Resourcization of Argillaceous Limestone with Mn3O4 Modification for Efficient Adsorption of Lead, Copper, and Nickel

Argillaceous limestone (AL) is comprised of carbonate minerals and clay minerals and is widely distributed throughout the Earth’s crust. However, owing to its low surface area and poorly active sites, AL has been largely neglected. Herein, manganic manganous oxide (Mn3O4) was used to modify AL by an in-situ deposition strategy through manganese chloride and alkali stepwise treatment to improve the surface area of AL and enable its utilization as an efficient adsorbent for heavy metals removal. The surface area and cation exchange capacity (CEC) were enhanced from 3.49 to 24.5 m2/g and 5.87 to 31.5 cmoL(+)/kg with modification, respectively. The maximum adsorption capacities of lead (Pb2+), copper (Cu2+), and nickel (Ni2+) ions on Mn3O4-modified argillaceous limestone (Mn3O4–AL) in mono-metal systems were 148.73, 41.30, and 60.87 mg/g, respectively. In addition, the adsorption selectivity in multi-metal systems was Pb2+ > Cu2+ > Ni2+ in order. The adsorption process conforms to the pseudo-second-order model. In the multi-metal system, the adsorption reaches equilibrium at about 360 min. The adsorption mechanisms may involve ion exchange, precipitation, electrostatic interaction, and complexation by hydroxyl groups. These results demonstrate that Mn3O4 modification realized argillaceous limestone resourcization as an ideal adsorbent. Mn3O4-modified argillaceous limestone was promising for heavy metal-polluted water and soil treatment.


Introduction
Over the years, large amounts of wastewater containing lead (Pb 2+ ), copper (Cu 2+ ), nickel (Ni 2+ ), and other heavy metal ions have been discharged into surface and underground water.These heavy metals, originating from metal electroplating, mining, battery manufacturing, chemical industries, and other industries [1], are widely recognized as one of the most severe threats to human health, animal well-being, and plants, due to their carcinogenicity, toxicity, non-degradability, persistence in the environment, and ability to bioaccumulate within food chains [2][3][4].Consequently, there is an urgent need for effective removal of heavy metal ions from wastewater.
One of the most efficient techniques for remediating water contaminated with heavy metals is adsorption due to its advantages, such as availability, cost-effectiveness, and ease of operation [5,6].The key aspect of the adsorption technique is to select an ecofriendly, inexpensive, and efficient adsorbent.Natural clays, along with other minerals, are considered green adsorbents due to their affordability and abundance as a natural resource [7].At present, various studies have focused on utilizing minerals for wastewater treatment.Du et al. (2016) investigated the absorption behavior of Pb 2+ and Cd 2+ in water using a bacteria-montmorillonite composite material [8].Zhao et al. (2022) synthesized a silicate-based composite material and demonstrated its remarkable capacity to adsorb Cd 2+ (63.80 mg/g) [9].
Argillaceous limestone (AL), comprised of carbonate and clay minerals, is widely distributed throughout the Earth's crust.Due to the precipitation properties of the carbonate minerals and the high cation exchange capacity of the clay minerals, both are effective in removing heavy metals [7,10,11].However, research on AL as a heavy metal adsorbent remains relatively limited [12].This is primarily attributed to its low content of functional components, low surface area, and poor active sites [13].Therefore, it is necessary to modify or functionalize AL before it can be applied as a heavy metal adsorbent.Additionally, AL commonly coexists with grey limestone as an associated mineral.After extracting grey limestone for building materials purposes, AL is often abandoned [14].Therefore, AL has the advantage of low cost.The modification of AL as an adsorbent for heavy metal removal holds great significance for resource utilization.
Manganese oxide-modified adsorbents have attracted researchers' attention due to their high electronegativity, surface area, and other unique physicochemical properties [6,[15][16][17][18][19].Moreover, manganese oxides can be firmly loaded onto silica-based surfaces [18].Additionally, manganese oxides are abundant in geological reserves and therefore, also serve as a low-cost material [19].To date, reports have confirmed the potential application of Mn 3 O 4 as a good adsorbent to remove metal ions from water.Lingamdinne et al. (2022) found that Mn 3 O 4 -anchored reduced graphene oxide had significant adsorption efficiency for lead and chromium ions and the maximum sequestration capacities were 130.28 and 138.51 mg/g, respectively [6].In addition, the research of Zou et al. (2016) showed that the maximum adsorption of antimony ions on Mn 3 O 4 -modified reduced graphene oxide was 151.84 mg/g [17].Lee et al. prepared Mn 3 O 4 -coated activated carbon (Mn 3 O 4 /AC) for use in the removal of Pb and Cu ions, and showed that the adsorption capacities of Mn 3 O 4 /AC were enhanced 2.2 and 6.1 times for Pb and Cu ions, respectively, compared with pure active carbon [15].However, to the best of our knowledge, limited studies have been conducted on the adsorption of heavy metals by the Mn 3 O 4 -modified AL (Mn 3 O 4 -AL).Previous research has indicated that environmental factors, such as pH, ionic strength, and organic matter, can influence the adsorption capacity of materials.It has been demonstrated that an increase in pH significantly enhances the adsorption of heavy metals [6,20].Nonetheless, the effect of coexisting ions on adsorption is inconsistent and may hinder or promote the adsorption process [21].In addition, it has been reported that humic acid can affect the adsorption behavior of heavy metal ions by mineral or synthetic adsorbents [22].Therefore, it is crucial to investigate the effects of these environmental factors on adsorption for practical applications.
In this study, we modified AL with Mn 3 O 4 using an in-situ deposition strategy through a manganese chloride (MnCl 2 ) and alkali stepwise treatment to improve its adsorption capacity.Consequently, the purposes of this study were to: (1) synthesize and characterize the Mn 3 O 4 -AL; (2) explore the capacity of Mn 3 O 4 -AL to adsorb Pb 2+ , Cu 2+ , and Ni 2+ in mono-or multi-heavy metal systems; (3) examine the impact of environmental factors (pH, ionic strength, and humic acid) on adsorption; and (4) clarify the adsorption mechanism of the adsorption process.The results obtained from this research are expected to provide valuable insights into understanding the ability of Mn 3 O 4 -AL to adsorb Pb 2+ , Cu 2+ , and Ni 2+ and contribute towards resource utilization of AL.This work indicated that the Mn 3 O 4 -AL was a promising candidate for efficient heavy metal removal.

