Evaluation of the utility of bacterial extracellular polymers for treatment of metal-contaminated soils: Polymer persistence, mobility, and the influence of lead

Abstract

Toxic transition metals are common contaminants at Superfund sites. Metals adsorb strongly to soil surfaces and, therefore, are not easily removed by pump and treat remediation. Metal mobility is also an important factor in risk assessment. Bacterial extracellular polymers occur naturally in soils and have well-documented metal-binding properties. Therefore, extracellular polymers that are soluble in water and do not adsorb strongly to the soil matrix have the potential of increasing metal mobility in soils. The transport and fate of bacterial polymers have not received an extensive evaluation. This research focused on the microbial degradation of bacterial polymers and their mobility in porous media, processes that must be understood if polymers are going to be considered for inclusion in risk assessments or for use in remediation efforts. Mineralization of 14C-labeled bacterial polymers was studied using bacterial inocula obtained from four sources (aquifer sand, seep sediment, surface soil, and unchlorinated secondary effluent from a municipal wastewater treatment plant). The bacterial population in secondary effluent was found to be one of the most effective at extracellular polymer degradation. After 10 days, 20|X% of the 14C-labeled polymer was mineralized by the secondary effluent inoculum, compared with 55% of the 14C-labeled glucose in a parallel experiment. Column experiments were used to determine the mobility of extracellular polymer during flow through a porous medium. An aquifer sand was used as the solid (stationary) phase in these experiments. The polymer was found to have a retardation factor ( R ⩽ 21) orders of magnitude lower than those reported for lead ( R = 19,000) and cadmium ( R = 4700) in the sand medium. The polymer was partially mineralized during transport through the porous media. The inhibitory effect of lead on polymer degradation was studied in both batch and column experiments. In column tests, the polymer degradation rate was reduced in the presence of lead. Degradation rate constants ranged from 0.00057/min to 0.0048/min in the absence of lead and from 0.00056/min to 0.0018/min in the presence of lead (values varied depending on the assumptions used in model calculations). Lead inhibition of polymer degradation was also observed in some batch experiments. The high mobility and relatively low biodegradability of bacterial polymers demonstrated by this work indicate that such polymers deserve further evaluation as facilitated-transport carriers of metals in contaminated groundwater.