The effect of humic acid on the transport of volatile chlorinated hydrocarbons in soil

Abstract

Flow-through column experiments were conducted to estimate the change of the concentration of three chlorinated VOCs (chloroform, trichloroethylene and tetrachloroethylene) in two types of soil. The two soils were a terra rosa with organic carbon content of 1.32% and a sandy clay loam with organic carbon content of 0.26%. The experiments took place for 116 μg/l single solutes in the absence and presence of humic acid (10 and 30 mg/l). Effluent concentrations depended on the individual compound, the carbon content of the soil, and the presence of humic acid. In the absence of humic acid, chloroform showed the highest effluent concentration, or equivalently the smallest extent of adsorption. When humic acid was added to the feed solution, the VOC effluent concentration increased. This was more severe for chloroform and trichloroethylene. The higher the concentration of humic acid in the feed solution, the higher the concentration of each of the VOCs in the soil bed effluent. The increased mobility was attributed to the binding of the organic contaminants to the humic molecules affecting their solubility in the soil aqueous phase. In the case of tetrachloroethylene a small degree of biodegradation was also observed as indicated by the appearance of trichloroethylene.