The Adsorption Kinetic Parameters of Co 2 + Ions by α-C 2 SH

In this work, the kinetic parameters of Co2+ ion adsorption by α-C2SH were determined. α-C2SH was synthesized under hydrothermal conditions at 175 ◦C, when the duration of isothermal curing was 24 h and the molar ratio of primary mixture was CaO/SiO2 = 1.5. This research allows us to state that the adsorption reactions proceed according to the chemisorption process. In order to determine adsorption kinetic parameters, kinetics models have been developed and fitted for these reactions. Additionally, it was determined that adsorbed Co2+ ions have a significant influence on the stability of α-C2SH. These results were confirmed by XRD, STA, and atomic absorption spectroscopy methods.


Introduction
Heavy metal pollution in wastewater is one of the most serious environmental problems because most of it is toxic, even at very low concentrations [1][2][3][4].This pollution appears in many industries, such as metal plating, mining operations, chlor-alkali, radiator manufacturing, and alloy industries [2,5,6].Methods for removing metal ions from aqueous solutions mainly consist of physical, chemical, and biological technologies [1,7].Various treatment processes have been developed for the removal of metal ions: chemical precipitation, filtration, ion exchange, electrochemical treatment, membrane technologies, and evaporation [8][9][10].Among these methods, adsorption is attractive due to its low cost, high efficiency, and simple operation [11,12].The common adsorbents primarily include activated carbon, zeolites, bio macromolecules, and calcium silicate hydrates [12][13][14][15].However, some adsorbents show low adsorption capacities and short operation time [3,12].In addition, when these adsorbents reach their adsorption capacity limit, they are usually stored to a landfill and discarded, which eventually causes secondary pollution due to the risk of leaching and the subsequent release of non-biodegradable heavy metals [16].For these reasons, the research on new adsorbents that can be used as additives in ordinary Portland cement or in the manufacture of alternative cementitious materials is in progress [17].
Many studies have been made regarding the properties of α-C 2 SH. because of its basis a new hydraulic cementitious materials family has been created ("Celitement", "solidia Cement") [27][28][29].The main feature of these materials is that the CO 2 emission and energy consumption associated with cement production can be reduced by up to 30-70%.The production of new cementitious materials is based on a multi-stage process: (1) the synthesis of higher basicity calcium silicate hydrate; (2) activation of the synthesis products and stabilization of highly reactive C 2 S forms ("Cellitement"); (3) the cement curing in a water or CO 2 atmosphere.Moreover, Siauciunas et al. [30] found that an additive of gyrolite intercalated with Cd 2+ ions can be successfully utilized in alternative cement based on α-C 2 SH.For these reasons, it is possible to use α-C 2 SH with incorporated heavy metal ions in the synthesis of alternative cementitious materials.However, in order to successfully use such material, the first step is to determine and understand their adsorption capacity and its properties.
The aim of this study is to explore the adsorption kinetic parameters of α-C 2 SH for Co 2+ ions and its properties.