Materials and Chemicals
Argillaceous limestone (AL) was collected from the lower Cambrian Mantou formation in the Dabeiwang area of Jiangsu province, China.Our previous research showed that the raw minerals are mainly composed of carbonate minerals, accounting for 41.8%.In addition, clay minerals account for around 12.2% [23].

Modification of Argillaceous Limestone
The modification process for argillaceous limestone (AL) was carried out following the previous study of Sarı et al. (2012) [19].The synthesis pathway of modifying argillaceous limestone with Mn 3 O 4 is shown in Figure 1a.Detailed synthesis steps are as follows: 20 g AL was activated with 80 mL of 6 mol/L NaOH at 90 • C for 2 h.Next, the obtained mixture was added to a 100 mL solution of 2.5 mol/L MnCl 2 (with the pH adjusted to 2.0 using 1 M HCl), and placed at room temperature (25 • C) for 10 h.The mixture was filtered and the resulting precipitate was mixed with 40 mL of a 6 mol/L NaOH solution at 25 • C for 10 h to obtain a manganese hydroxide (Mn(OH) 2 ) precipitate.The mixture was exposed to the air to oxidize the manganese hydroxide.Then, the modified argillaceous limestone was washed with deionized water and dried at 105 • C.

Materials and Chemicals
Argillaceous limestone (AL) was collected from the lower Cambrian Mantou for mation in the Dabeiwang area of Jiangsu province, China.Our previous research showed that the raw minerals are mainly composed of carbonate minerals, accounting for 41.8%In addition, clay minerals account for around 12.2% [23].

Modification of Argillaceous Limestone
The modification process for argillaceous limestone (AL) was carried out following the previous study of Sarı et al. (2012) [19].The synthesis pathway of modifying argillaceous limestone with Mn3O4 is shown in Figure 1a.Detailed synthesis steps are as follows 20 g AL was activated with 80 mL of 6 mol/L NaOH at 90 °C for 2 h.Next, the obtained mixture was added to a 100 mL solution of 2.5 mol/L MnCl2 (with the pH adjusted to 2.0 using 1M HCl), and placed at room temperature (25 °C) for 10 h.The mixture was filtered and the resulting precipitate was mixed with 40 mL of a 6 mol/L NaOH solution at 25 °C for 10 h to obtain a manganese hydroxide (Mn(OH)2) precipitate.The mixture was ex posed to the air to oxidize the manganese hydroxide.Then, the modified argillaceous limestone was washed with deionized water and dried at 105 °C.

Batch Adsorption Experiments
Stock solutions of 1000 mg/L Pb 2+ , Cu 2+ , and Ni 2+ were prepared using Pb(NO 3 ) 2 , Cu(NO 3 ) 2 •5H 2 O, Ni(NO 3 ) 2 •6H 2 O.The desired pH of the suspensions was controlled using 0.1 mol/L NaOH or HNO 3 .NaNO 3 and Ca(NO 3 ) 2 were used as background electrolytes to adjust the ionic strength of the suspensions.Adsorption kinetic experiments were conducted by adding 0.0075 g of the Mn 3 O 4 -AL into 50 mL solutions containing either 50 mg/L Pb 2+ , Cu 2+ , or Ni 2+ or all three metals.The mixtures were shaken at 25 • C for 0-1080 min.To investigate the impact of the initial concentration and to compare the mono-and multi-heavy metal adsorption capacity of the Mn 3 O 4 -AL, adsorption isotherm experiments were carried out in multi-heavy metal and single-heavy metal solutions with initial concentrations in the range of 1-200 mg/L.Various concentrations of environmental factors, including pH (2.0, 3.0, 4.0, 5.0, 6.0), ionic strength (sodium ions(Na + ), calcium ions (Ca 2+ ), chloride ions (Cl − ), and sulfate ions (SO 4 2− ); 0, 0.01, and 0.1 mol/L), and humic acid (HA) (0, 0.1, 0.5, 1, 3, 5, 8, 10, 15, 20, 30 mg/L), were individually controlled to investigate their influence on heavy metal adsorption.In the isotherm and environmental factor experiments, the mixture was shaken for 12 h, and the other details were the same as in the adsorption kinetic experiments.
In addition, to explore the adsorption capacity of the Mn 3 O 4 -AL in the natural environment, lake water was used as the background water to simulate natural environment conditions.The lake water was collected from Zhaoyang Lake, at the South China Agricultural University, Guangdong Province, China.The pH of the lake water was 7.28, and the dissolved organic carbon (DOC) was 4.02 mg/L.The initial concentrations of total Cd, Pb, Cu, Ni, and Zn of the lake water were 0.08, 1.36, 1.88, 2.06, and 1.22 µg/L, respectively, which was below the environmental quality standards for surface water in China (GB 3838-2002) [24].The mixed solution was prepared using Pb(NO 3 ) 2 , Cu , and Cd(NO 3 ) 2 •4H 2 O.The initial concentrations of Pb 2+ , Cu 2+ , Ni 2+ , Cd 2+ , and Zn 2+ in the multi-metal solution were each 50 mg/L.The mixture was shaken for 12 h, and other details were the same as in the adsorption kinetic experiments.
After shaking, all samples were immediately filtered with 0.22 µm filters.The concentrations of Pb 2+ , Cu 2+ , and Ni 2+ were determined using flame atomic absorption spectroscopy (AAS) (Z2300, Hitachi, Tokyo, Japan).To ensure the reproducibility and accuracy of data, all of the experiments in this study were performed in triplicate.
where Q e and Q t are the adsorption capacity at equilibrium and time t (mg/g), respectively.k 1 is the rate constant of pseudo-first-order (min −1 ), and k 2 is the rate constant of pseudosecond-order (g/(mg min)).
In addition, the Langmuir and Freundlich models (Equations ( 3) and ( 4)) were used to simulate the adsorption isotherm data [6]. log where, C e is the equilibrium concentration of the heavy metal ions in solution (mg/L).Q e and Q m are the equilibrium and maximum adsorption capacity, respectively (mg/g).K L is the Langmuir constant (L/mg), K F is the Freundlich constant (mg 1−(1/n) L 1/n /g), and 1/n is the heterogeneity factor.

