/urea mixture (CA-Mox-u, CA-Nox-u) as oxidizing agents. The physical–chemical characteristics of activated carbons were determined from nitrogen sorption isotherms, SEM-EDX, FTIR, pH metric titrations, the Boehm titration method and elemental analysis. The results of batch experiments indicate that maximum adsorption can be achieved in broad pH ranges: 4–8 for Co(II) and 4–10 for Sr(II). The maximum adsorption capacities of Co(II) and Sr(II) on oxidized activated carbons at pH = 4 are: CA-Mox, 0.085 and 0.076 mmol/g; CA-Mox-u, 0.056 and 0.041 mmol/g; and CA-Nox-u, 0.041 and 0.034 mmol/g, respectively. The mathematical models (pseudo-ﬁrst-order, pseudo-second-order and intraparticle diffusion kinetic models, and Langmuir, Freundlich, Dubinin–Radushkevich, and Temkin–Pyzhev isotherm models) were used to explain the adsorption kinetics, to study the adsorption mechanism and predict maximum adsorption capacity of the adsorbents. The adsorption mechanisms of toxic metal ions on activated carbons were proposed.


Introduction
Several studies have confirmed that heavy metals are toxic to different life forms (e.g., plants, aquatic organisms and humans) and can generate a significant environmental impact.With the increase in the number of nuclear reactors, radioactive pollution of water and soil is increasing.Heavy and radioactive metals (such as lead, cadmium, arsenic, cesium, strontium, cobalt, mercury, chromium, etc.) are persistent pollutants with potential hazards, which have high toxicity, are nonbiodegradable and easily accumulate in organisms from food and/or drinking water [1][2][3][4][5][6].Several acute and chronic toxic effects of heavy metals affect different body organs.Gastrointestinal and kidney dysfunction, nervous system disorders, skin lesions, vascular damage, immune system dysfunction, birth defects and cancer are examples of the complications of heavy metals toxic effects [1,5,6].Radioactive heavy metals have a more complex effect on the living organism, as the ionizing radiation of the incorporated ions can break down DNA, proteins and other biomolecules [7,8].Adsorption on activated carbons is one of the often-used procedures to remove toxic metals from fluids [9][10][11][12][13][14][15][16][17][18].This separation process implies the transfer of metal ions from the fluid (i.e., water or industrial effluent) to the surface of a solid matrix (i.e., adsorbent) that should have a tailored surface chemistry and porosity to reach an effective separation.The effectiveness of the adsorption of heavy metal ions is affected by several operating variables like contact time, adsorbent amount, temperature, initial metal concentration and C 2023, 9, 71 2 of 21 pH [11][12][13]18].Also, textural parameters and surface chemistry of the activated carbon are essential to achieve the successful removal of toxic metals.
Activated carbons from plant-derived raw (agro-based) materials are low-cost adsorbents and are considered in agricultural countries with large amounts of agricultural waste produced every year.Among the plant-derived materials used to obtain activated carbons are nut shells, grape seeds, plum stones, peach stones, tree wood, rice husk, sawdust, wheat straw, etc. [3,4,[12][13][14][16][17][18][19].As was mentioned above, among the important activated carbons characteristics that influence the adsorption of metals is surface chemistry.In the literature, several types of surface tailoring and modification of activated carbons were reported, including nitric acid, peroxide and ozone oxidation, sulphuration and nitrogenating treatment, as well as anchoring coordination ligands [13,14,16,20].
In the current work, nitric acid was selected as an oxidation agent for the modification of activated carbon surface chemistry.In addition to the traditional method, a modified method was also applied, using as an oxidizing agent the mixture of nitric acid and urea.The indirect role of urea is to protect the activated carbon surface from excessive degradation with the formation of humic acids [21].
Preliminary results of adsorption of toxic metal ions (such as lead, mercury, cobalt, cadmium, strontium and caesium) from solution on walnut shells (CA-N) and apple wood (CA-M)-activated carbons highlight samples modified by oxidation, and cobalt, lead, cadmium and strontium ions are best adsorbed [22].
The present study aimed to investigate the adsorption behavior of Co(II) and Sr(II) ions on the surface of plant-derived activated carbons (obtained from walnut shells and apple wood, and modified by oxidation with nitric acid and nitric acid/urea mixture).The pseudo-first-order, pseudo-second-order and intraparticle diffusion models were used to explain the adsorption kinetics and accessibility of the heavy metal ions, while Langmuir, Freundlich, Dubinin-Radushkevich, and Temkin-Pyzhev isotherm models were used to study the adsorption mechanism and predict maximum adsorption capacity of the adsorbents.By summarizing the obtained results, the adsorption mechanisms of toxic metal ions on activated carbons were proposed.

Modification and Characterization of Activated Carbons
In this research, carbonaceous adsorbents of local origin were used; activated carbons obtained from walnuts shells (CA-N) and apple wood (CA-M) by physical-chemical activation method with water steam were provided by LLC Ecosorbent (Republic of Moldova).

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Modification of activated carbons surface by oxidation with nitric acid The oxidation process of activated carbons by the classical method was carried out as follows [13,23,24]: concentrated nitric acid solution (63%) was added to 500 g of granular activated carbon CA-M, fraction 0.63 ÷ 0.8 mm (at a solid (g):liquid (mL) ratio of 1:3), which was placed in a glass flask on the water bath (the scheme installation is presented in Appendix S1).The mixture of nitric acid and activated carbon was kept at 95 • C for 8 h.The released nitrogen oxides (off-gases) were collected in the collection vessel filled with sodium hydroxide solution.After the oxidation process was finished, the mixture was cooled and decanted.Humic and fulvic acids formed during the oxidation process were removed with a 1.0 N potassium hydroxide solution, and then by repeated washing with distilled water until the washing solution color changed from brown to yellow.After removing the humic acids, the oxidized activated carbon was converted to the H + form by treatment with a 1.0 N solution of hydrochloric acid.Afterward, the oxidized activated C 2023, 9, 71 3 of 21 carbon was washed with distilled water until the negative reaction of chlorine ions, then dried at 110 ± 5 • C and labeled as CA-Mox.

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Modification of activated carbons surface by oxidation with nitric acid/urea mixture The oxidation with nitric acid/urea mixture was carried out as follows: activated carbon (500 g) (CA-N or CA-M) was introduced into the flask to which the mixture of nitric acid and urea in a molar ratio of 3:1 was added (at a solid (g):liquid (mL) ratio of 1:3) (Appendix S1).The oxidation process took about 10-12 h at a temperature of 95 • C. The released gases were collected in the collection vessel filled with sodium hydroxide solution.After the oxidation process was finished, the mixture was cooled and decanted.The formation of humic and fulvic acids was checked with a 1.0 N potassium hydroxide solution and, since the respective acids were not formed, the steps of washing the oxidized activated carbon with the basic solution and its conversion into H form were omitted.The oxidized activated carbons were washed with distilled water until the negative reaction of nitrate/nitrite ions, then dried at 110 ± 5 • C and labeled as CA-Mox-u, CA-Nox-u.
During the oxidation of activated carbon with nitric acid, the presence of urea is necessary, which, acts as a buffer (nitric acid in combination with urea forms a soluble complex H 2 NCONH 2 •HNO 3 that reduces the oxidation capacity of nitric acid), protecting the activated carbon by absorbing nitrogen oxides released during oxidation, thus preventing the formation of humic compounds [13,21,24,25] (a detailed explanation is presented in Appendix S2).
For the experiments, an activated carbons grain fraction of 0.63 ÷ 0.8 mm was selected.Prior to all experiments activated carbons samples were dried in an oven at 110 ± 5 • C for at least 3 h.