Materials and Methods
Fine ground SiO 2 •nH 2 O and calcium oxide were used as starting materials.α-C 2 SH was synthesized at 175 • C when the duration of isothermal curing was 24 h.The molar ratio of primary mixture (CaO/SiO 2 ) was equal to 1.5, while the water/solid ratio was 10.The detailed methodology, the preparation of raw materials, and the formation sequence of intermediate compounds were previously described in works [24][25][26].
It was determined that the dibasic calcium silicate-α-C 2 SH (PDF 04-009-6343) and semicrystalline C-S-H type compounds (PDF 00-033-0306 and PDF 00-034-0002) were formed in the synthesis products (Figure 1a).In addition, a small intensity diffraction peaks of calcium carbonate were also identified.X-ray diffraction analysis (XRD) data were proved by the results of simultaneous thermal analysis (STA) (Figure 1b).In the DSC curve, the first endothermic effect (50-200 • C) can be assigned to the loss of crystallization water in calcium silicate hydrates.Meanwhile, the decomposition of α-C 2 SH is observed in a 400-500 • C temperature interval, in which ~5.26% of mass were lost (Figure 1b, Curve 1).According to this data, it was calculated that ~55.5% of α-C 2 SH was formed.In addition, two exothermic effects at ~852 and ~872 • C, which are characteristic of the recrystallization of C-S-H(I) and C-S-H(II) to wollastonite, were identified (Figure 1b, Curve 2).Meanwhile, an endothermic effect at ~679 • C was assigned to the decomposition of calcium carbonate.It should be noted that, in further stages of this work, the synthesis product, in which α-C 2 SH is the dominant compound, is simply referred to as α-C 2 SH.The mineralogical composition of samples was determined by X-ray diffraction analysis, while thermal stability was determined by simultaneous thermal analysis, methods that were previously described in works [24][25][26].
According to S. Zadaviciute et al.'s [16,31] work, the adsorption mechanism of Co 2+ ions by synthetic adsorbents can be determined by pseudo-first-order and pseudo-second-order equations.The pseudo-first-order can be represented by the following equation [31]: where q e and q t are amounts of adsorbed ions at equilibrium and at time t e , respectively (mg/g), k 1 is the rate constant of pseudo-first-order adsorption (min −1 ).After integration and applying boundary conditions 0 ≤ t ≤ t e and 0 ≤ q t ≤ q e , the integrated form becomes log(q e − q t ) = log q e − k 1 2.303 t. ( The pseudo-second-order adsorption kinetic rate equation can be expressed as [31] where k 2 is the rate constant of the pseudo-second-order adsorption (g/(mg•min)).For the boundary conditions 0 ≤ t ≤ t e and 0 ≤ q t ≤ q e , the integrated form of the equation (the integrated rate law for the pseudo-second-order reaction) becomes