Characterization Method
The mineralogical compositions of the Mn 3 O 4 -AL before and after adsorption were analyzed by X-ray diffraction (XRD) (D8 Advance, Bruker, Germany).The morphologies of the Mn 3 O 4 -AL before and after adsorption were characterized using scanning electron microscopy (SEM) and X-ray energy dispersive spectrometry (EDS) (Quanta FEG 650, FEI, Houston, TX, USA).The surface area of the Mn 3 O 4 -AL was measured using a gas sorption analyzer (Gemini VII 2390 V1.03, Micromeritics Instrument Corp, Norcross, GA, USA) by the Brunauer-Emmett-Teller (BET) method.A FT-IR instrument (Nicolet iS10, Thermo Fisher, Waltham, MA, USA) was used to obtain the change of functional groups before and after adsorption.Surface chemical compositions of the Mn 3 O 4 -AL before and after adsorption were evaluated by X-ray photoelectron spectroscopy (XPS) (ESCALAB 250Xi, Thermo Fisher, USA).The zeta potential of the Mn 3 O 4 -AL under different background solutions was analyzed by a dynamic light scattering analyzer (Nano-ZS90 Zetasizer, Malvern Instruments, Malvern, UK).The total organic carbon (TOC) of the Mn 3 O 4 -AL was determined with a TOC analyzer (Vario TOC, Elementar, Langenselbold, Germany).The cation exchange capacity (CEC) of the Mn 3 O 4 -AL was determined based on He et al. ( 2018) [23].The pH and dissolved organic carbon (DOC) were determined using the procedure of Zhang et al. (2017) [21].

Statistical Analysis
Data are presented as the mean ± standard deviation (SD).All data were statistically assessed by one-way ANOVA with the minimum level of significance set at p < 0.05.In addition, all of the functional graphs were produced using OriginPro 2021 software.

Characterization of Mn 3 O 4 Modified Argillaceous Limestone
To obtain a satisfactory and economical adsorbent, raw argillaceous limestone was modified by in-situ deposition of Mn 3 O 4 from a MnCl 2 precursor solution with alkalized treatment.The morphology and chemical composition of the Mn 3 O 4 -AL were analyzed using SEM and EDS (Figure 1b,c).Previous research indicated that raw argillaceous limestone exhibited dense massive structures [23].However, after Mn 3 O 4 modification, the surface of the argillaceous limestone became fluffy and featured the distribution of numerous small granular particles.This transformation can be attributed to the acidic atmosphere created by the strong acid and weak alkali salt MnCl 2 , which dissolved carbonates.Consequently, argillaceous limestone was broken down into smaller particles, leading to an increase in its specific surface area.This observation was further supported by a significant rise in the specific surface area from 3.49 to 24.5 m 2 /g after modification (Table S1).EDS spectra results revealed that the Mn 3 O 4 -AL mainly consisted of manganese (Mn), silicon (Si), oxygen (O), calcium (Ca), magnesium (Mg), aluminum (Al), potassium (K), and carbon (C) (Figure 1c).Notably different from our previous research on AL [23], there was a distinct peak corresponding to Mn present in the EDS of the Mn 3 O 4 -AL.In contrast, there is no Mn peak in AL.These findings indicated the successful loading of Mn onto raw argillaceous limestone.
The FT-IR spectra of AL and the Mn 3 O 4 -AL are depicted in Figure 2a.For the AL, the broad band at 3437 cm −1 can be attributed to the stretching vibration of -OH groups [25,26].The peaks at 1396 cm −1 and 1615 cm −1 were the characteristic peaks for bCO 3 2− and mCO 3  2− , respectively [21,27].Negatively charged functional groups (-OH and COO-) existed on the surface of AL, which facilitates the adsorption of heavy metal ions through electrostatic adsorption and complexation reactions [21].The bands at 1008 cm −1 and 521 cm −1 refer to Si-O symmetric stretching [28,29].After modification, the signals of -OH and COO-were still maintained in the Mn 3 O 4 -AL curve.The intensity of CO at 1473 cm −1 became weaker, indicating partial dissolution of the carbonate mineral [30].This is consistent with the result of the increasing specific surface area of the Mn 3 O 4 -AL.Additionally, a new peak appeared at 635 cm −1 , which is consistent with the Mn-O-Mn stretching vibration [31][32][33].It demonstrated that Mn was loaded onto the argillaceous limestone in the form of manganese oxide.
tionally, a new peak appeared at 635 cm , which is consistent with the Mn-O-Mn stretch-ing vibration [31][32][33].It demonstrated that Mn was loaded onto the argillaceous limestone in the form of manganese oxide.XPS was applied to analyze the chemical states of the Mn3O4-AL (Figure 2).The C 1s spectrum exhibited three characteristic peaks at 284.8, 286.3, and 288.8 eV (Figure 2c), corresponding to C-C, C-O-H/C-O-C, and O-C=O, respectively.Combined with the FT-IR result mentioned earlier, it can be concluded that the main form of C was carbonate.Three peaks were observed at 529.9, 531.2, and 532.3 eV in terms of O 1s high resolution (Figure 2d), corresponding to lattice oxygen (Mn-O-Mn), -OH, and C=O [34,35], respectively.The peaks at 641.7 eV and 652.7 eV in the detailed spectra of Mn 2p (Figure 2e) were attributed to 2p3/2 and 2p1/2 Mn (III), and the peaks at 644.5 eV and 653.6 eV were attributed to Mn(II) [17,36,37].Combined with the results of FT-IR, we can speculate that Mn exists in the form of Mn3O4 in the Mn3O4-AL [35].The mineralogical compositions of AL before and after modification with Mn3O4 were determined by XRD (Figure 2f).The XRD curve depicted that AL was mainly comprised of dolomite (CaMg(CO3)2) (2θ = 31.20°)(PDF No. 03-0864) and quartz (PDF No. 82-0513).The peaks at 2θ = 28.98° and 35.98° were regarded as akermanite (Ca2MgSi2O7) (PDF No. .With the treatment of NaOH and MnCl2 sequentially, the location of the dominating diffraction peaks in the Mn3O4-AL had not obviously changed, but the relative intensity of those peaks weakened, illustrating that part of the SiO2 and carbonate minerals had reacted.The diffraction peaks at 2θ = 45.4° and 57.6° were related to hausmannite (Mn3O4) (PDF No. 24-0734), which confirmed that Mn3O4 had been loaded onto the argillaceous limestone [15].These results were consistent with those of XPS and FT-IR.
The other physiochemical characterizations of Mn3O4-AL are presented in Table S1.With modification, the pH and point of zero charge (PZC) were changed from 8.32 (AL) to 7.92 (Mn3O4-AL) and from 1.95 (AL) to 3.20 (Mn3O4-AL), respectively.The specific XPS was applied to analyze the chemical states of the Mn 3 O 4 -AL (Figure 2).The C 1s spectrum exhibited three characteristic peaks at 284.8, 286.3, and 288.8 eV (Figure 2c), corresponding to C-C, C-O-H/C-O-C, and O-C=O, respectively.Combined with the FT-IR result mentioned earlier, it can be concluded that the main form of C was carbonate.Three peaks were observed at 529.9, 531.2, and 532.3 eV in terms of O 1s high resolution (Figure 2d), corresponding to lattice oxygen (Mn-O-Mn), -OH, and C=O [34,35], respectively.The peaks at 641.7 eV and 652.7 eV in the detailed spectra of Mn 2p (Figure 2e) were attributed to 2p 3/2 and 2p 1/2 Mn (III), and the peaks at 644.5 eV and 653.6 eV were attributed to Mn(II) [17,36,37].Combined with the results of FT-IR, we can speculate that Mn exists in the form of Mn 3 O 4 in the Mn 3 O 4 -AL [35].
The mineralogical compositions of AL before and after modification with Mn 3 O 4 were determined by XRD (Figure 2f).The XRD curve depicted that AL was mainly comprised of dolomite (CaMg(CO The other physiochemical characterizations of Mn 3 O 4 -AL are presented in Table S1.With modification, the pH and point of zero charge (PZC) were changed from 8.32 (AL) to 7.92 (Mn 3 O 4 -AL) and from 1.95 (AL) to 3.20 (Mn 3 O 4 -AL), respectively.The specific surface area was increased from 3.49 (AL) to 24.5 m 2 /g (Mn 3 O 4 -AL) which was consistent with the morphology of the Mn 3 O 4 -AL as evidenced by SEM (Figure 1a).Those changes were mainly ascribed to the addition of a large amount of acidic MnCl 2 solution in the preparation process of Mn 3 O 4 -AL, which etched the surface of AL by the neutralization of some carbonate and hydroxyl functional groups.The increased specific surface area can expose more adsorption sites on the surface, which is conducive to the adsorption of heavy metals.In addition, the initial content of total Pb, Cu, and Ni in the Mn 3 O 4 -AL were 38.1, 12.1, and 20.9 mg/kg, respectively, which were below the environmental quality standard for soils in China (GB 15618-2018) [38].Therefore, the environmental risk during application of Mn 3 O 4 -AL can be negligible.Moreover, the cation exchange concentration (CEC) of the Mn 3 O 4 -AL and AL were 31.5 and 5.87 cmoL(+)/kg, respectively.The higher CEC of the Mn 3 O 4 -AL could be ascribed to the increased exposure of ion exchange sites, which were not available before Mn modification.The increased CEC value of the Mn 3 O 4 -AL can further enhance the adsorption capacity of Pb 2+ , Cu 2+ , and Ni 2+ [21].