Characterization of activated carbons
The physical-chemical characteristics of activated carbons were evaluated by general indices (ash A, humidity U, and elemental analysis).Humidity and ash content in activated carbons were determined by standard methods [26].The elemental analysis (C, H, N, S) was determined on the Elemental analyzer CHNS Vario EL III (Elementar Analysensysteme GmbH, Langenselbold, Germany).SEM-EDX profiles of the samples were obtained by the Scanning Electron Microscope (SEM), model VEGA II LSH (TESCAN Co., Libušina, Czech Republic) coupled with Energy Dispersive X-ray detector (EDX), type QUANTAX QX2 (BRUKER/ROENTEC Co., Berlin, Germany).
Porous structure characteristics of active carbons were determined from sorption isotherms of nitrogen at 77 K measured using Autosorb-1MP (Quantachrome Instruments, Boynton Beach, FL, USA).The specific surface area (S BET ) was calculated using the Brunauer-Emmett-Teller (BET) equation.The total pore volume (V total ) was calculated by converting the amount of nitrogen gas adsorbed at a relative pressure of 0.99 to the equivalent liquid volume of the adsorbate.The volume of micropores (V micro ) was determined using the t-method, and the volume of mesopores (V meso ) was determined from the difference between the total volume and the volume of micropores.
The surface chemistry of the activated carbons has been evaluated by jointing infrared spectroscopy (FTIR), pH metric titrations and the Boehm titration method.The IR spectra of activated carbons were recorded by FT-IR Spectrum 100 apparatus (PerkinElmer, Shelton, CT, USA), using dilutions in potassium bromide.The Boehm titration method (proposed by H.P. Boehm in 1966 [27,28]) is based on the theory that acid and base in an aqueous solution react with different basic and acidic functional groups on the surface of activated carbons which are derived from different oxygenated functional groups [13,27,28].Experimental details are presented in the literature [27][28][29] (Appendix S3).The results of pH metric titrations were used to determine pH pzc and acidity constants of surface groups of activated carbons [29,30].The activated carbon pH pzc was determined from the intersection of proton-binding isotherms obtained for three concentrations of background electrolyte (0.1 M, 0.05 M and 0.01 M NaCl) [29] (Appendix S4).Proton affinity distributions, F(pKa), C 2023, 9, 71 4 of 21 were calculated from proton-binding isotherms by solving the adsorption integral equation using the CONTIN method [31][32][33][34].

Determination of metals
Determination of cobalt(II) and strontium(II) ions, as well as metals on activated carbons ash, was performed on AAS-1N (Carl Zeiss, Industrielle Messtechnik GmbH, Oberkochen, Germany) and Shimadzu AA7000 (Shimadzu Co., Kyoto, Japan) atomic absorption spectrometers, respectively, depending on metals concentration in solution.

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Determination of metals species in solution as pH The cobalt and strontium speciation in the solution was calculated/estimated by Visual MINTEQ software, version 4.0 (4.04) [35].

Adsorption Experiments
The metals salts (Co(NO 3 ) 2 ), Sr(NO 3 ) 2 )) solutions (0.2-2 mmolL −1 ) were added to 0.05 g of adsorbent dosage, at a solid (g):liquid (mL) ratio of 1:100.The mixtures of adsorbent-adsorbate were agitated in a shaker, between 10 and 600 min, at a temperature of 20 ± 2 • C. The samples were filtered to separate the adsorbent from the metal solution.The concentration of metal ions in the solution was determined by AAS at a wavelength of 240.7 nm for cobalt(II) and 460 nm for strontium(II).The pH of the solutions was adjusted using HNO 3 and NaOH solutions.
where C 0 is the initial concentration (mmolL −1 ); and C e is the equilibrium concentration (mmolL −1 ); m is mass of adsorbent; and V the volume of solution (L).
The pseudo-first-order (Lagergren) [36], pseudo-second-order (Ho and McKay) [37] and intraparticle diffusion (Weber-Morris) [38,39] models were used to analyze the kinetic data.The linearized expressions of the theoretical models are shown in Equations ( 3)-( 5), respectively.ln(q e − q t ) = ln(q e ) − k 1 t where q e and q t is the adsorption capacity (mmol/g) at equilibrium and time t (min); k 1 is the pseudo-first-order constant (min −1 ); k 2 is the pseudo-second-order constant (mmolg −1 min −1 ); and k i is the intraparticle constant (mmol/g min 1 2 ).The adsorption isotherms data were fitted to the mathematical models Langmuir [40], Freundlich [41], Dubinin-Radushkevich [42] and Temkin-Pyzhev [43], as presented in Equations ( 6)- (9).The essential characteristics of Langmuir isotherm are expressed in terms of a dimensionless constant separation factor R L (equilibrium parameters) that is given in Equation (10). ) C 2023, 9, 71 5 of 21 q e = (q s ) exp − k ads ε 2 ( 8) where q e is the adsorption capacity (mmol/g) at equilibrium; Q 0 is the monolayer capacity (mmol/g); K L is the Langmuir adsorption constant (L/mmol); K F is the Freundlich equilibrium constant; q s is the saturation capacity (mmol/g); K ads and ε are the constants of the Dubinin-Radushkevich isotherm; K T is the constant of Temkin-Pyzhev model; n is the parameter characterizing the system heterogeneity.

Characterization of the Activated Carbons
The modification of activated carbons via oxidation with concentrated nitric acid (CA-Mox sample) and with nitric acid/urea mixture (CA-Mox-u and CA-Nox-u samples) leads to a decrease in ash content from ~3% to ~0.6% (Table 1).The content of metals determined via AAS, and completed by the SEM-EDX results, reveals that after the oxidation process, traces of metals remain, while all the manganese, sodium and iron compounds are mostly removed (Table S1, Figure S1).The porous structure characteristics of the samples determined from the nitrogen sorption-desorption isotherms are presented in Table 2.The results show that after the oxidation process, either for activated carbon obtained from nut shells (CA-N), as well as, for activated carbon obtained from apple wood (CA-M), there is a decrease in the values of S BET of the carbonaceous adsorbents, V total and V meso , whereas V micro is almost unchanged (Table 2).The decrease in V total is mainly due to V meso , and this can be explained by the formation of functional groups on the surface of mesopores in the process of oxidation of the activated carbon.Functional groups cannot form in the micropores because of their length compared to the size of the micropores.According to data from the literature, there is a decrease in S BET and V total after the oxidation process, which in certain cases is explained by the formation of functional groups and pores being blocked [13].The presence of functional groups on the surface of activated carbons obtained from walnut shells and apple wood has been determined by IR spectroscopy, pH metric titrations and the Boehm method.The IR spectra of CA-M and CA-N samples contain absorptions characteristic of unmodified activated carbons, whilst the IR spectra of the samples oxidized with nitric acid (CA-Mox) and nitric acid/urea mixture (CA-Mox-u and CA-Nox-u) display additional absorption bands, which are attributed to the OH group from alcohols, phenols and carboxylic acids.Moreover, the adsorption increases in intensity at ~1700 cm −1 due to vibrations in the C=O bond of the carboxylic, ketones and aldehydes groups (Figure S2) [44][45][46][47].
The pH pzc and dissociation constants of the functional groups (pK) on activated carbons surface were determined from the pH metric titrations.The obtained results show that the pH pzc value of the studied activated carbons follows the series: CA-Mox (2.3) > CA-Mox-u (3.3) > CA-Nox-u (3.9) > CA-M (6.9) > CA-N (8.3) [24].
The curves of distribution of functional groups according to the pK value (Figure S3) show that both initial activated carbon samples (CA-N and CA-M) have only basic functional groups of 0.98 meq/g for the activated carbon CA-M and 0.75 meq/g for the activated carbon CA-N.In addition, it should be noted that most of the peaks belong to the CA-Mox sample (basic functional groups 0.45 meq/g and acid functional groups 1.33 meq/g) [48].Four peaks are distinguishable for each of the activated carbon samples: CA-Mox-u (basic functional groups 0.58 meq/g and acidic functional groups 0.75 meq/g) and for CA-Nox-u (basic functional groups 0.60 meq/g and acidic functional groups 0.55 meq/g) [48].
The acidic or basic character of the functional groups on the surface of activated carbons determined by pH metric titrations is in good agreement with the type of functional groups determined via the Boehm method.Using the Boehm method [29,49], the following acidic groups have been identified on the surface of activated carbon: strongly carboxylic groups (CA-Mox, 0.95 meq/g; CA-Mox-u, 0.44 meq/g; CA-Nox-u, 0.32 meq/g), weakly carboxylic and phenolic groups (Table 3).The small discrepancies in values can be explained by different approaches in the determination of functional groups via these two methods.