Results and Discussion
In the first part of this work, the adsorption experiment was carried out in a Co(NO 3 ) 2 •6H 2 O solution containing 0.25 g Co 2+ /dm 3 .It was determined that the latter process proceeds intensively at the beginning because, after 15 s of reaction, about 50% of cobalt ions were incorporated into the structure of α-C 2 SH (Figure 2).Moreover, the adsorbed amount of these ions increased and, after 5 min, was equal to 24.68 mg Co 2+ /g α-C 2 SH.However, the further increment in experiment duration did not affect the uptake of Co 2+ ions because, after 30 min of reaction, the amount of absorbed ions remained almost the same and was equal to 24.73 mg Co 2+ /g α-C 2 SH (Figure 2).For this reason, it can be stated that, after 5 min of adsorption, the equilibrium was attained and the adsorption of Co 2+ ions was complete (Figure 2).
It is worth noting that, at the beginning of experiment, about 7.2% (30 mg) of Ca 2+ ions were released from the structure of adsorbent, showing that this dibasic calcium silicate hydrate α-C 2 SH is unstable in the acidic Co(NO 3 ) 2 •6H 2 O solution (Figure 3a).These data can be explained by evaluating the crystal structure of the latter compound.It is well-known that α-C 2 SH with isolated silicate tetrahedrons possess the lowest degree of polymerization, and its crystal structure consists of isolated acidic SiO 3 (OH) tetrahedra, which share edges with alkaline Ca(O,OH) 6 and Ca(O,OH) 7 polyhedra [32,33].For this reason, both Ca 2+ cations and OH − anions are released, when the structure of this compound is destroyed in an acidic environment.These data are in a good agreement with the value of pH of liquid medium, which increased from 4.91 (before adsorption) to 6.07 (after 30 min of adsorption).As expected, due to the good adsorption properties of calcium silicate hydrates for metals ions, the concertation of Ca 2+ ions in the liquid medium decreased to 4.56% (18.6 mg) by prolonging the adsorption to 15-30 min.Thus, it is clearly seen that all Co 2+ ions present in liquid medium were adsorbed by α-C 2 SH when a lower concentration of Co(NO 3 ) 2 •6H 2 O solution (0.25 g/cm 3 ) was used.For this reason, in the next stage of this work, it was raised to 1 g of Co 2+ /dm 3 .
It was determined that a higher metal ion concentration had a positive effect on the incorporation of Co 2+ ions (Figure 4).After 30 s of adsorption, α-C 2 SH adsorbed 90% of initial Co 2+ ions (90.01 mg Co 2+ /1 g α-C 2 SH) (Figure 4) and released 22.1% (89.84 mg) of Ca 2+ ions (Figure 3b).When the duration of the experiment was extended to 10 min, the equilibrium was attained and the amount of adsorbed ions reached 99.61 mg Co 2+ /1 g α-C 2 SH (Figure 4).Additionally, as in the previous case, the decrease in concentration of desorbed Ca 2+ ions was observed (Figure 3b).It is worth mentioning that, due to a higher destruction of adsorbents, the pH value of liquid medium increased from 4.91 to 10.97.In order to confirm the intercalation of Co 2+ ions, XRF analysis of the adsorbent was performed after 30 min.It was found that the α-C 2 SH contained 14.6% of Co 2+ ions.
Thus, the previous results indicated that the interaction between adsorbent and adsorptive followed the addition reaction α-C 2 SH + Co 2+ → α-C 2 SH − Co.
In order to determine the maximum adsorption capacity of α-C 2 SH for Co 2+ ions, the concentration of Co(NO 3 ) 2 •6H 2 O solution was increased to 10 g/dm 3 .
It was found that the latter parameter (Σx) was equal to 288 mg Co 2+ /g after 30 min of adsorption, but ~50% of Ca 2+ ions were released to a liquid medium within the same duration.Thus, due to these results, it is not recommended that more than 3 g Co 2+ /dm 3   According to the literature, the adsorption capacity of lower basicity calcium silicate hydrates (gyrolite, xonotlite, and tobermorite) depends on the nature of adsorptive.Kasperaviciute et al. [34] determined that the maximum amount of adsorbed Cu 2+ ions by gyrolite is equal to 1480 mg Cu 2+ /g.Meanwhile, Bankauskaite et al. [35] showed that the adsorption capacity of this compound for Co 2+ ions is equal only to 14 mg Co 2+ /g.For this reason, it can be stated that synthetic α-C 2 SH can be used as an adsorbent for cobalt ions.
In order to determine if adsorption is a reversible process or not in a neutral liquid medium, α-C 2 SH sample with intercalated Co 2+ ions (1 g Co 2+ /dm 3 ) was dried up and immersed in distilled water.It was measured that, after 60 min at 25 • C, Co 2+ ion concentrations in the solution did not exceed ~0.008%.It is worth mentioning that, after the desorption process, 14.6 wt % Co 2+ ions were detected in the α-C 2 SH structure via XRF analysis.
Furthermore, in the next stage of this research, the kinetic adsorption parameters were determined by applying pseudo-first-order and pseudo-second-order equations.Solutions containing 0.25 g and 1 g Co 2+ /dm 3 were chosen.The aforementioned equation suitability for the adsorption reaction was verified by comparing the calculated equilibrium concentrations (q e(cal) ) of Co 2+ ions to the experimental concentrations (q e(exp) ).In addition, the equation is suitable only if the errors between q e(cal) and q e(exp) do not exceed 5%.
Using the pseudo-first-order equation, it was found that the values of R 2 were lower (0.84, 0.86) and that the values of q e(cal) disagreed with the experimental q e(exp) ones (Table 1).Thus, the pseudo-first-order equation did not fit well with the Co 2+ ion adsorption mechanism description.Different results were obtained using the pseudo-second-order kinetics equation: the values of R 2 were near 1 and equal to 0.99 (0.25 g Co 2+ /dm 3 ) and 1.00 (1 g Co 2+ /dm 3 ) (Figure 5).In addition, the calculated q e(cal) values were close to the experimental q e(exp) ones (Table 1).Thus, it can be stated that pseudo-second-order kinetics model adequately describes the Co 2+ ion adsorption mechanism of α-C 2 SH.Moreover, it was calculated that the Co 2+ ion adsorption rate (k 2 = 0.139 g/(mg•min)) is higher in the solution containing 0.25 g Co 2+ /dm 3 than in the solution containing 1 g Co 2+ /dm 3 (k 2 = 0.091 g/(mg•min)) (Table 1).It should be noted that these data agree with the experimental results and confirm that Co 2+ ions are chemisorbed by α-C 2 SH.The same phenomenon can be found in the literature [31,[34][35][36][37][38].
In order to evaluate the stability of the adsorbent, α-C 2 SH was described by X-ray diffraction and simultaneous thermal analysis.It was determined that, after the adsorption process, in the solution containing 0.25 and 1 g Co 2+ /dm 3 , the intensity of the main diffraction peaks characteristic to α-C 2 SH decreased by 1.77 and 2.53 times, respectively (Figures 1a and 6).In addition, 89.84 mg of Ca 2+ ions were desorbed from the crystal structure of α-C 2 SH.However, the structure of the latter compound remained stable.Meanwhile, after adsorption in the solution containing 10 g Co 2+ /dm 3 , α-C 2 SH fully decomposed because diffraction peaks typical to this compound are not detected in XRD patterns (Figure 6).It is worth mentioning that two diffraction peaks characteristic to a new compound were observed, but it was impossible to identify it.XRD results were supported by STA analysis data.It was found that the absorbed heat of the first endothermic effect increased from 84.26 (pure α-C 2 SH) (Figure 1b) to 175.78 J/g (α-C 2 SH after adsorption in solution containing 10 g Co 2+ /dm 3 ) (Figure 7).These data can be associated with the amorphization of the synthesis product.Meanwhile, the quantity of α-C 2 SH decreased by 1.8 times after adsorption in solutions with lower Co 2+ ion concentrations.Meanwhile, the endothermic effect typical to this compound was not identified in DSC curves of sample, in which the 10 g/dm 3 Co(NO 3 ) 2 •6H 2 O solution was used (Figure 7c).This clearly shows that α-C 2 SH is fully decomposed in such an acidic environment.It was found that semicrystalline calcium silicate hydrates-C-S-H(I) and C-S-H(II)-decomposed after adsorption in solution containing 1 g Co 2+ /dm 3 (Figure 7).It is worth mentioning that, during the adsorption process, carbonization appeared, and the quantity of calcium carbonate increased to 6.75-9% (Figure 7).In further research, α-C 2 SH with incorporated Co 2+ ions will be used in alternative cementitious materials, and the properties of these binder materials such as hydration, mineralogical composition, and porosity will be examined.