Adsorption Kinetics
Adsorption kinetics is a significant indicator for evaluating the potential of adsorbents for practical applications.A rapid adsorption process could reduce the cost of remediation.The kinetics and fitting curves of Pb 2+ , Cu 2+ , and Ni 2+ were investigated in a multi-metal system.The results are depicted in Figure 3, while the corresponding parameter values and correlation coefficients are presented in Table 1.Consistent with previous research, the adsorption rate of Pb 2+ , Cu 2+ , and Ni 2+ in a multi-metal system increased rapidly during the first 120 min, followed by a slower adsorption stage before reaching equilibrium [39].Finally, the adsorption equilibrium was reached after 360 min and the adsorption capacity stabilized at 42.48, 17.31, and 6.272 mg/g for Pb 2+ , Cu 2+ , and Ni 2+ , respectively.This was mainly attributed to the large number of adsorption sites on the Mn 3 O 4 -AL adsorbent and the higher driving force of concentration gradients in the adsorption primary stage of adsorption.As the free adsorption sites were gradually occupied and the repulsive force on the solid-liquid interface increased, the adsorption rate decreased until equilibrium was reached [21,40].Compared to pseudo-first-order kinetics, the pseudo-second-order kinetic model fit the adsorption kinetic behavior of Pb 2+ , Cu 2+ , and Ni 2+ on the Mn 3 O 4 -AL in a multi-metal system more closely, as indicated by the higher regression coefficient (R 2 ).The theoretical equilibrium adsorption values (Q e ) were closer to experimental values (Table 1).The results indicate that chemical adsorption may be the rate-controlling mechanism for the adsorption of Pb 2+ , Cu 2+ , and Ni 2+ onto Mn 3 O 4 -AL in multi-metal systems.Table 1.Kinetic parameters of pseudo-first-order and pseudo-second-order models for competitive adsorption.

