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 (Ca52(HAsO4)x(AsO4)∙yH2O, Ca5H2x(AsO4)∙yH2O, or Ca32(AsO4)∙10 H2O) 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. [Display omitted] •Changes of soil pH with Ca materials affect As(V) phytoavailability.•As(V) is immobilized by phosphogypsum faster and easier than calcite.•A new XRD pattern of Ca-As(V) precipitation is discovered by calcite.•Sulfate (SO42-) of phosphogypsum is exchanged with As(V) (HAsO42-).•Substitution is the effective mechanism of As(V) immobilization in upland arable soil.