Conclusions
It was obtained, that the adsorption capacity of α-C 2 SH and intrusion of Co 2+ ions in its structure depends on the reaction time and the initial concentration of metal ions.It was determined that the highest value of adsorbed metal ions by 1 g of α-C 2 SH was equal to 288 mg.It was found that interaction between adsorbent and adsorptive follows the addition reaction: α-C 2 SH + Co 2+ → α-C 2 SH − Co.
It was observed that the pseudo-second-order model fit well with the cobalt ion adsorption mechanism description, because the calculated q e(cal) values were close to the experimental q e(exp) ones.By the way, it was found that the Co 2+ ion adsorption rate (k 2 ) depends on the initial metal ion concentration.It was determined that the adsorption reactions are not reversible in a neutral liquid medium, because after desorption experiment, Co 2+ ion concentrations in the solution did not exceed ~0.008%.

Figure 2 .
Figure 2. Integral kinetic Curve of Co 2+ ion adsorption by α-C 2 SH, when the initial concentration of Co 2+ was equal to 0.25 g/dm 3 .

Figure 3 .
Figure 3.The amount of released calcium ions from adsorbent, when the initial concentration of Co 2+ was equal to 0.25 g/dm 3 (a) and 1 g/dm 3 (b).
of Co(NO 3 ) 2 •6H 2 O solution is used for the adsorption experiments with α-C 2 SH.

Figure 4 .
Figure 4. Integral kinetic Curve of Co 2+ ion adsorption by α-C 2 SH when the initial concentration of Co 2+ was equal to 1 g/dm 3 .

Table 1 .
The kinetic parameters of the pseudo-first-and pseudo-second-order kinetic models.