Desorption of Exchangeable Cations at Adsorption of Lead Ions by Chernozem in the Presence of Attendant Anions

Corresponding Author: Mandzhieva, S.S. Southern Federal University, 344090, 194/1 prosp. Stachki, Rostov-on-Don, Russia Email: msaglara@mail.ru Abstract: The effect of Cl, Ac ̅ and NO3 ̅ anions on the adsorption of Pb cations by an ordinary chernozem and desorption of exchangeable cations in the soil-solution system has been assessed under model laboratory conditions. It has been shown that the attendant anions affect the shape of the adsorption isotherms, which are described by the Langmuir equation. The greatest influence on the affinity constant of Pb with soil has anion Ac ̅. It has been found that the uptake of the Pb cations by chernozem from solutions of different salts is accompanied by the displacement of the exchangeable cations to the solution in the following order: Ca > Mg > Na > K. The sum of the displaced exchangeable cations in most cases exceeds the amount of the adsorbed metal cations. The content of Pb cations desorbed by the ammonium acetate buffer solution (pH 4.8) makes up 6-39% of the total Pb adsorbed cations. According to the effect of the anions on the displacing capacity of cations for Pb the following series are formed: Cl ̄ > Ac ̅ > NO3 ̄.


Introduction
At the present, the contamination with Heavy Metals (HMs) constitutes a serious environmental danger. Among the HMs, copper is one of the priority environmental pollutants. Lead is a highly technophilic element, which finds wide use in industry. It arrives into the environment from technogenic sources and actively affects all the environmental components. Soils are subject to the maximum load (Panin and Siromlya, 2005). This is related to the capacity of soils to strongly fix Pb into low-mobile compounds and thus fulfill the role of natural buffer and detoxicant.
It is known that most of HMs emitted by industrial enterprises and motor vehicles as aerosols arrive into the soil in the form of oxides, oxyhydroxides, carbonates, chlorides, sulfates and phosphates. However, problems related to the effect of the chemical form of a HM on its adsorption by the soil remain to be studied (Wang and Harrell, 2005;Davis and Leckie, 1978;Guilherme et al., 2003;Puls and Bohn, 1988).
The transformation of technogenic Pb compounds in the soil starts from their dissolution in soil water. For the hardly soluble Pb compounds, this is the first and the slowest stage of their transformation (Ladonin and Karpukhin, 2011). According to Zaplina (1994), the time of the complete dissolution of HM oxides added to the soil in the amounts corresponding to the actual contamination levels can be from six months to tens of years. Other authors (Parnunina, 1983;Gorbatov, 1988) studying the transformation kinetics of Pb, Cd and Zn oxides, as well as of silt containing these HMs, in soils, on the contrary, noted the high dissolution rate of metal oxides and the adsorption of their ions on the surface of soil particles.
The equivalence of cation exchange in soils still remains a debatable problem. The effect of salt anions on the balance of the displaced cations is worst understood. In accordance with the ion exchange theory, the adsorption of some cations should be accompanied by the displacement of an equivalent amount of other cations, including H + , from the Soil Exchange Complex (SEC). However, real experiments frequently give contradictory results. Panin and Siromly (2005) generally noted an equivalence of ion-exchange processes with consideration for the released hydrogen. However, some studies were performed with monoionic Ca-saturated soils (Minkina et al., 2011;Pinskii, 1997), which makes incorrect the comparison of their results with the data for soils with a natural composition of the exchangeable cations.
The involvement of all the cations and different anions present in the system and capable of exchange in the sorption is an important aspect of adsorption processes in natural soils . The anionic composition of the salt containing a Heavy Metal (HM) can have a qualitative and quantitative effect on the exchange of cations in the soil-solution system.
The aim of this work was to study the balance of Pb cations adsorbed from different salt solutions and the amount of exchangeable cations displaced from the SEC.

Adsorption Experiment
The soil in the natural ionic form sieved through a 1 mm sieve was used in the studies. The concentrations of the initial Pb (Cini) solutions varied in the range from 0.05 to 1 mM L −1 . The selection of the metal concentration range is determined by the existing levels of the soil contamination with pollutants. The samples of soil in the natural cationic form were treated with solutions of Pb nitrates, acetates and chlorides at a soil: Solution ratio of 1: 10. The suspensions were shaken for 1 h and left to stand for 24 h. The pH values were measured in the equilibrium solutions by potentiometry. Then, the suspensions were filtered. The contents of Ca 2+ , Mg 2+ , Na + and K + were determined by the Shaimukhametov method (AMSS, 1990). The content of Pb 2+ in the filtrates was determined by atomic absorption spectroscopy (Kvant-2A). The contents of the adsorbed Pb cations were calculated from the difference between the metal concentrations in the initial and equilibrium solutions. Sorption isotherms were constructed by plotting the sorbed metal content of the soil (mM per of dry soil) against the concentration of the metal in solution at equilibrium (mM L −1 ).
The desorption of the adsorbed Pb in the soil (their mobile form) was assessed by their extraction with a 1 N ammonium acetate solution at pH 4.8 with a soil: Solution ratio of 1: 5. The time of the extraction was 18 h (Minkina et al., 2008).