Adsorption Studies
The kinetics of cobalt and strontium ions has been described using the following theoretical models: pseudo-first order, pseudo-second order and intraparticle diffusion (Weber-Morris model) models [36][37][38][39].Figure 1 shows the kinetic curves of the adsorption process of cobalt ions on oxidized activated carbons (CA-Mox, CA-Mox-u and CA-Nox-u) and the pH value of the solutions.The concentration of cobalt(II) ions decreases rapidly to approx.50-60 min of phase contact, after which the speed decreases until a relatively constant value is reached.The adsorption equilibrium of cobalt(II) ions on oxidized activated carbons (CA-Mox, CA-Mox-u and CA-Nox-u samples) is established for approx.100 min, as seen from the kinetic curves of variation of the concentration of cobalt(II) ions in the solution, and the pH value of the solutions (Figure 1, Table 4).The adsorption of cobalt ions on oxidized activated carbons is very well described by the pseudo-secondorder kinetic model, since the calculated adsorption values are close to the experimental ones, and the value of the correlation coefficient (R 2 ) is close to 1 (0.999) compared to the R 2 obtained for the pseudo-first-order kinetic model (<0.921) (Tables 4 and S2).The mechanism of adsorption of cobalt ions on activated carbons, as described by the pseudosecond-order kinetic model, was also identified by other authors [50][51][52].The applicability of the pseudo-second-order kinetic model suggests that the adsorption of cobalt ions on oxidized activated carbons is based on chemisorption, involving an exchange of electrons between the adsorbate and the adsorbent where the cobalt ions are attached to the activated carbon surface by chemical bonding [50][51][52][53].
tion process of cobalt ions on oxidized activated carbons (CA-Mox, CA-Mox-u and CA-Nox-u) and the pH value of the solutions.The concentration of cobalt(II) ions decreases rapidly to approx.50-60 min of phase contact, after which the speed decreases until a relatively constant value is reached.The adsorption equilibrium of cobalt(II) ions on oxidized activated carbons (CA-Mox, CA-Mox-u and CA-Nox-u samples) is established for approx.100 min, as seen from the kinetic curves of variation of the concentration of cobalt(II) ions in the solution, and the pH value of the solutions (Figure 1, Table 4).The adsorption of cobalt ions on oxidized activated carbons is very well described by the pseudo-second-order kinetic model, since the calculated adsorption values are close to the experimental ones, and the value of the correlation coefficient (R 2 ) is close to 1 (0.999) compared to the R 2 obtained for the pseudo-first-order kinetic model (<0.921) (Tables 4  and S2).The mechanism of adsorption of cobalt ions on activated carbons, as described by the pseudo-second-order kinetic model, was also identified by other authors [50][51][52].The applicability of the pseudo-second-order kinetic model suggests that the adsorption of cobalt ions on oxidized activated carbons is based on chemisorption, involving an exchange of electrons between the adsorbate and the adsorbent where the cobalt ions are attached to the activated carbon surface by chemical bonding [50][51][52][53].The prediction of the rate-limiting step is an important factor to consider in the adsorption process.The adsorption mechanism could follow multiple kinetic orders that are changed during the adsorption process [54].For adsorption at the solid/liquid interface, the solute transfer process is usually characterized by external mass transfer (diffusion into the solution film from the surface of the activated carbon particle) or intraparti-  The prediction of the rate-limiting step is an important factor to consider in the adsorption process.The adsorption mechanism could follow multiple kinetic orders that are changed during the adsorption process [54].For adsorption at the solid/liquid interface, the solute transfer process is usually characterized by external mass transfer (diffusion into the solution film from the surface of the activated carbon particle) or intraparticle diffusion, or both.A better interpretation of the adsorption mechanisms can be obtained in the plots of q t versus t 1/2 .Figures 2 and S4 show the dependence of cobalt ions adsorption on the square root of time, according to the Weber-Morris model [39].All the oxidized activated carbons showed a multilinearity of the lines, which implies that the adsorption process involves more than one kinetic stage (Figures 2 and S4).All the oxidized activated carbons showed a multilinearity of the lines, which implies that the adsorption process involves more than one kinetic stage (Figures 2 and S4).The first stage for the sample CA-Mox (Figure 2) suggests that the adsorption process proceeds through the stage of adsorption on the activated carbon particle surface (external adsorption), and the second C 2023, 9, 71 8 of 21 one corresponds to intraparticle diffusion (gradual diffusion within the pores).Then, the adsorption equilibrium has established [39].
tained in the plots of qt versus t .Figures 2 and S4 show the dependence of cobalt ions adsorption on the square root of time, according to the Weber-Morris model [39].All the oxidized activated carbons showed a multilinearity of the lines, which implies that the adsorption process involves more than one kinetic stage (Figures 2 and S4).All the oxidized activated carbons showed a multilinearity of the lines, which implies that the adsorption process involves more than one kinetic stage (Figures 2 and S4).The first stage for the sample CA-Mox (Figure 2) suggests that the adsorption process proceeds through the stage of adsorption on the activated carbon particle surface (external adsorption), and the second one corresponds to intraparticle diffusion (gradual diffusion within the pores).Then, the adsorption equilibrium has established [39].For the case of the strontium(II) ions adsorption from aqueous solutions (concentration of approx. 1 mmol/L) the adsorption equilibrium, both for the initial activated carbons (CA-N, CA-M) and for the oxidized activated carbons (CA-Nox-u, CA-Mox-u, CA-Mox), is established after approx.100 min (Figures 3 and S5-S7).The pH value of the strontium solution after contact with CA-M activated carbon increases from 4.0 to 5.2 (Figure 3b), also on the dependence 2 inflections are observed.Such inflections were also observed by other authors, who assume that the adsorption of strontium ions takes place in several steps [55].Among the oxidized activated carbon samples, as in the case of cobalt ions adsorption, the CA-Mox sample is distinguished, which shows the highest adsorption capacity for strontium ions (Figure 3, Table 5).The behavior of oxidized activated carbons (CA-Mox, CA-Mox-u and CA-Nox-u) in strontium(II) solutions is interesting, as the sample CA-Mox tends to decrease the pH value from 4.0 to 3.5 3, and 5 in the first minutes of contact and gradually increases up to approx.3.7, which corresponds to the adsorption equilibrium establishing time (Figure 3b).For CA-Mox-u and CA-Nox-u samples, the pH value increases from 4.0 to 4.5 and 5.7, respectively (Figures S6 and S7).This suggests that the pH value of the solution tends towards the pHpzs value of activated carbons.
The adsorption of strontium ions on the studied activated carbons is described by the pseudo-second-order kinetic model; the calculated adsorption value is close to that determined experimentally, and the correlation coefficients for the initial and oxidized activated carbons are close to 1, in contrast to the pseudo-first-order kinetic model (R 2 = 0.524-0.915)(Tables 5 and S3).According to the data published in the specialized literature, the kinetics of the adsorption process of strontium ions on activated carbons is best described by the pseudo-second-order kinetic model [56][57][58].For the case of the strontium(II) ions adsorption from aqueous solutions (concentration of approx. 1 mmol/L) the adsorption equilibrium, both for the initial activated carbons (CA-N, CA-M) and for the oxidized activated carbons (CA-Nox-u, CA-Mox-u, CA-Mox), is established after approx.100 min (Figures 3 and S5-S7).The pH value of the strontium solution after contact with CA-M activated carbon increases from 4.0 to 5.2 (Figure 3b), also on the dependence 2 inflections are observed.Such inflections were also observed by other authors, who assume that the adsorption of strontium ions takes place in several steps [55].Among the oxidized activated carbon samples, as in the case of cobalt ions adsorption, the CA-Mox sample is distinguished, which shows the highest adsorption capacity for strontium ions (Figure 3, Table 5).The behavior of oxidized activated carbons (CA-Mox, CA-Mox-u and CA-Nox-u) in strontium(II) solutions is interesting, as the sample CA-Mox tends to decrease the pH value from 4.0 to 3.5 in the first minutes of contact and gradually increases up to approx.3.7, which corresponds to the adsorption equilibrium establishing time (Figure 3b).For CA-Mox-u and CA-Nox-u samples, the pH value increases from 4.0 to 4.5 and 5.7, respectively (Figures S6 and S7).This suggests that the pH value of the solution tends towards the pH pzs value of activated carbons.
The adsorption of strontium ions on the studied activated carbons is described by the pseudo-second-order kinetic model; the calculated adsorption value is close to that determined experimentally, and the correlation coefficients for the initial and oxidized activated carbons are close to 1, in contrast to the pseudo-first-order kinetic model (R 2 = 0.524-0.915)(Tables 5 and S3).According to the data published in the specialized literature, the kinetics of the adsorption process of strontium ions on activated carbons is best described by the pseudo-second-order kinetic model [56][57][58].
The kinetic adjustment of the experimental data with the intraparticle diffusion model (Weber-Morris) pointed out that the adsorption process of strontium ions on activated carbons takes place in two stages: the first stage, linear for sample CA-M and curved for CA-Mox, is followed by a second linear stage and the curves do not pass through the origin (Figure 4).These two steps of the intraparticle diffusion model suggest that the adsorption process proceeds through the sorption of strontium(II) ions on the activated carbon surface and intraparticle diffusion.The initial curved portion of the dependence indicates the effect of the solution layer or film on the surface of the adsorbent particles, while the second, linear portion is due to intraparticle or pore diffusion [59].The kinetic adjustment of the experimental data with the intraparticle diffusion model (Weber-Morris) pointed out that the adsorption process of strontium ions on activated carbons takes place in two stages: the first stage, linear for sample CA-M and curved for CA-Mox, is followed by a second linear stage and the curves do not pass through the origin (Figure 4).These two steps of the intraparticle diffusion model suggest that the adsorption process proceeds through the sorption of strontium(II) ions on the activated carbon surface and intraparticle diffusion.The initial curved portion of the dependence indicates the effect of the solution layer or film on the surface of the adsorbent particles, while the second, linear portion is due to intraparticle or pore diffusion [59].The kinetic adjustment of the experimental data with the intraparticle diffusion model (Weber-Morris) pointed out that the adsorption process of strontium ions on activated carbons takes place in two stages: the first stage, linear for sample CA-M and curved for CA-Mox, is followed by a second linear stage and the curves do not pass through the origin (Figure 4).These two steps of the intraparticle diffusion model suggest that the adsorption process proceeds through the sorption of strontium(II) ions on the activated carbon surface and intraparticle diffusion.The initial curved portion of the dependence indicates the effect of the solution layer or film on the surface of the adsorbent particles, while the second, linear portion is due to intraparticle or pore diffusion [59].The results of the adsorption equilibrium data of the cobalt(II) and strontium(II) ions were fitted with theoretic models Langmuir, Freundlich, Temkin-Pyzhev and Dubinin-Radushkevich.The experimental data for the cobalt ions adsorption on oxidized activated carbon samples (CA-Mox, CA-Mox-u and CA-Nox-u) are best described by the Langmuir isotherm model (Figure 5 and Table 6) and also correlate well with data from the literature.
According to adsorption isotherms, the CA-Mox sample adsorbs approx.0.079 mmol/g, CA-Mox-u approx.0.051 mmol/g and CA-Nox-u approx.0.038 mmol/g of cobalt ions.The R 2 value of the four theoretical models applied to describe the adsorption of cobalt(II) ions on oxidized activated carbons decreases in the order: Langmuir > Temkin-Pyzhev > Dubinin-Radushkevich > Freundlich.It is clear that the Langmuir model corresponds to a dominant electrostatic attraction, ion exchange and complexation mechanism, while the order of the models shows that the adsorption process involved the physical adsorption and complexation process at the external heterogeneous surface of oxidized activated carbons.Acidic functional groups on the surface of activated carbons play an important role in the adsorption of metal ions from solutions [13,14,51,60].When contacting activated carbon with an aqueous solution, the acidic functional groups on the surface are ionized, H + ions are released into the solution and the activated carbon surface becomes negatively charged, on the surface appear negatively charged adsorption centers [61].The greater the number of acidic groups on the surface of the activated carbon, the more negative the charge on the surface of the activated carbon will be.This leads to increased electrostatic attraction between the negatively charged activated carbon surface and the positively charged metal cations [13,14,23,61].
The pH value of the cobalt nitrate solution after contact with the oxidized activated carbons decreases due to the acid functional groups on the surface and their ionization effect.For the CA-Mox sample, the pH value of cobalt(II) solutions at equilibrium gradually decreases from approx. 4 to approx.3, when the adsorption equilibrium of cobalt ions is also reached, and for the CA-Mox-u and CA-Nox-u samples the pH value of the solutions drops to 3.8, indicating the fact that adsorption occurs by ion exchange (Figure S8).
In the case of the adsorption of strontium(II) ions from the solution on the oxidized activated carbons, from the adsorption isotherms it is observed that the activated carbon CA-Mox adsorbs about 0.073 mmol/g, the activated carbon CA-Mox-u adsorbs approx.0.033 mmol/g and CA-Nox-u adsorbs 0.028 mmol/g of strontium ions (Figure 6).The maximum monolayer adsorption capacity of oxidized activated carbons for cobalt(II) ions (calculated according to the Langmuir model) is 0.075 mmol/g for the CA-Mox sample, 0.069 mmol/g for the CA-Mox-u sample and 0.040 mmol/g g for the CA-Nox-u sample (Table 6).For all oxidized activated carbons (CA-Mox, CA-Mox-u and CA-Nox-u) the R L value fell within the limits of 6.1 × 10 −6 ÷ 2.2 × 10 −4 , which means that the process of adsorption of cobalt(II) ions on the studied samples is very favorable [14,[50][51][52]].The free energy calculated according to the Dubinin-Radushkevich model denotes that chemical sorption takes place in the modeled system (Table 6).
Acidic functional groups on the surface of activated carbons play an important role in the adsorption of metal ions from solutions [13,14,51,60].When contacting activated carbon with an aqueous solution, the acidic functional groups on the surface are ionized, H + ions are released into the solution and the activated carbon surface becomes negatively charged, on the surface appear negatively charged adsorption centers [61].The greater the number of acidic groups on the surface of the activated carbon, the more negative the charge on the surface of the activated carbon will be.This leads to increased electrostatic attraction between the negatively charged activated carbon surface and the positively charged metal cations [13,14,23,61].
The pH value of the cobalt nitrate solution after contact with the oxidized activated carbons decreases due to the acid functional groups on the surface and their ionization effect.For the CA-Mox sample, the pH value of cobalt(II) solutions at equilibrium gradually decreases from approx. 4 to approx.3, when the adsorption equilibrium of cobalt ions is also reached, and for the CA-Mox-u and CA-Nox-u samples the pH value of the solutions drops to 3.8, indicating the fact that adsorption occurs by ion exchange (Figure S8).
In the case of the adsorption of strontium(II) ions from the solution on the oxidized activated carbons, from the adsorption isotherms it is observed that the activated carbon CA-Mox adsorbs about 0.073 mmol/g, the activated carbon CA-Mox-u adsorbs approx.0.033 mmol/g and CA-Nox-u adsorbs 0.028 mmol/g of strontium ions (Figure 6).Acidic functional groups on the surface of activated carbons play an important role in the adsorption of metal ions from solutions [13,14,51,60].When contacting activated carbon with an aqueous solution, the acidic functional groups on the surface are ionized, H + ions are released into the solution and the activated carbon surface becomes negatively charged, on the surface appear negatively charged adsorption centers [61].The greater the number of acidic groups on the surface of the activated carbon, the more negative the charge on the surface of the activated carbon will be.This leads to increased electrostatic attraction between the negatively charged activated carbon surface and the positively charged metal cations [13,14,23,61].