Adsorption Isotherm
To gain information on the mechanisms of the adsorption effect, experiments on adsorption isotherms in mono-and multi-heavy metal systems were conducted.Figure 4 depicts the adsorption isotherms and fitting curves of the Langmuir and Freundlich models for adsorption of mono-and multi-heavy metal ions (Pb 2+ , Cu 2+ , and Ni 2+ ) by the Mn 3 O 4 -AL.The corresponding parameters are shown in Table S2.In mono-metal systems, the R 2 values obtained using the Langmuir model were higher than that of the Freundlich model, and the Q m for Pb 2+ , Cu 2+ , and Ni 2+ were 148.73, 41.30, and 60.87 mg/g (Table S2), respectively.Compared to the Freundlich fitting results, the experimental data could be better fitted by the Langmuir model, suggesting the significant role of the monolayer adsorption process in the Mn 3 O 4 -AL adsorption of Pb 2+ , Cu 2+ , and Ni 2+ .The maximum adsorption capacities of the Mn3O4-AL in this study were compared with those of other mineral adsorbents (Table S3) [21,23,29,[44][45][46][47][48][49][50][51][52][53][54][55][56][57][58].Although compared to CAL (chitosan-coated argillaceous limestone) or FAL (Fe 3+ -modified argillaceous limestone) [21,23], the Mn3O4-AL had lower adsorption of Cu 2+ and Pb 2+ , the Mn3O4-AL had satisfactory adsorption of Ni 2+ .Furthermore, the adsorption capacities of the Mn3O4-AL for the three metals were greater than those of most natural and inorganically-modified mineral adsorbents, such as palygorskite, natural and Na-bentonite, goethite, Na-montmorillonite, and zeolite.Some organic material-modified minerals showed higher adsorption capacity than the Mn3O4-AL, such as cationic surfactant-modified bentonite, polyacrylamide/sodium montmorillonite, and sodium polyacrylate-grafted bentonite.However, the inorganic modifier has the advantage of being more cost-effective and naturally abundant compared to organic modifiers [21].Therefore, the Mn3O4-AL is a promising adsorbent for removing heavy metals.

Effects of Environmental Factors
The initial solution pH is a significant factor influencing the adsorption process.Acidity can impact the surface charge of the adsorbent and adsorbate and ultimately affects the ability of the adsorbent to adsorb pollutants [42].The pH of the solution may affect its surface charge due to the exchange of H + [59].Therefore, the effect of varying the initial solution pH on the adsorption capacity of the Mn3O4-AL was investigated to explore the adsorption mechanisms.Figure 5a shows the adsorption behaviors of mono-and multiheavy metal ions (Pb 2+ , Cu 2+ , Ni 2+ ) onto the Mn3O4AL from pH 2.0 to 6.0.The adsorption capacity (Qe) for Pb 2+ , Cu 2+ , and Ni 2+ by the Mn3O4-AL increased from pH 2.0 to 4.0 and reached a plateau at a pH value of 6.0.At low pH, the adsorption of Pb 2+ , Cu 2+ , and Ni 2+ was inhibited due to the competitive adsorption between H + and Pb 2+ , Cu 2+ , and Ni 2+ for In the multi-metal system, the adsorption of Pb 2+ , Cu 2+ , and Ni 2+ by the Mn 3 O 4 -AL could also be better described by the Langmuir model due to its higher R 2 values compared to the Freundlich model.In addition, the Q m values of Pb 2+ , Cu 2+ , and Ni 2+ were 56.96, 23.77, and 5.78 mg/g, respectively.These values represent 38%, 58%, and 9% of their values in mono-metal systems (Figure 4d).The adsorption selectivity of the Mn 3 O 4 -AL in the multi-metal system was in the order of Pb 2+ > Cu 2+ >Ni 2+ , indicating that the Mn 3 O 4 -AL exhibited better adsorption selectivity for Pb 2+ .Studies have shown that metal ions with higher electronegativity or a smaller hydrated radius exhibit a greater affinity for adsorption [41,42].Metal ions with a smaller hydrated radius may have smaller steric hindrance during transport and adsorption to the inner surface of adsorbents [42].The hydrated ionic radius and hydration energy of the three heavy metal ions were in the order of Pb 2+ > Cu 2+ >Ni 2+ [41,43].Evidently, the adsorption selectivity of the Mn 3 O 4 -AL was in accord with the hydrated ionic radius.

