Feasibility of using bentonite, lime and fly ash in permeable reactive barriers for acid sulphate soils

Abstract

Acidic groundwater resulting from the poorly-planned use of acid sulfate soils has become a major environmental issue in coastal Australia. Use of permeable reactive barriers (PRBs) designed to generate alkalinity by promoting sulfate reduction has recently become popular as an alternative solution to this problem. Although different materials suitable for PRBs have recently been proposed and tested for remediation of acidic groundwater, there are still several aspects that need to be addressed. One of them is chemical precipitation and clogging of pore space that may significantly decrease the hydraulic conductivity of a PRB over a period of time. This study seeks to explore the feasibility of using bentonite in addition to fly ash and lime to form mixtures with a high buffer capacity and permeability that would enable groundwater flow through PRBs over a substantial period of time. To investigate the acid neutralization capacity of fly ash, lime, and bentonite mixtures, a series of laboratory tests were performed on different soil samples leached with solutions of sulphuric acid. It was found that the ability of mixtures to neutralize acidic fluids was mostly controlled by the content of lime. Laboratory data also indicated that an addition of bentonite to lime-fly ash mixtures may decrease the buffer capacity of soil. Results of hydraulic conductivity tests showed that the permeability of fly ash-lime specimens with bentonite may increase over a period of time, probably due to the changes in the diffuse double layer of bentonite particles.