Deep removal of arsenic from arsenic-bearing gypsum using a seed-induced crystal control technique and its mechanisms

Abstract

Arsenic-bearing gypsum (ABG), a byproduct from the lime/ferric sulfate process in the waste acid treatment of the heavy non-ferrous industry, represents a substantial hazardous arsenic (As) waste issue in China. Due to the potential arsenic leakage, the disposal of ABG poses an urgent challenge to the surrounding environment. The elevated As content and the low-value-added recycling products are the primary hurdles hindering ABG recycling. In this study, precise control over the gypsum crystal and As species in Na2SO4-H2SO4 solutions under mild conditions facilitated the deep removal of As (with a leaching efficiency of 92.45 % and a final product As concentration of 80.32 ppm) and simultaneous preparation of high-strength α-bassanite (with a compressive strength of 35.38 MPa) from the pre-treated ABG. This process occurred during the recrystallization from dihydrate gypsum (CaSO4·2H2O, DH) to α-bassanite (α-CaSO4·0.5H2O, α-HH), involving DH dissolution followed by α-HH crystallization. The encapsulated As was fully released during the dissolution process, crucial for the efficient elimination of As impurities. H2SO4 dissolved the released As, transforming it into the protonated species (H3AsO4) with a distinct structure and a less negative binding energy (ΔEB) to SO42-. This transformation prevented the chemical incorporation of the released As during the α-HH crystallization process. Additionally, the morphology and size of α-HH were controlled by seeding in the mixed solutions. The formation of large short-columnar α-HH crystals with a low specific surface area significantly reduced the surface adsorption of As, further enhancing As leaching efficiency. Importantly, the large short-columnar α-HH was identified as high-strength gypsum with high added value. This study provided innovative guidance for efficiently removing impurities from gypsum and pioneered a cost-effective approach for the clean and high-value utilization of industrial gypsum residues.