Effects of Environmental Factors
The initial solution pH is a significant factor influencing the adsorption process.Acidity can impact the surface charge of the adsorbent and adsorbate and ultimately affects the ability of the adsorbent to adsorb pollutants [42].The pH of the solution may affect its surface charge due to the exchange of H + [59].Therefore, the effect of varying the initial solution pH on the adsorption capacity of the Mn 3 O 4 -AL was investigated to explore the adsorption mechanisms.Figure 5a shows the adsorption behaviors of mono-and multi-heavy metal ions (Pb 2+ , Cu 2+ , Ni 2+ ) onto the Mn 3 O 4 AL from pH 2.0 to 6.0.The adsorption capacity (Q e ) for Pb 2+ , Cu 2+ , and Ni 2+ by the Mn 3 O 4 -AL increased from pH 2.0 to 4.0 and reached a plateau at a pH value of 6.0.At low pH, the adsorption of Pb 2+ , Cu 2+ , and Ni 2+ was inhibited due to the competitive adsorption between H + and Pb 2+ , Cu 2+ , and Ni 2+ for adsorption sites [60].In addition, Zeta potential analysis showed that Mn 3 O 4 -AL in deionized water carried a positive charge at pH levels below 4 (Figure 5b).Therefore, the electrostatic repulsion between the Mn 3 O 4 -AL and Pb 2+ , Cu 2+ , Ni 2+ also resulted in a reduction of adsorption capacity.As the initial pH increased beyond the pH pzc , the surface charge of the Mn 3 O 4 -AL became negative [61].Therefore, a stronger electrostatic attraction increases the adsorption capacity through electrostatic attraction [62].In addition, the surface functional groups of the adsorbent were protonated at low pH, which weakened the complexation between the adsorbent and the heavy metal ions [61].
The adsorption of Pb 2+ , Cu 2+ , and Ni 2+ by the Mn 3 O 4 -AL at different ionic strengths and alkali ions (i.e., Na + and Ca 2+ ) is presented in Figure 5c-e.The concentration of alkali ions had little effect on the adsorption of Cu 2+ by the Mn 3 O 4 -AL.This is similar to the influence of ionic strength on Cu adsorption by Na-bentonite [63] and carbon nanotube-hydroxyapatite [64].The adsorption capacity of Pb 2+ and Ni 2+ onto the Mn 3 O 4 -AL decreased with increasing concentrations of Na + and Ca 2+ .A similar trend was reported in a previous study, where montmorillonite was employed for heavy metal adsorption [65].This phenomenon mainly occurs because the adsorbed Pb 2+ and Ni 2+ may form outersphere surface complexes with the Mn 3 O 4 -AL.The electrolytes may compete with Pb 2+ and Ni 2+ for active sites on the Mn 3 O 4 -AL.In addition, the electrolytes may compress the electrical double layer thickness of the Mn 3 O 4 -AL, resulting in a decrease in the electrostatic attraction force between the Mn 3 O 4 -AL and heavy metal ions.This speculation could be supported by the higher inhibitory effect of Ca 2+ as compared with Na + .Divalent ions can compress the thickness of the electric double layer more effectively than monovalent ions, preventing Pb 2+ and Ni 2+ from reaching the adsorbent surface [21].Moreover, increasing the ionic strength may improve the aggregation of the Mn 3 O 4 -AL [66,67], which may further lead to a decrease in available adsorption sites of the Mn 3 O 4 -AL.In addition, the effects of different anionic concentrations were investigated and are shown in Figure S1.The concentration of Cl − had little effect on the adsorption of Pb 2+ , Cu 2+ , and Ni 2+ onto the Mn 3 O 4 -AL.The adsorption capacity of Pb 2+ , Cu 2+ , and Ni 2+ on the Mn 3 O 4 -AL increased with increasing concentrations of SO 4 2− , primarily due to the formation of sulfate [23].To simulate natural environmental conditions, lake water was used as the solvent to investigate the adsorption capacity of the Mn3O4-AL for Pb 2+ , Cu 2+ , Ni 2+ , Cd 2+ , and Zn 2+ in a multi-metal system (Figure 6).Results showed that the adsorption capacity of Pb 2+ , Cu 2+ , and Ni 2+ decreased to 36.72, 13.25, and 4.35 mg/g, respectively.The adsorption capacity of these heavy metals followed the order of Pb 2+ > Cu 2+ > Ni 2+ > Cd 2+ > Zn 2+ , which is consistent with previous research using montmorillonite and Sphagnum peat [41,65].These results indicated that the Mn3O4-AL can effectively remove various heavy metal ions from lake water-based simulated wastewater.Humic acid (HA), a component of humic substances, is widely distributed in surface and groundwater.The ability of HA to complex with heavy metals can enhance the migration of heavy metal ions in solution and impact the removal of heavy metal ions [68].Figure 5f illustrates the impact of HA on the heavy metal adsorption capacity of the Mn 3 O 4 -AL.The adsorption of Pb 2+ and Cu 2+ by the Mn 3 O 4 -AL increased as the concentration of HA increased, and reached a plateau at 15 mg/L.However, the adsorption of Ni 2+ onto the Mn 3 O 4 -AL decreased as the concentration of HA increased.This distinct difference in the effect of HA can be explained by the following reasons.On the one hand, HA contains numerous functional groups, including phenols (-OH) and carboxylic groups (-COOH), which can offer extra adsorption sites and improve the adsorption of heavy metals [69].Heavy metals can react with HA in a solution to create stable complexes [70,71].Chaturvedi et al. (2007) also reported that humic acid can enhance the adsorption capacity of heavy metals on mineral surfaces through the formation of ternary mineral surface-metal-organic ligand complexes [72].Therefore, the formation of Mn 3 O 4 -AL-HA-metal ion ternary complexes may enhance the adsorption of heavy metals.Research has shown that the affinities of these ions followed the order of Pb 2+ > Cu 2+ > Ni 2+ [71].The promotional effect of HA on the adsorption of Pb 2+ and Cu 2+ is greater than that of Ni 2+ .On the other hand, studies have shown that HA with a large size and low solubility, can accumulate at the solid/water interface to form a coating on the solid phase.HA attached to the solid phase surface will occupy adsorption sites and reduce the adsorption of heavy metals [73].Therefore, we hypothesize that the HA adsorbed onto the Mn 3 O 4 -AL tends to combine with Pb 2+ and Cu 2+ rather than Ni 2+ to form Mn 3 O 4 -AL-HA-Pb and Mn 3 O 4 -AL-HA-Cu complexes.This results in an increase in the adsorption capacity of Pb 2+ and Cu 2+ on the Mn 3 O 4 -AL with higher concentrations of HA, while the adsorption capacity of Ni 2+ decreases.

Adsorption Mechanisms
To simulate natural environmental conditions, lake water was used as the solvent to investigate the adsorption capacity of the Mn 3 O 4 -AL for Pb 2+ , Cu 2+ , Ni 2+ , Cd 2+ , and Zn 2+ in a multi-metal system (Figure 6).Results showed that the adsorption capacity of Pb 2+ , Cu 2+ , and Ni 2+ decreased to 36.72, 13.25, and 4.35 mg/g, respectively.The adsorption capacity of these heavy metals followed the order of Pb 2+ > Cu 2+ > Ni 2+ > Cd 2+ > Zn 2+ , which is consistent with previous research using montmorillonite and Sphagnum peat [41,65].These results indicated that the Mn 3 O 4 -AL can effectively remove various heavy metal ions from lake water-based simulated wastewater.To simulate natural environmental conditions, lake water was used as the solvent to investigate the adsorption capacity of the Mn3O4-AL for Pb 2+ , Cu 2+ , Ni 2+ , Cd 2+ , and Zn 2+ in a multi-metal system (Figure 6).Results showed that the adsorption capacity of Pb 2+ , Cu 2+ , and Ni 2+ decreased to 36.72, 13.25, and 4.35 mg/g, respectively.The adsorption capacity of these heavy metals followed the order of Pb 2+ > Cu 2+ > Ni 2+ > Cd 2+ > Zn 2+ , which is consistent with previous research using montmorillonite and Sphagnum peat [41,65].These results indicated that the Mn3O4-AL can effectively remove various heavy metal ions from lake water-based simulated wastewater.