Statistical Analysis
The experiments were carried out in triplicate. All statistical calculations were performed using Microsoft Excel 2010. The approximation of the experimental isotherms by the Langmuir equation was performed using the SigmaPlot 2001 statistics package at a confidence probability of 0.95.

Results
The adsorption isotherms of Pb cations from chloride, nitrate and acetate solutions are given in the Fig. 1. The isotherm's shape is determined by the character of the adsorbate-adsorbent interaction: The more convex the isotherm, the stronger the adsorbate is retained by the adsorbent.
The adsorption of Pb by the soils is most frequently described by the Langmuir equation. The Langmuir equation is derived from the model concept of the adsorption process and has the form: Where: C ads = The content of the adsorbed cations С max = The maximum adsorption of the HM, mM kg −1 soil K L = The affinity constant, L mM −1 C = The concentration of the HM in the equilibrium solution, mM L −1 The parameters of the Pb adsorption calculated from these equations are given in Table 1. The approximation quality of the experimental isotherms by these equations is characterized by high determination coefficients (R 2 ). The analysis of the data (Table 1) shows that the effect of the attendant anions on the values of C max for the studied Pb cations varies in the following series: Cl‾ > NO 3 ‾ > Ac‾.  Table 3. Data on the pH of the salt solutions used (above the line) and the equilibrium suspensions of an ordinary chernozem during the adsorption of Pb (under the line) Cini, mmol L −1 PbCl 2 Pb(CH 3 COO) 2 Pb(NO 3 ) 2 0.05 5.6/6.8 6.4/7.0 6.3/7.0 0.08 5.5/6.8 6.4/7.0 6.1/6.7 0.1 5.5/6.7 6.3/6.7 5.9/6.7 0.3 5.4/6.6 6.2/6.5 5.8/6.5 0.5 5.3/6.6 6.1/6.4 5.6/6.4 0.8 5.3/6.5 6.0/6.3 5.4/6.3 1.0 5.3/6.4 6.0/5.9 5.2/6.2 The values of K L vary in the following series: Ac‾ > Cl‾ > NO 3 ‾.
The adsorption of metal by the ordinary chernozem was accompanied by the displacement of Са 2+ , Mg 2+ , K + , Na + and H + into the solution ( Table 2).
The value of the suspension pH varies during the adsorption of Pb (Table 3). According to the effect on the pH value, the acid anions form the following series: NO 3 ‾ > Ac‾≥ Cl‾.
Chemical properties of the anions associated not only affect the adsorption characteristics metal quantitative, but also on the mechanisms of their interaction with the surface of soil particles and metals in the form of finding the solid phase (Minkina et al., 2008). To answer this question studied desorption exchange forms of Pb cations adsorbed from soil solutions of chlorides, acetates and nitrates, ammonium acetate buffer (pH 4.8).
For the effect of the anions on the extraction of the adsorbed cations from the soil, it could be noted that the extraction of Pb progressively decreases in the series: 3 Cl¯Ac¯NO> > Fig. 1. Isotherms of the adsorption of Pb by an ordinary chernozem at their addition in the form of (1) chlorides, (2) nitrates (3) and acetates

Lead Adsorption
The positions of the anions in the affinity series for the values of Cmax and KL do not coincide. The soluble complexes of the studied HM cations with nitrate ions are the least stable and, hence, least affect the behavior of the Pb in adsorption processes, while the acetate anions form relatively stable complexes with the studied cation. In this case, the adsorption of the associated Pb forms with changes in the interaction mechanism is possible. The studied system, which includes only three anions, is incomplete. Actually, the behavior of the studied HMs is also affected by other anions always present in natural solutions-CO 3 2 ‾, HCO 3 ‾ and OH‾ as well as anions of dissolved organic acids (Kurochkina and Pinskii, 2004).
Thus, the effect of the attendant anions on the adsorption of HM cations by soils generally is retained and corresponds to the stability constants of the complexes formed by the studied metals with the attendant anions present in the liquid phase of the studied systems (Lur'e, 1971;Pagenkopf, 1978): CO 3 2 ‾ > OH‾ > HCO 3 ‾ > SO 4 2 ‾ > Ac‾ > Cl‾ > NO 3 ‾. The Pb complexes with Cl‾ and NO 3 ‾ anions are the least stable.

