INFLUENCE OF SOLUTION COMPOSITION ON PHOSPHATE SORPTION ON GYPSUM

Phosphate is common in aquatic systems at concentrations typically below 20 ug L-1. In impacted systems, concentrations may reach 1 mg L-1 or more, causing excess growth of phytoplankton and ultimately eutrophication. In an effort to minimize the impact of anthropogenic sources of phosphate, we investigated its sorption by gypsum.

Batch sorption experiments were completed at pH 6 and 7 in a background electrolyte solution of either 0.001 M CaCl2 or 0.002 M NaCl. Additionally, a batch study was completed at pH 7 in 0.01 M CaCl2. The solutions were spiked with phosphate at eight different concentrations ranging from 3.1 ug L-1 to 620 ug L-1 to simulate a range of natural to impacted conditions. Gypsum was prepared by crushing and sieving with the 0.125 to 0.250 mm fraction retained for the batch experiments. A 2 g sample of gypsum was reacted with 40 ml of solution for 1 hour. Following the reaction period, solutions were analyzed for phosphate by autoanalyzer using the ascorbic acid method.

Results in all the dilute electrolyte solutions generally conform to an L-shaped isotherm. This suggests that gypsum has a strong affinity for phosphate, but a limited number of sorption sites exist. Evaluating the data using the Langmuir isotherm indicates that phosphate sorption is slightly preferred at pH 6 for both the Ca and Na background electrolyte, and little difference in sorption is seen between the Ca and Na solutions at equivalent pH values. Sorption did not fit well to a Langmuir isotherm for the 0.01 M Ca solution. At this higher Ca concentration, phosphates affinity for Ca may be producing the precipitation of a Ca-phosphate phase.