Phosphate Recovery from Arsenic-Laden Groundwater Treatment Sludge

Abstract

Abstract In this work, we investigated phosphate (P) recovery via calcium phosphate (CaP) precipitation from P-rich solutions produced synthetically and derived from arsenic (As)-laden groundwater treatment sludges. We systematically altered key variables in synthetic experiments (e.g., pH, Ca:P ratio, co-occurring solutes, time) and found that >95% P recovery was possible at a dosed Ca:P ratio of 1.67. High P recovery (>90%) also disproportionately increased the solid-phase As content (>500 mg As/kg) of the synthetic CaP end-products. All sludge-derived experiments recovered >80% P, but the solid-phase As content varied (75 ± 27 to 3337 ± 31 mg As/kg) depending on the initial As concentration. Pair distribution function analysis revealed that all CaP end-products resembled poorly ordered hydroxyapatite, with the sludge-derived solids displaying more structural disorder than synthetic CaPs. Using scanning transmission X-ray microscopy, we attributed the lower crystallinity of the sludge-derived CaPs to the presence of saccharides extracted from the initial As-laden groundwater treatment sludge. Additionally, our structural data suggested As co-uptake occurred via structural incorporation into the CaP solids. This detailed knowledge on P recovery efficiency and the structure of the CaP solids is critical to design resource recovery systems for As-laden groundwater treatment sludge and for subsequent application of the recovered CaP end-products.