Technical PaperModification of the swelling characteristics and phosphorus retention of bentonite clay using alum

The neutralization of negatively charged clay particles using positively charged aluminum species is a well-known process. However, few studies explain the impact of the process on the Swelling Index (SWI) of clays over a wide range of alum and pH values. In this study, the SWI test was used to assess the effect of alum on the swelling characteristics of highly expansive bentonite clay and the saturation of the clay surfaces with Al species to improve the phosphorus retention characteristics of clays. The experiments included varying the pH and alum dosages and measuring the resulting SWI. When alum was not present, varying the pH was seen to have a profound impact on the swelling characteristics of the clay. Below pH 2 and above pH 11, the SWI was low because of an abundance of H+ at low pH values and an abundance of NaOH at high pH values. Alum was effective below pH 5 in reducing the SWI from 20 mL/g without alum down to 5 mL/g using increasing quantities of alum up to 1 g of alum per 2 g of clay in 100 mL of water. Within the pH range of 5-9, alum was not effective in controlling the SWI. Above pH 11, aluminum precipitated and the SWI was low. The addition of clay competed with the availability of Al to precipitate phosphorus, and the use of modified clay coagulated with alum resulted in significant phosphorus removal. The results demonstrated the effectiveness of alum to control clay swelling and the impact of wide variations in pH on the process.