Adsorption Mechanisms
To gain further insight into the adsorption mechanisms of the Mn3O4-AL for Pb 2+ , Cu 2+ , and Ni 2+ , the chemical composition of the Mn3O4-AL after the adsorption process of multi-heavy metal ions (Pb 2+ , Cu 2+ , Ni 2+ ) was determined using SEM-EDS, XRD, and XPS The metallic state of elements in the Mn 3 O 4 -AL after competitive adsorptions was analyzed using XPS analysis (Figure 7c-i).The presence of Na, Mn, O, Ca, C, and Si elements in the Mn 3 O 4 -AL before competitive adsorption can be observed (Figure 2b).After competitive adsorptions, three new peaks Cu 2p, Ni 2p, and Pb 4f appeared in the Mn 3 O 4 -AL (Figure 7c), indicating the successful adsorption of Pb 2+ , Cu 2+ , and Ni 2+ [19].Meanwhile, the disappearance of Na 1s after competitive adsorption demonstrated that ion exchange was an important mechanism of the ability of the Mn 3 O 4 -AL to adsorb Pb 2+ , Cu 2+ , and Ni 2+ [20,21].The high CEC of the Mn 3 O 4 -AL (31.5 ± 1.82 cmoL(+)/kg) can be attributed in part to the presence of clay mineral components, which also confirmed that the Mn 3 O 4 -AL has a high ion exchange potential [21].In addition, we proved that alkali ions can significantly influence the adsorption of Ni and Pb in the study mentioned above (Figure 5).This further confirmed that ion exchange was the primary adsorption process of competitive adsorptions.After adsorption, the C 1s peak was observed at 284.8, 286.3, and 288.8 eV (Figure 7d), which was consistent with the peaks observed before adsorption, indicating that the existing form of carbonate had negligible change after adsorption.Whereas, the O 1s peak changed from 529.9, 531.2, and 532.3 eV to 530.3, 531.9, and 533.4 eV.The O 1s peak shifted to a higher binding energy after adsorption.This shift in binding energy position may be due to electron transfer between the heavy metal ions (M 2+ ) and the Mn 3 O 4 -AL, indicating the formation of -Mn-O-M 2+ , -Si-O-M 2+ and -COO-M 2+ compounds [21,74,75].Furthermore, detailed spectra of the peaks of Pb, Cu, and Ni are shown in Figure 7.The main peaks of Pb 4f 7/2 and Pb 4f 5/2 were observed at 138.4 eV and 143.3 eV, indicating the formation of lead complex compounds during adsorption [76].The Pb 4f 7/2 peak occurred between the lead hydroxide (137.3 eV) and lead carbonate (138.7 eV) binding energy centers (Figure 7g), suggesting the formation of two lead species as a result of the adsorption by the Mn 3 O 4 -AL [76,77].These findings were consistent with the XRD analysis results, which confirmed the formation of hydrocerussite (Pb 2 (OH) 2 (CO 3 ) 2 ) and cerussite (PbCO 3 ).These results demonstrate that the precipitation of lead carbonate played a crucial role in the adsorption process of Pb 2+ by the Mn 3 O 4 -AL.In the XPS spectra of Cu (Figure 7h), peaks at 941.6 and 944.9 eV were attributed to shake-up satellite peaks of Cu 2p 3/2 , which were caused by the charge transfer between Cu 2+ and the ligand [77].The peak at 962.3 eV confirmed the existing form of Cu(OH) 2 , while peaks at 934.5 and 954.1 eV were attributed to the formation of copper complex compounds, Mn 3 O 4 -AL-O-Cu [77].Therefore, complexation and precipitation were the main mechanisms of Cu 2+ adsorption [78].As for Ni, the peaks at 856.1 and 873.7 eV corresponded to the peaks of Ni 2p 3/2 and Ni 2p 1/2 , which were attributed to Ni(OH) 2 [67,79].The peaks that appeared at 860.2 eV belonged to NiO(OH) and 879.1 eV peak was attributed to the corresponding satellite peak of Ni 2p [67,80], indicating that precipitation was an important mechanism of Ni adsorption on Mn 3 O 4 -AL.

Conclusions
In this study, we provided a strategy for the resourcization of argillaceous limestone modified with Mn3O4.The CEC and specific surface area increased significantly after modification.Mn3O4-modified argillaceous limestone has a high adsorption capacity for Pb 2+ , Cu 2+ , and Ni 2+ in both mono-metal and multi-metal systems.At pH 2-4, the adsorption capacity increased with increasing pH values.At pH 4-6, the adsorption capacity was Furthermore, the introduction of negatively charged Mn 3 O 4 makes AL more electronegative, leading to an electrostatic attraction between the Mn 3 O 4 -AL and heavy metal ions [81].In addition, the increased specific surface area after Mn 3 O 4 modification led to an increase in the number of contact sites on the Mn 3 O 4 -AL, which is favorable for the adsorption of Pb 2+ , Cu 2+ , and Ni 2+ .
In summary, the adsorption mechanisms of the Mn 3 O 4 -AL for Pb 2+ , Cu 2+ , and Ni 2+ are as follows: (1) ion exchange between alkali ions and heavy metal ions; (2) precipitation to form PbCO 3 , Pb 2 (OH) 2 (CO 3 ) 2 , Cu(OH) 2 , Ni(OH) 2 , and NiO(OH) due to the large proportion of carbonate and other basic functional groups in the Mn 3 O 4 -AL; (3) electrostatic interaction between the Mn 3 O 4 -AL and heavy metal ions due to the high negative charge of Mn 3 O 4 ; and 4) formation of heavy metal complexes Mn-O-M 2+ , -Si-O-M 2+ and -COO-M 2+ by hydroxyl groups on the clay mineral components of the Mn 3 O 4 -AL.The Mn 3 O 4 -AL exhibited excellent performance in removing Pb 2+ , Cu 2+ , and Ni 2+ , which could significantly contribute to the remediation of heavy metal-contaminated water and soil.

Conclusions
In this study, we provided a strategy for the resourcization of argillaceous limestone modified with Mn 3 O 4 .The CEC and specific surface area increased significantly after modification.Mn 3 O 4 -modified argillaceous limestone has a high adsorption capacity for Pb 2+ , Cu 2+ , and Ni 2+ in both mono-metal and multi-metal systems.At pH 2-4, the adsorption capacity increased with increasing pH values.At pH 4-6, the adsorption capacity was maximized and remained stable, indicating that the Mn 3 O 4 -AL is suitable for removing heavy metals in weakly acidic soil or neutral wastewater.As the concentration of HA increased, the adsorption of Pb 2+ and Cu 2+ increased, but the adsorption of Ni 2+ decreased.Higher ionic strength was not conducive to the removal of Pb 2+ , and Ni 2+ , but it had no significant effect on the removal of Cu 2+ .Ion exchange, electrostatic interaction, precipitation, and complexation were the primary adsorption mechanisms.Of course, due to the limitations of laboratory-scale experiments and the intricacy of natural soil, further research is needed to assess the potential of the Mn 3 O 4 -AL in remediating heavy metal-contaminated soil.