The pH value of the cobalt nitrate solution after contact with the oxidized activated carbons decreases due to the acid functional groups on the surface and their ionization effect.For the CA-Mox sample, the pH value of cobalt(II) solutions at equilibrium gradually decreases from approx. 4 to approx.3, when the adsorption equilibrium of cobalt ions is also reached, and for the CA-Mox-u and CA-Nox-u samples the pH value of the solutions drops to 3.8, indicating the fact that adsorption occurs by ion exchange (Figure S8).
In the case of the adsorption of strontium(II) ions from the solution on the oxidized activated carbons, from the adsorption isotherms it is observed that the activated carbon CA-Mox adsorbs about 0.073 mmol/g, the activated carbon CA-Mox-u adsorbs approx.0.033 mmol/g and CA-Nox-u adsorbs 0.028 mmol/g of strontium ions (Figure 6).The pH value of the equilibrium solutions also differs for the strontium nitrate solutions after contact with CA-Mox, as the pH value at the lowest equilibrium concentrations gradually decreases from 4 to approx.3.6 and to approx.3.4 around reaching equilibrium (Figure S9).In the case of the adsorption isotherms of strontium ions on CA-Mox-u and CA-Nox-u samples in equilibrium solutions, the pH trend is totally different.The pH value from 4.0 (in the initial solutions) increases to 5.8 and then gradually decreases to approx.4.6 for the CA-Mox-u sample, and for the CA-Nox-u sample the pH value increases from 4.0 to 6.2 and decreases to approx.5.8, upon reaching equilibrium (Figure S9) [24].Although the results obtained for the pH value of the equilibrium solutions for the CA-Mox-u and CA-Nox-u samples seem to contradict those obtained for the CA-Mox sample, they are consistent with the pH value of the solutions at reaching equilibrium to study the adsorption kinetics of strontium ions.
The parameters and constants of the theoretical isotherms calculated for the adsorption of strontium(II) ions on the oxidized activated carbons CA-Mox, CA-Mox-u and CA-Nox-u are presented in Table 7.According to the correlation coefficients, the adsorption isotherms of strontium(II) ions on oxidized activated carbons correspond to the Langmuir model.According to data from the literature, the adsorption process of strontium(II) ions on activated charcoal from rice straw (non-oxidized) is described by the Dubinin-Radushkevich model; at the same time, the authors say that the adsorption is a physical one, with a free energy of 9.7 kJ/mol [56].Similar results of applicability of the Dubinin-Radushkevich model for describing the adsorption of strontium(II) ions on modified activated carbons are presented for a limited range of equilibrium concentrations 0.016-0.084mmol/L [60].The R L value demonstrates that the adsorption process of strontium(II) ions on oxidized activated carbons is favorable, also being confirmed with the help of the empirical parameter (1/n) calculated for the theoretical Freundlich model (0 < 1/n < 1).The free energy calculated according to the Dubinin-Radushkevich model identifies that in the modeled system a chemical sorption occurs.
By summarizing the obtained results, the adsorption capacity of the studied activated carbons (CA-Mox, CA-Mox-u and CA-Nox-u) for both cobalt(II) and strontium(II) ions, determined from kinetics measurements and adsorption isotherms correlates well.The data from the literature regarding the maximum adsorption capacities of different carbonaceous adsorbents for cobalt(II) and strontium(II) ions removal are summarized in Table S5.These results show that the adsorption capacity of CA-Mox, CA-Mox-u and CA-Mox-u for adsorption of cobalt(II) and strontium(II) are comparative to some adsorbents and lower with others, but it is quite challenging to draw a conclusion because the experimental conditions are very different (regarding initial concentration, pH value temperature, etc.).
In some research, several factors influencing the adsorption of metal ions on adsorbents are discussed, such as ion radius, hydrated ion radius, hydration energy, and electronegativity [53,57,62].In general, the larger the radius of the ion, the greater the affinity for the adsorbent surface.
Metal cations with lower hydration energy will have a greater tendency to be adsorbed on activated carbons [53].Ions with a higher electronegativity will be more tightly attracted to the surface of the adsorbent [53].In aqueous solutions cobalt(II) and strontium(II) ions  [62,63].The radii of hydrated cobalt(II) and strontium(II) ions are very close in value, 0.412 nm and 0.423 nm, respectively (Table S4).At the same time, the hydration-free energy is lower for cobalt(II) ions, and the electronegativity is higher, which means that cobalt(II) ions will have a higher affinity [63] to the activated carbon surface (Table S4).Under the conditions of the experiments in the paper, according to the obtained data, the adsorption capacity of activated carbons for cobalt(II) ions is comparable to that for strontium(II) ions.
The pH value is an important parameter for the adsorption of metal ions from aqueous solutions because it affects the metal ions solubility, the concentration of ions associated with the adsorbent functional groups and the ionization degree of functional groups during the reaction [64].In cases when the solution pH is higher than the adsorbent pH pzc , the negative charge from the surface ensures favorable electrostatic interactions for the cationic species adsorption.For the samples of activated carbons used in the Co(II) and Sr(II) ions adsorption from aqueous solutions, the pH pzc value increases as follows: CA-Mox (2.3) > CA-Mox-u (3.3) > CA-Nox-u (3.9) > CA-M (6.9) > CA-N (8.3).
The pH influence on the adsorption (removal) of cobalt(II) and strontium(II) ions from the solution in the presence of activated carbons is represented in Figure 7, Figures S9 and S10.It should be noted that the values of pH and electrical conductivity of the final solutions are different than those of the initial solutions.In the case of cobalt(II) ions removal, the pH of initial solutions varies from 1 to 10 (the conductivity value ranges from 118 to 128 µScm −1 ), while pH at equilibrium varies in steps.As shown in Figure 7 and Figure S9, for the removal rate of cobalt(II) ions (variation of pH and conductibility at equilibrium), the behavior of initial activated carbons, CA-M and CA-N, and the oxidized ones, CA-Mox-u and CA-Nox-u, is similar: nothing happens on the pH range from 1 to 2, the pH value is constant and the cobalt ions removal rate is null.This fact is also demonstrated by the conductivity of the solutions.
adsorbents are discussed, such as ion radius, hydrated ion radius, hydration energy, and electronegativity [53,57,62].In general, the larger the radius of the ion, the greater the affinity for the adsorbent surface.
Metal cations with lower hydration energy will have a greater tendency to be ad sorbed on activated carbons [53].Ions with a higher electronegativity will be more tight ly attracted to the surface of the adsorbent [53].In aqueous solutions cobalt(II) and stron tium(II) ions are hydrated; cobalt(II) ions are surrounded by six water molecules [Co(H2O)6] 2+ and strontium ions are octahydrated [Sr(H2O)8] 2+ [62,63].The radii of hy drated cobalt(II) and strontium(II) ions are very close in value, 0.412 nm and 0.423 nm respectively (Table S4).At the same time, the hydration-free energy is lower for cobalt(II ions, and the electronegativity is higher, which means that cobalt(II) ions will have a higher affinity [63] to the activated carbon surface (Table S4).Under the conditions of the experiments in the paper, according to the obtained data, the adsorption capacity of ac tivated carbons for cobalt(II) ions is comparable to that for strontium(II) ions.
The pH value is an important parameter for the adsorption of metal ions from aqueous solutions because it affects the metal ions solubility, the concentration of ions associated with the adsorbent functional groups and the ionization degree of functiona groups during the reaction [64].In cases when the solution pH is higher than the adsor bent pHpzc, the negative charge from the surface ensures favorable electrostatic interac tions for the cationic species adsorption.For the samples of activated carbons used in the Co(II) and Sr(II) ions adsorption from aqueous solutions, the pHpzc value increases as fol lows: CA-Mox (2.3)> CA-Mox-u (3.3)> CA-Nox-u (3.9)> CA-M (6.9)> CA-N (8.3).
The pH influence on the adsorption (removal) of cobalt(II) and strontium(II) ions from the solution in the presence of activated carbons is represented in Figures 7, S9 and S10.It should be noted that the values of pH and electrical conductivity of the final solu tions are different than those of the initial solutions.In the case of cobalt(II) ions remov al, the pH of initial solutions varies from 1 to 10 (the conductivity value ranges from 118 to 128 µScm −1 ), while pH at equilibrium varies in steps.As shown in Figures 7 and S9 for the removal rate of cobalt(II) ions (variation of pH and conductibility at equilibrium) the behavior of initial activated carbons, CA-M and CA-N, and the oxidized ones, CA Mox-u and CA-Nox-u, is similar: nothing happens on the pH range from 1 to 2, the pH value is constant and the cobalt ions removal rate is null.This fact is also demonstrated by the conductivity of the solutions.On the second pH interval 3-5, the equilibrium pH varies between 3.25-3.70.On th same interval, one observes an increase in the removal rate of cobalt ions, comparabl values for these four types of activated carbons (CA-M, CA-N, CA-Mox-u and CA-Nox u).It is unclear if acidic functional groups from the activated carbon surface were in volved in the cobalt ions adsorption.The pHpzc value of the oxidized samples is 3.3 (CA Mox-u) and 3.9 (CA-Nox-u) and in this pH of solutions, some acidic functional group values for these four types of activated carbons (CA-M, CA-N, CA-Mox-u and CA-Nox-u).It is unclear if acidic functional groups from the activated carbon surface were involved in the cobalt ions adsorption.The pH pzc value of the oxidized samples is 3.3 (CA-Mox-u) and 3.9 (CA-Nox-u) and in this pH of solutions, some acidic functional groups from the oxidized activated carbon surface should be ionized.For solutions with an initial pH of 6, the value of pH at equilibrium suddenly increases up to ~6.8.This phenomenon can be explained by the presence of various forms of cobalt ions in solution, e.g., Co(OH) + , Co(OH) 2 and Co(OH) 3 − , at pH near 8; therefore, the effect of cobalt ions removal is decreased (Figure S12) [50].For the next stage (pH 7-10), the initial pH of the solutions decreases to 7.0-7.5 at equilibrium.On this interval, most probably, the precipitation of cobalt ions takes place but not the adsorption, because the conductivity value is nearly constant in these solutions (Figure 8).On the diagram of cobalt ions removal from solution, at pH values of 9 and 10 the removal rate increases until ~85%.However, this is due to the precipitation of the cobalt ions in the hydroxide form (Figure 7a).The influence of the solution pH on cobalt ions adsorption on various adsorbents had been studied by several researchers.Nevertheless, the pH interval when cobalt ions adsorption occurs is different and some have reported that maximum adsorption occurs in the pH interval 3-5, 6.5 and 7 [50,52,[65][66][67][68].On the second pH interval 3-5, the equilibrium pH varies between 3.25-3.70.On the same interval, one observes an increase in the removal rate of cobalt ions, comparable values for these four types of activated carbons (CA-M, CA-N, CA-Mox-u and CA-Noxu).It is unclear if acidic functional groups from the activated carbon surface were involved in the cobalt ions adsorption.The pHpzc value of the oxidized samples is 3.3 (CA-Mox-u) and 3.9 (CA-Nox-u) and in this pH of solutions, some acidic functional groups from the oxidized activated carbon surface should be ionized.For solutions with an initial pH of 6, the value of pH at equilibrium suddenly increases up to ~6.8.This phenomenon can be explained by the presence of various forms of cobalt ions in solution, e.g., Co(OH) + , Co(OH)2 and Co(OH)3 − , at pH near 8; therefore, the effect of cobalt ions removal is decreased (Figure S12) [50].For the next stage (pH 7-10), the initial pH of the solutions decreases to 7.0-7.5 at equilibrium.On this interval, most probably, the precipitation of cobalt ions takes place but not the adsorption, because the conductivity value is nearly constant in these solutions (Figure 8).On the diagram of cobalt ions removal from solution, at pH values of 9 and 10 the removal rate increases until ~85%.However, this is due to the precipitation of the cobalt ions in the hydroxide form (Figure 7a).The influence of the solution pH on cobalt ions adsorption on various adsorbents had been studied by several researchers.Nevertheless, the pH interval when cobalt ions adsorption occurs is different and some have reported that maximum adsorption occurs in the pH interval 3-5, 6.5 and 7 [50,52,[65][66][67][68].The oxidized activated carbon CA-Mox, when compared to other samples, behaves slightly differently in solution.On the pH interval 1-2, the pH value at equilibrium remains constant, and the removal rate of cobalt ions is null (Figure 8).With the increase in the initial solution pH (pH interval 3-5), the pH at equilibrium decreases to ~3 (2.98-3.16).This pH interval is higher than the pH pzc of this activated carbon (2.3) and the cobalt ions adsorption depends on the dissociation state of weak acidic functional groups.At pH higher than 5, the number of competitive hydrogen ions is lower and more functional groups are dissociated, the charge on the surface of the activated carbon CA-Mox becomes more negative and, therefore, contributes to the cobalt ions adsorption via ion exchange.For solution pH of 6, the pH at equilibrium decreases to 5.1 (Figure 8b), also, the increase in the adsorption rate of cobalt ions is observed (Figure 7a).This can be explained by two effects: (i) at pH 6, the dissociation of strong acidic functional groups from the surface of CA-Mox begins, hydrogen ions are released in solution (solution pH decreases) and adsorption of cobalt cations occurs; (ii) pH 6 > pH pzc of activated carbon CA-Mox, then the surface is more negatively charged and attracts positively charged cobalt ions.From Figure 8, the variation of the pH value at equilibrium (and of the conductivity [24]), it is highlighted that cobalt ions adsorption occurs up to pH 8 of the initial solutions (equilibrium pH of 5.9-6.1)(Figure 7a).Generally, functional groups from the activated carbons' surface possess affinity and adsorption capacity for heavy metal ions because they are able to donate a pair of electrons, thus bonding the metal via chelation or complexation.The adsorption mechanism is explained by surface complexation, diffusion in adsorbent micropores, chemisorption, ion exchange and electrostatic interactions, which may occur in the singular or complex modes [13,69].
The obtained results show that acidic functional groups formed on the activated carbon surface CA-Mox (via oxidation with nitric acid) had led to the increase in the ion exchange capacity among the cobalt(II) ions and the following functional groups: hydroxyl (-OH), carboxyl (-COOH) and carbonyl (-C=O).Other researchers have also tried to explain a dependence between the point of zero charge (PZC), solutions pH and the character of surface functional groups [14,68,70].The studies have revealed that the modification of the solution pH leads to the change of the activated carbon surface charge, which is different for different solution pH values [68,71].It is also mentioned that carboxylic and lactone groups dissociate in aqueous solutions, the H + ions transfer in the solution and the activated carbon surface remains negatively charged (-COO − ) [68,72].
In the case of strontium(II) ions adsorption onto activated carbons (CA-M, CA-N, CA-Mox-u, CA-Nox-u and CA-Mox), the adsorption capacity depends on the solution's pH value.At an initial solution pH < 3, the value of pH at equilibrium is not modified for all the investigated samples, and the strontium ions adsorption does not occur, which suggests the complete protonation of the functional groups from the activated carbon surface (Figure 8b and Figure S11).For higher pH values (4-7), the removal rate of strontium ions from the solution is this observation is in agreement with the conductivity value at equilibrium for this pH range.On the pH interval 8 ÷ 10 of the initial solution, the removal rate of strontium ions slightly increases, but the pH value at equilibrium remains constant at around 6 for CA-M, CA-N, CA-Mox-u and CA-Nox-u carbons and in the range of 4 and 5 for CA-Mox.For all samples of activated carbons, the solution conductivity at equilibrium tends to decrease.In aqueous solutions, strontium is found as Sr 2+ ions and SrOH + species on a broad pH interval (pH 6-10, Figure S13), but SrOH + is the main form at pH > 12.8 [63].The strontium ions removal capacity at pH higher than 7 may be due to a combined effect of adsorption and precipitation on the activated carbon surface [73].
Generally, according to the results obtained, there is not a good agreement between the amount of acid functional groups on the surface of oxidized activated carbons (determined by the Boehm method and electrometric titrations) and the amount of adsorbed metals.This was also observed by other authors and can be explained by the fact that not all acidic functional groups are accessible to hydrated metal ions.The acidic functional groups in the pores often interact with each other and block the pores [13].
In the literature, for the adsorption of strontium ions from solutions on activated carbon from rice straw, modified chitosan, activated carbon obtained from almond shells and activated carbon from pecan nuts, the ion exchange mechanism between carboxylic, phenolic and lactone acidic functional groups has been proposed and strontium ions [57,58,[73][74][75] or the combination of two mechanisms, dipole-dipole interaction and ion exchange [60].