Displacing of Exchangeable Cations at the Adsorption of Pb
The contents of the displaced exchangeable cations form the following series: Ca 2+ > Mg 2+ > Na + > K + >> H + , which corresponds to their proportions in the SEC. The content of hydrogen atoms involved in the exchange processes is significantly lower than the concentrations of the other cations under study and has almost no effect on their sum; therefore, it is not considered in the balance calculations (Table 2).

Changes in the Soil Suspension pH at the Interaction with Pb Salts
The changes in the suspension pH at the addition of Pb salts to the soil are due to the following main processes: The hydrolysis of HM salts, the adsorption of HM ions and protons by the SEC and the association of HM cations with the anions present in the solution: OH -, SO 4 2-, Cl‾, NO 3 and Ac -. The comparison of the data on the changes in the pH of the soil suspensions due to the hydrolysis of the original salts and the adsorption of metal by the soil (Table 3) shows that these processes can activate or inhibit each other. Thus, the hydrolysis of the studied salts decreases the solution pH with the increasing concentration of salts. This difference is primary due to the different Bronsted-Lowry acidities of the attendant anions: Sulfuric, hydrochloric and nitric are strong acids and acetic is a weak acid.
The calculation of the state of the Pb cations in the equilibrium solutions of the soils studied in the pH range of 5.8-7.2 showed that the acetate ions present in the solution fix up to 20% into relatively stable charged MeAc + complexes (Minkina et al., 2011). The concentration of the neutral carbonate complexes PbCO 3 0 also increases with the increasing pH. The addition of soil to the solution significantly changes the situation. The HM cations are capable of displacing the exchangeable hydrogen from the SEC and, hence, acidifying the equilibrium solutions. An inverse process occurs simultaneously: Protons, which have an extremely high affinity for the weak acidic components of the SEC, are absorbed by them (Pinskii, 1997). In addition, acetate anions are actively adsorbed by soils and the acidification enhances this process due to the protonation of the surface of highly dispersed phases. The superposition of these processes forms a relatively complex pattern of pH changes at the adsorption of HMs from different salt solutions by soils. The situation is even more complicated because of the participation of the alkali earth cations (Ca 2+ and Mg 2+ displaced into the solution) in these processes. Analogous processes also occur at the adsorption of the Pb 2+ cations. In particular, the precipitation of insoluble carbonates, oxycarbonates and hydroxides is typical for Pb (Forano et al., 2006). The use of the synchronous Xray technique allowed experimentally proving the existence of some stable Pb compounds in soils. These are primarily organic metal chelate complexes, forms sorbed by iron and manganese hydroxides and insoluble phosphates (Kretzschmar et al., 1999;Lamoureux et al., 2001;Manceau et al., 2002;1996;Vodyanitskii, 2006). The possibility of the formation of insoluble Pb hydroxypyromorphite was shown earlier (Pinskii and Fiala, 1991).

Assessing the Equivalence of the Exchange between the Pb and the SEC Cations
The sum of the exchangeable cations displaced into the solution is generally significantly larger than the amount of the adsorbed metal and the ratio between the sum of the exchangeable cations displaced into the solution and the amount of the adsorbed HMs is almost always higher than 1 and decreases with the increasing amount of the adsorbed HM. Ion exchange processes in soils by definition occur in equivalent proportions; therefore, the question arises concerning the reasons for the observed disagreement between the contents of the metal adsorbed by the soil and the exchangeable cations displaced into the solution.
The content of the exchangeable cations released into the solution depends on the properties of the adsorbed Pb cations and the attendant anions (Table  2), as well as on the composition and properties of the SEC (Minkina et al., 2009). From the mean total concentrations of the exchangeable cations in the equilibrium solutions, the displacing capacities of the studied metal can be assessed.
By the effect of the anions on the displacing capacity of the Pb cations, the following series are formed: Cl‾ >> NO 3 ‾ > Ac‾.
The association of HM cations with the solution components plays an important though uncertain role not only in the adsorption of HM cations but also in the interaction with the exchangeable cations released into the solution Pinskii and Zolotareva, 2004;Pinskii et al., 2014). The effect of the attendant cations on the displacing capacity of the Pb does not correlate with the stability constants of the complexes of the metal with the corresponding anions (Lur'e, 1971;Pagenkopf, 1978). MeAc + and MeSO 4 0 are the most stable complexes of the studied metal, while the MeNO 3 + and MeCl + complexes are less stable and are formed in smaller amounts. On the other hand, the HCO 3 and OHanions, which are always present in the solutions, also form relatively stable charged complexes with Pb 2+ and thus affect the behavior of HMs in the systems studied. The formation of charged metal complexes with inorganic and organic anions in the solution changes the mechanisms of interaction with the solid phase surface, which increases the amount of the adsorbed Pb. More complex interactions of the associated HM cations with the solid phase surface are also possible (Davis and Leckie, 1978;Minkina et al., 2009;Benjamin and Leckie, 1982;Pinskii, 1988).
The super equivalent displacement of exchangeable cations at the adsorption of metal by the soil is explained by the presence of some amounts of Ca and Mg carbonates, as well as precipitates of the soluble salts of the main exchangeable cations in the original soil. The excess of the salts displaced into the solution regularly decreases with the increasing amount of the adsorbed Pb, because the exchange of cations occurs in equivalent proportions and the content of soluble salts remains constant. As a result, the Σ displaced exchangeable cations/C ads ratio decreases, but it usually remains higher than 1. The adsorption of associated metal forms MeL + (where L is a singly charged ligand) also can cause a decrease of this ratio, because it is formally accompanied by an increase in the content of the adsorbed HM cations and, hence, the denominator of the ratio. The formation of associates can significantly change the mechanism of metal adsorption.
Another mechanism of the unexchangeable adsorption of HMs by the soil is the formation of insoluble compounds localized on the surface. The probability of such processes was experimentally proved using EXAFS spectroscopy (Vodyanitskii, 2006). Calculations showed (Minkina et al., 2009) that the precipitation of Pb hydroxide is possible under these experimental conditions. A significant contribution is made by the hydrolysis of compounds on the surface of soil particles, which proceeds relatively rapidly. This is also a source of additional ions in the equilibrium solution (Minkin et al., 1982).