Figure 1 .
Figure 1.Schematic route of the modification of Mn 3 O 4 -AL (a).SEM (b,c) and EDS (d) images of Mn 3 O 4 -AL.

Figure 2 .
Figure 2. FT-IR spectra of Mn3O4-AL and AL (a).XPS spectra of wide scan, O 1s, C 1s, and Mn 2p of Mn3O4-AL after modification (b-e).XRD patterns spectra of Mn3O4-AL and AL (f).

Figure 2 .
Figure 2. FT-IR spectra of Mn 3 O 4 -AL and AL (a).XPS spectra of wide scan, O 1s, C 1s, and Mn 2p of Mn 3 O 4 -AL after modification (b-e).XRD patterns spectra of Mn 3 O 4 -AL and AL (f).

Figure 4 .
Figure 4. Adsorption isotherms of mono-and multi-heavy metal ions (a-c); Comparison of the maximum adsorption capacity of mono-and multi-heavy metal ions (d).

Figure 4 .
Figure 4. Adsorption isotherms of mono-and multi-heavy metal ions (a-c); Comparison of the maximum adsorption capacity of mono-and multi-heavy metal ions (d).

Figure 5 .
Figure 5.Effect of pH on the adsorption of mono-and multi-heavy metal ions (a); zeta potential of Mn3O4-AL at different ionic strengths and pH of solutions (b); Effect of ionic strength and pH on the adsorption of multi-heavy metal ions (c-e); Effect of HA on the adsorption process (f).

Figure 5 .
Figure 5.Effect of pH on the adsorption of mono-and multi-heavy metal ions (a); zeta potential of Mn 3 O 4 -AL at different ionic strengths and pH of solutions (b); Effect of ionic strength and pH on the adsorption of multi-heavy metal ions (c-e); Effect of HA on the adsorption process (f).

Figure 5 .
Figure 5.Effect of pH on the adsorption of mono-and multi-heavy metal ions (a); zeta potential of Mn3O4-AL at different ionic strengths and pH of solutions (b); Effect of ionic strength and pH on the adsorption of multi-heavy metal ions (c-e); Effect of HA on the adsorption process (f).

Figure 6 .
Figure 6.Comparison of the adsorption capacity of Pb 2+ , Cu 2+ , Ni 2+ , Cd 2+ , and Zn 2+ in lake waterbased simulated wastewater.3.2.4.Adsorption Mechanisms To gain further insight into the adsorption mechanisms of the Mn 3 O 4 -AL for Pb 2+ , Cu 2+ , and Ni 2+ , the chemical composition of the Mn 3 O 4 -AL after the adsorption process of multi-heavy metal ions (Pb 2+ , Cu 2+ , Ni 2+ ) was determined using SEM-EDS, XRD, and XPS (Figure 7).Peaks of Pb, Cu, and Ni were obvious in the SEM-EDS curves after adsorption, indicating successful fixation of Pb, Cu, and Ni onto the Mn 3 O 4 -AL.The mineralogical composition of the Mn 3 O 4 -AL after competitive adsorption was characterized by powder XRD (Figure 7b).New diffraction peaks of hydrocerussite (2θ = 24.74• , 27.16 • , and 51.0 • ) (PDF No. 28-0529) and cerussite (2θ = 44.1 • , 48.94 • , and 46.9 • ) (PDF No. 85-1088) appeared after adsorption, indicating the formation of Pb 3 (CO 3 ) 2 (OH) 2 and PbCO 3 [21].The results indicated that the precipitation of lead carbonate plays a crucial role in the process of the Mn 3 O 4 -AL adsorbing Pb 2+ .The metallic state of elements in the Mn 3 O 4 -AL after competitive adsorptions was analyzed using XPS analysis (Figure7c-i).The presence of Na, Mn, O, Ca, C, and Si elements in the Mn 3 O 4 -AL before competitive adsorption can be observed (Figure2b).After competitive adsorptions, three new peaks Cu 2p, Ni 2p, and Pb 4f appeared in the Mn 3 O 4 -AL (Figure7c), indicating the successful adsorption of Pb 2+ , Cu 2+ , and Ni 2+[19].Meanwhile, the disappearance of Na 1s after competitive adsorption demonstrated that ion exchange was an important mechanism of the ability of the Mn 3 O 4 -AL to adsorb Pb 2+ , Cu 2+ , and Ni 2+[20,21].The high CEC of the Mn 3 O 4 -AL (31.5 ± 1.82 cmoL(+)/kg) can be attributed in part to the presence of clay mineral components, which also confirmed that the Mn 3 O 4 -AL has a high ion exchange potential[21].In addition, we proved that alkali ions can significantly influence the adsorption of Ni and Pb in the study mentioned above (Figure5).This further confirmed that ion exchange was the primary adsorption process of competitive adsorptions.After adsorption, the C 1s peak was observed at 284.8, 286.3, and 288.8 eV (Figure7d), which was consistent with the peaks observed before adsorption, indicating that the existing form of carbonate had negligible change after

Toxics 2024 ,
12, x FOR PEER REVIEW 13 of 17proportion of carbonate and other basic functional groups in the Mn3O4-AL; (3) electrostatic interaction between the Mn3O4-AL and heavy metal ions due to the high negative charge of Mn3O4; and 4) formation of heavy metal complexes Mn-O-M 2+ , -Si-O-M 2+ and -COO-M 2+ by hydroxyl groups on the clay mineral components of the Mn3O4-AL.The Mn3O4-AL exhibited excellent performance in removing Pb 2+ , Cu 2+ , and Ni 2+ , which could significantly contribute to the remediation of heavy metal-contaminated water and soil.

Figure 7 .
Figure 7. EDS images of Mn 3 O 4 -AL after competitive adsorption (a).XRD patterns of Mn 3 O 4 -AL after adsorption (b).XPS scan of Mn 3 O 4 -AL after competitive adsorption (c-i).

Table 1 .
Kinetic parameters of pseudo-first-order and pseudo-second-order models for competitive adsorption.