The Suggested Mechanisms for the Adsorption of Co(II)/Sr(II) Ions on Activated Carbons
By corroborating the obtained results regarding the characterization of carbonaceous adsorbents surface chemistry and the metal ions' adsorption capacity as a function of the solution pH, mechanisms of adsorption are proposed.
For the metal ions' adsorption onto activated carbons at pH < 3, there is competition between the H + and Me 2+ ions for the same adsorption site.
The interval pH < pH pzc of activated carbons.Generally, at values of pH < pH pzc , the metal ions' adsorption on activated carbons may occur via the complexation mechanism (between the oxygen atoms from C=O groups from the activated carbon surface and the metal ions) and electrostatic interactions (Figure 9).
of the solution pH, mechanisms of adsorption are proposed.
For the metal ions' adsorption onto activated carbons at pH < 3, there is competition between the H + and Me 2+ ions for the same adsorption site.
The interval pH < pHpzc of activated carbons.Generally, at values of pH < pHpzc, the metal ions' adsorption on activated carbons may occur via the complexation mechanism (between the oxygen atoms from C=O groups from the activated carbon surface and the metal ions) and electrostatic interactions (Figure 9).The interactions Cπ-Me n+ (cation) are attributed to electrostatic interactions between the aromatic rings of carbonaceous adsorbents (with the basic surface) and metal cations.On the activated carbon surface, according to some authors, the π (Cπ) electrons possess weak negative charge properties [76].The positions with Cπ electronic density could correspond to the basic functional groups determined via the Boehm method (chromene, ketones and pyrones structures) [76].