Pb Sorbtion-Desorbtion
The data of Table 4 shows that in all cases the amount of desorbed cations substantially less than the amount absorbed. Hence, the single extraction of the exchangeable Pb cations by this extracting agent does not ensure their total transfer into the solution. Sadovnikova (1997) noted that a single treatment with 1 N NH 4 Ac does not completely displace the exchangeable forms of Pb, because it extracts no more than 50% of the exchangeable alkaline-earth cations, which are significantly weaker retained by the soil than the HMs.
When the content of the adsorbed Pb cations increases, both the absolute amount of the desorbed Pb cations and the percentage of their desorption increase. This fact confirms the common idea that the binding of the adsorbed cations to the soil particles becomes weaker because of the energetic heterogeneity of the surface and the mutual effect of the adsorbed cations. As far as the surface is filled with adsorbed cations, they occupy positions with progressively lower affinities for these cations, which increases their mobility and availability to plants and microorganisms. The

Conclusion
In general, the behavior of the metal in the soil, the solution greatly depends on the individual circumstances. The Cl‾, Ac‾ and NO 3 ‾ anions significantly affect the adsorption of Pb cations by ordinary chernozem. Almost all isotherms are adequately described by the Langmuir equation. Soluble complexes investigated Pb cations from nitrate ions, the least stable and hence less likely affect the behavior of the metal in the adsorption process, while the anions of acetic acid to form sufficiently stable complexes with Pb. The chloride anion occupies an intermediate position.
The involvement of all the cations and different anions present in the system and capable of exchange in the sorption is an important aspect of adsorption processes in natural soils. The anionic composition of the salt containing a HM can have a qualitative and quantitative effect on the exchange of cations in the soil-solution system.
It was shown that the adsorption of Pb by the soil from different salt solutions is accompanied by the displacement of exchangeable cations and protons into the solution in proportions typical for the studied soil: Ca 2+ > Mg 2+ > Na + > K + >> H + . The sum of the displaced exchangeable cations always exceeds the amount of the adsorbed HM cations. This is related to the presence of soluble salt precipitates in the soil.
The effect of the attendant anions on the pH of the equilibrium solutions is uncertain because of the simultaneous action of three main processes: The hydrolysis of HM salts, the adsorption (displacement) of protons from the SEC at the adsorption of HMs and the association of HM cations with the anions present in the solution: OH -, SO 4 2-, Cl‾, NO 3 ‾ and Ac -. According to the effect on the pH value, the acid anions form the following series: NO 3 ‾ > Ac‾ ≥ Cl‾.
The sum of the exchangeable cations displaced into the solution always exceeds the amount of the adsorbed metal and the ratio between the sum of the exchangeable cations displaced into the solution and the amount of the adsorbed Pb is always higher than 1 and decreases with the increasing content of the adsorbed metal.
The content of Pb cations desorbed by the ammonium acetate buffer solution (pH 4.8) makes up 6-39% of the total Pb adsorbed cations. According to the effect of anions on the displacing capacity of Pb cations, the following series are found: Cl‾ > Ac -> NO 3 ‾.