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Several approaches for immobilization of arsenic (As) based on the transformation of its soluble forms (compounds) into highly insoluble arsenate apatite Ca5(AsO4)3 OH have been proposed. These immobilization techniques are successfully applied in treatment of industrial waste containing As. Quite the contrary, treatment of soil contaminated with As by apatite amendments, instead of immobilization of this toxic element, increases its mobility and bioavailability. The mechanism underlying these opposite effects still remains elusive. Here, the stability analysis of different calcium arsenates: Ca5(AsO4)3OH, Ca4(AsO4)2, Ca3(AsO 4)2 Ca5H2(AsO4) 2 and CaHAsO4 was performed, which is based on the calculation of the ion-ion interaction potential (HIP). It has been demonstrated earlier that IIIP, representing the main term of the cohesive energy, is a suitable parameter for assessment of mineral stability. According to the results of this analysis, arsenate apatite with IIIP value of -0.578 Ry represents the most stable chemical form among analyzed compounds. Based on this finding, we proposed a mechanism of formation of arsenate apatite in the presence of hydroxyapatite. This mechanism can explain the suitability of this approach for the treatment of industrial waste and its limitations for in situ treatment of soil and water contaminated with As.

Several approaches for immobilization of arsenic (As) based on the transformation of its soluble forms (compounds) into highly insoluble arsenate apatite Ca-5(AsO4)(3)OH have been proposed. These immobilization techniques are successfully applied in treatment of industrial waste containing As. Quite the contrary, treatment of soil contaminated with As by apatite amendments, instead of immobilization of this toxic element, increases its mobility and bioavailability. The mechanism underlying these opposite effects still remains elusive. Here, the stability analysis of different calcium arsenates: Ca-5(AsO4)(3)OH, Ca-4(AsO4)(2)(OH)(2), Ca-3(AsO4)(2) Ca5H2(AsO4)(2) and CaHASO(4) was performed, which is based on the calculation of the ion-ion interaction potential (IIIP). It has been demonstrated earlier that HIP, representing the main term of the cohesive energy, is a suitable parameter for assessment of mineral stability. According to the results of this analysis, arsenate apatite with IIIP value of -0.578 Ry represents the most stable chemical form among analyzed compounds. Based on this finding, we proposed a mechanism of formation of arsenate apatite in the presence of hydroxyapatite. This mechanism can explain the suitability of this approach for the treatment of industrial waste and its limitations for in situ treatment of soil and water contaminated with As.

Several approaches for immobilization of arsenic (As) based on the transformation of its soluble forms (compounds) into highly insoluble arsenate apatite Ca-5(AsO4)(3)OH have been proposed. These immobilization techniques are successfully applied in treatment of industrial waste containing As. Quite the contrary, treatment of soil contaminated with As by apatite amendments, instead of immobilization of this toxic element, increases its mobility and bioavailability. The mechanism underlying these opposite effects still remains elusive. Here, the stability analysis of different calcium arsenates: Ca-5(AsO4)(3)OH, Ca-4(AsO4)(2)(OH)(2), Ca-3(AsO4)(2) Ca5H2(AsO4)(2) and CaHASO(4) was performed, which is based on the calculation of the ion-ion interaction potential (IIIP). It has been demonstrated earlier that HIP, representing the main term of the cohesive energy, is a suitable parameter for assessment of mineral stability. According to the results of this analysis, arsenate apatite with IIIP value of -0.578 Ry represents the most stable chemical form among analyzed compounds. Based on this finding, we proposed a mechanism of formation of arsenate apatite in the presence of hydroxyapatite. This mechanism can explain the suitability of this approach for the treatment of industrial waste and its limitations for in situ treatment of soil and water contaminated with As.