•
The interval of pH 3 ÷ 5.In this pH interval, the oxidized activated carbons adsorb metal ions by ion exchange and the formation of metal-ligand complexes on the surface (Figure 10).The interactions C π -Me n+ (cation) are attributed to electrostatic interactions between the aromatic rings of carbonaceous adsorbents (with the basic surface) and metal cations.On the activated carbon surface, according to some authors, the π (C π ) electrons possess weak negative charge properties [76].The positions with C π electronic density could correspond to the basic functional groups determined via the Boehm method (chromene, ketones and pyrones structures) [76].

•
The interval of pH 3 ÷ 5.In this pH interval, the oxidized activated carbons adsorb metal ions by ion exchange and the formation of metal-ligand complexes on the surface (Figure 10).The pH interval of pH > 7.For oxidized activated carbons, within this interval, the dissociation of hydroxyl groups (from phenols and alcohols, -OH; pK~9-13) begins, also in this interval Co(OH)2 and Sr(OH)2 hydroxides are formed.Moreover, the increase in the removal rate of Co(II) and Sr(II) ions is mostly due to the precipitate formation of these hydroxides in solution or on the activated carbon surface.In conclusion, under the modeled conditions, for the studied metal ions, hydroxyl groups do not participate in the adsorption process.The interval of pH > 7.For oxidized activated carbons, within this interval, the dissociation of hydroxyl groups (from phenols and alcohols, -OH; pK~9-13) begins, also in this interval Co(OH) 2 and Sr(OH) 2 hydroxides are formed.Moreover, the increase in the removal rate of Co(II) and Sr(II) ions is mostly due to the precipitate formation of these hydroxides in solution or on the activated carbon surface.In conclusion, under the modeled conditions, for the studied metal ions, hydroxyl groups do not participate in the adsorption process.

