Arsenate Sorption on Calcareous Soils.

The strong affinity of calcite for arsenic predicts its importance for arsenic sorption on calcareous soils.  In calcareous soils, the role of calcite as a sorbent might depend on its concentration because more sorption sites are provided by higher amounts of calcite.  Batch sorption experiments were conducted on sorbent mixtures composed of acid-washed silica sand and reagent grade calcite powder, ranging from 2% to 8% calcite content.  Arsenate sorption was investigated on all sorbent mixtures with 0.01 M NaCl as the background buffer to determine if arsenate sorption increases as the calcite content increases.  Wastewater effluent was used as the buffer to study its effect on arsenate sorption on sorbent mixtures with 3%, 4%, and 8% calcite content.  The concern with wastewater effluent is that it contains other anions that might compete with arsenic for sorption onto calcite, causing arsenic to remain in solution.  Sorption was analyzed using inductively coupled plasma optical emission spectroscopy (ICP-OES).  Possible sorption maxima of 1 mgKg^-1 were observed on 2% and 3% calcite.  Significant concentrations of arsenate were measured on 4% and 8% calcite, but only in NaCl buffer.  A calcite content of 4% sorbed up to 20 mgKg^-1 and a calcite content of 8% showed a sorption capacity of 70 mgKg^-1.  Sorption of arsenate on calcite was inhibited when applied with wastewater effluent and it is probably unable to compete for sorption sites until present at concentrations above 50 mgL^-1.  Major species of potential competitive anions present in wastewater effluent were modeled with PHREEQC as HCO^3-, SO₄^2-, Cl^-, F^-, HPO₄^2-, and NO₃^-.  Observed trends from sorption isotherms are of interest due to the possible application of wastewater effluent that contains arsenic residuals on calcareous soils.  Formation of calcium arsenates was investigated but not observed from analysis with x-ray diffraction (XRD) and scanning electron microscopy/energy dispersive spectroscopy (SEM-EDS).