Weathering of clay minerals induced by fluoride-containing solutions from phosphogypsum by-product

Abstract

The production of phosphate fertilizer from phosphate rock results in the formation of phosphogypsum (PG) and HF. Soluble F− ends up in process water and in the pore space of the PG by-product. This study determined changes in the amount, properties and mineral composition of soil clay upon reaction with acidic PG solutions. Calcareous and non calcareous soils were reacted with PG leachates (PG-L, 30 mg F− L−1) and process water (PG-PW, 6070 mg F− L−1) using a sequential batch equilibration method. Phosphogypsum-L did not alter the clay fraction as much as PG-PW, which dissolved as much as two-thirds of the clay fraction. Phosphogypsum-PW reduced surface area of clay fractions from about 500 to 150 m2 g−1; Cation exchange capacity CEC was decreased from about 60 to 17 cmolc kg−1 clay. Reduction in surface area and CEC in PG-PW treated soil corresponded to near complete dissolution of smectite and a negative enrichment of mica. Kaolinite in coarse clay of the PG-PW treatment was reduced to 20% of the control and was completely destroyed in fine clay fractions. Amorphous silica was produced from the F−-induced dissolution of clay. The results indicate there is likely to be considerable alteration of minerals in subsoils beneath PG waste repositories and of clay liners used at some storage facilities. Key words: Clay liners, landfills, dissolution, waste management, leachates, geochemical reactions, mineral stability