Conclusions
New carbonaceous adsorbents had been obtained from walnut shells and apple wood by modifying the surface via oxidation with nitric acid and nitric acid/urea mixture.The presence of functional groups on the surface of activated carbons obtained from nut shells and apple wood has been determined by using IR spectroscopy, pH metric titrations and the Boehm method.The following functional groups have been identified on the surface of activated carbon: strongly acidic groups (CA-Mox, 0.95 meq/g; CA-Mox-u, 0.44 meq/g; CA-Nox-u, 0.32 meq/g) and weakly acidic and phenolic groups, while the pH pzc value of the studied activated carbons increase as follows: CA-Mox (2.3) > CA-Mox-u (3.3) > CA-Nox-u (3.9) > CA-M (6.9) > CA-N (8.3).
The adsorption process of strontium(II) and cobalt(II) ions on activated carbons has been studied by applying the following models: pseudo-first-order model, pseudo-secondorder model and intraparticle diffusion model, as well as theoretic isotherms models of Langmuir, Freundlich, Temkin-Pyzhev and Dubinin-Radushkevich.The experimental data are best described by the pseudo-second kinetic model suggesting that the adsorption of metal ions (cobalt(II) and strontium(II)) onto oxidized activated carbons is based on chemisorption.The constants determined for the Freundlich and Langmuir models and the energy values estimated by Dubinin-Radushkevich demonstrate that the adsorption process of strontium(II) and cobalt(II) ions onto oxidized activated carbons is favorable and dominated by chemisorption of the energetically homogenous surface in the modeled system.
The investigations highlight the important role of the solution pH, of the activated carbons functional groups and the pH pzc value for the adsorption process of cobalt(II) and strontium(II) ions.It should be noted that depending on the surface chemistry and solution pH, the adsorption of strontium(II) and cobalt(II) ions occurs via different mechanisms:

•
For pH < pHpzc values, the adsorption of metal ions on activated carbons may occur through the complexation mechanism (between oxygen atoms from C=O groups from the activated carbon surface and metal ions) and electrostatic interactions;

•
For the adsorption of metal ions on initial activated carbons (with the basic surface, on the pH interval 3 ÷ 5), the mechanism of adsorption via electrostatic interactions between the Cπ sites and metal ions has been proposed.For oxidized activated carbons, the mechanism of ion exchange between the carboxylic groups and metal ions and the formation of metal-ligand complexes on the surface has been proposed; • Within the pH interval 5 ÷ 7, the weak acidic functional groups of lactone type dissociate and the mechanism of adsorption of cobalt(II) ion species on the surface of the oxidized activated carbon via ion exchange and complexation has been proposed.

Figure 2 .
Figure 2. Intraparticle diffusion kinetic model (Weber-Morris) for cobalt ions adsorption on oxidized activated carbon CA-Mox.(a) The plot of a t versus t 1/2 ; (b) the plot of pH t versus t 1/2 .

Figure 3 .
Figure 3. Kinetics of strontium ions adsorption on activated carbons: CA-M (1) s , i CA-Mox (2).(a) The variation of strontium ions concentration in solution with time.(b) The pH value of solutions after adsorption of strontium ions on activated carbons.Solid:liquid ratio-1:100, pH = 4.

Figure 7 .
Figure 7.The pH influence on the adsorption (removal) of cobalt(II) and strontium(II) ions from the solution in the presence of oxidized activated carbons.Removal rate (R, %) of cobalt(II) ions (a and strontium(II) ions (b) from solutions.

Figure 7 .
Figure 7.The pH influence on the adsorption (removal) of cobalt(II) and strontium(II) ions from the solution in the presence of oxidized activated carbons.Removal rate (R, %) of cobalt(II) ions (a) and strontium(II) ions (b) from solutions.On the second pH interval 3-5, the equilibrium pH varies between 3.25-3.70.On the same interval, one observes an increase in the removal rate of cobalt ions, comparable

Figure 7 .
Figure 7.The pH influence on the adsorption (removal) of cobalt(II) and strontium(II) ions from the solution in the presence of oxidized activated carbons.Removal rate (R, %) of cobalt(II) ions (a) and strontium(II) ions (b) from solutions.

Figure 8 .Figure 8 .
Figure 8.The pH influence on the adsorption (removal) of cobalt(II) ions from solutions in the presence of oxidized activated carbons: Δ CA-Nox-u, • CA-Mox-u, ○ CA-Mox.(a) and (b) pH variation in equilibrium solutions.

Figure 9 .
Figure 9. Schematic presentation of the adsorption of metal ions on activated carbons by complexation.

Figure 9 .
Figure 9. Schematic presentation of the adsorption of metal ions on activated carbons by complexation.

Figure 10 .
Figure 10.Schematic presentation of the adsorption of metal ions on oxidized activated carbons by ion exchange and the formation of metal-ligand complexes on the surface.• The pH interval of 5 ÷ 7.For oxidized activated carbon, within this interval, the weak acidic functional groups dissociate, e.g., lactones.At the same time, the species of Co(II), Co(OH) + , Co(OH)2 and Co(OH)3 -begin to form in the solution.The schematic presentation of the adsorption of Co(II) ion species on the surface of oxidized activated carbon by ion exchange and complexation is shown in Figure 11.

Figure 10 .
Figure 10.Schematic presentation of the adsorption of metal ions on oxidized activated carbons by ion exchange and the formation of metal-ligand complexes on the surface.•ThepH interval of 5 ÷ 7.For oxidized activated carbon, within this interval, the weak acidic functional groups dissociate, e.g., lactones.At the same time, the species of Co(II), Co(OH) + , Co(OH) 2 and Co(OH) 3 − begin to form in the solution.The schematic

Figure 10 .
Figure 10.Schematic presentation of the adsorption of metal ions on oxidized activated carbons by ion exchange and the formation of metal-ligand complexes on the surface.•ThepH interval of 5 ÷ 7.For oxidized activated carbon, within this interval, the weak acidic functional groups dissociate, e.g., lactones.At the same time, the species of Co(II), Co(OH) + , Co(OH)2 and Co(OH)3 -begin to form in the solution.The schematic presentation of the adsorption of Co(II) ion species on the surface of oxidized activated carbon by ion exchange and complexation is shown in Figure11.

Figure 11 .
Figure 11.Schematic presentation of the adsorption of Co(II) ion species on the surface of oxidized activated carbon by ion exchange and complexation.

Figure 11 .
Figure 11.Schematic presentation of the adsorption of Co(II) ion species on the surface of oxidized activated carbon by ion exchange and complexation.

Table 1 .
The general characteristics of activated carbons.

Table 2 .
Porous structure characteristics of activated carbons, determined from N 2 sorption isotherms.

Table 3 .
The quantity and character of functional groups (meq/g) on the surface of activated carbons, determined by the Boehm method.

Table 4 .
Kinetic parameters and qe values of the adsorption process of cobalt ions (C0 = 2 mmol/L) on oxidized activated carbons.Pseudo-second kinetic model (Ho and McKay).

Table 4 .
Kinetic parameters and q e values of the adsorption process of cobalt ions (C 0 = 2 mmol/L) on oxidized activated carbons.Pseudo-second kinetic model(Ho and McKay).

Table 5 .
Kinetic parameters and qe values of the adsorption process of strontium ions (C0= 1 mmol/L) on activated carbons.Pseudo-second-order kinetic model (Ho and McKay).

Table 5 .
Kinetic parameters and q e values of the adsorption process of strontium ions (C 0 = 1 mmol/L) on activated carbons.Pseudo-second-order kinetic model(Ho and McKay).

Table 5 .
Kinetic parameters and qe values of the adsorption process of strontium ions (C0= 1 mmol/L) on activated carbons.Pseudo-second-order kinetic model (Ho and McKay).