Several approaches for immobilization of arsenic (As) based on the transformation of its soluble forms (compounds) into highly insoluble arsenate apatite Ca5(AsO4)3 OH have been proposed. These immobilization techniques are successfully applied in treatment of industrial waste containing As. Quite the contrary, treatment of soil contaminated with As by apatite amendments, instead of immobilization of this toxic element, increases its mobility and bioavailability. The mechanism underlying these opposite effects still remains elusive. Here, the stability analysis of different calcium arsenates: Ca5(AsO4)3OH, Ca4(AsO4)2, Ca3(AsO 4)2 Ca5H2(AsO4) 2 and CaHAsO4 was performed, which is based on the calculation of the ion-ion interaction potential (HIP). It has been demonstrated earlier that IIIP, representing the main term of the cohesive energy, is a suitable parameter for assessment of mineral stability. According to the results of this analysis, arsenate apatite with IIIP value of -0.578 Ry represents the most stable chemical form among analyzed compounds. Based on this finding, we proposed a mechanism of formation of arsenate apatite in the presence of hydroxyapatite. This mechanism can explain the suitability of this approach for the treatment of industrial waste and its limitations for in situ treatment of soil and water contaminated with As.

Apatites are well known matrix for heavy metal immobilization and remediation of ground water and contaminated soils. In this work, capacities of different synthetic hydroxyapatite (HAP) samples for Pb, Cd, Sr and Zn sorption were analyzed, from single metal solutions and their equimolar mixture. Initial concentration of each metal in single metal solutions and total metal concentration in the mixture was 10(-2) mol/dm(3). Initial pH was adjusted to 5.0 +/- 0.1. After 24 h equilibration of HAP samples with heavy metal solutions (solid/liquid ratio 1:200), the concentration of metals and of Ca2+ released from crystal lattice, as well as the final pH values were measured. The selectivity of all samples, regardless of the differences in their compositions, crystallinities, specific surface areas, points of zero charge and sorption capacities, was as follows: Pb GT Cd GT Zn GT Sr. The same order was obtained for both single metal solutions and their mixture. In all cases, the sorption was followed by a decrease in final pH values, which are lower than the pH(PZC) values obtained by equilibration of HAP samples with an inert electrolyte (KNO3). A linear relationship between the amount of metals sorbed and Ca2+ released from HAP was observed. Also, a correlation between sorption capacities and physicochemical properties of HAP samples was found.

Apatites are well known matrix for heavy metal immobilization and remediation of ground water and contaminated soils. In this work, capacities of different synthetic hydroxyapatite (HAP) samples for Pb, Cd, Sr and Zn sorption were analyzed, from single metal solutions and their equimolar mixture. Initial concentration of each metal in single metal solutions and total metal concentration in the mixture was 10-2 mol/dm3 . Initial pH was adjusted to 5.0 ± 0.1. After 24 h equilibration of HAP samples with heavy metal solutions (solid/liquid ratio 1:200), the concentration of metals and of Ca2+ released from crystal lattice, as well as the final pH values were measured. The selectivity of all samples, regardless of the differences in their compositions, crystallinities, specific surface areas, points of zero charge and sorption capacities, was as follows: Pb > Cd > Zn > Sr. The same order was obtained for both single metal solutions and their mixture. In all cases, the sorption was followed by a decrease in final pH values, which are lower than the pHPZC values obtained by equilibration of HAP samples with an inert electrolyte (KNO3). A linear relationship between the amount of metals sorbed and Ca2+ released from HAP was observed. Also, a correlation between sorption capacities and physicochemical properties of HAP samples was found.

Apatites are well known matrix for heavy metal immobilization and remediation of ground water and contaminated soils. In this work, capacities of different synthetic hydroxyapatite (HAP) samples for Pb, Cd, Sr and Zn sorption were analyzed, from single metal solutions and their equimolar mixture. Initial concentration of each metal in single metal solutions and total metal concentration in the mixture was 10(-2) mol/dm(3). Initial pH was adjusted to 5.0 +/- 0.1. After 24 h equilibration of HAP samples with heavy metal solutions (solid/liquid ratio 1:200), the concentration of metals and of Ca2+ released from crystal lattice, as well as the final pH values were measured. The selectivity of all samples, regardless of the differences in their compositions, crystallinities, specific surface areas, points of zero charge and sorption capacities, was as follows: Pb GT Cd GT Zn GT Sr. The same order was obtained for both single metal solutions and their mixture. In all cases, the sorption was followed by a decrease in final pH values, which are lower than the pH(PZC) values obtained by equilibration of HAP samples with an inert electrolyte (KNO3). A linear relationship between the amount of metals sorbed and Ca2+ released from HAP was observed. Also, a correlation between sorption capacities and physicochemical properties of HAP samples was found.