Optimizing calcium materials for minimizing arsenate phytoavailability in upland arable soil based on geochemical analysis

This study aimed to evaluate the reducing effects of calcite and phosphogypsum on arsenate [As(V)] availability to plants and elucidate the mechanisms of As(V) immobilization. The concentration of available As(V) to plants in upland arable soils with 1% calcite and phosphogypsum decreased to 17.4% and 36.9%, respectively, compared to the control. As(V) phytoavailability depends on the soil pH and calcium materials. The process of stabilizing As(V) (F3; anion exchange) with phosphogypsum is faster and easier compared to that with calcite (F4; bind to carbonate), but it results in a less stable form. New Ca-As(V) minerals (Ca(5)2(HAsO4)x(AsO4)center dot yH(2)O, Ca(5)H(2)x(AsO4)center dot yH(2)O, or Ca(3)2(AsO4)center dot 10H(2)O) were identified in X-ray diffraction (XRD) patterns with calcite treatment. Precipitation, the primary mechanism induced by calcite, was activated at a soil pH above 8.0. Based on the deconvolution of calcium and sulfur X-ray photoelectron spectroscopy spectra and the peak shift in the XRD pattern in phosphogypsum, the substitution in which SO42- is exchanged with HAsO42- is the primary mechanism for As(V) immobilization. Substitution induced by phosphogypsum is a suitable reaction in upland arable soils, the predominant form of As(V) in the soil, with a pH range of 5-7.

Automated summary

This study evaluated the reducing effects of calcite and phosphogypsum on the availability of arsenate [As(V)] to plants and investigated the mechanisms of As(V) immobilization. The presence of 1% calcite and phosphogypsum in upland arable soils decreased the availability of As(V) to plants by 17.4% and 36.9% respectively, compared to the control. The immobilization of As(V) depends on soil pH and the type of calcium material used.