Investigation into remediation of contaminated soil containing high sulphate and heavy metals concentration
This study involved the investigation of a contaminated soil problem in Gateshead, UK. The site was previously a dumping area from industrial activities for over a hundred years and generated problems of high sulphate concentration and heavy metals in both the soil and the leachate which discharges into the River Tyne. The combination of such contaminants has not been widely investigated in the area of contaminated soil. The study was therefore divided into 2 parts, namely bioremediation of the contaminated soil and leachate treatment by reverse osmosis. The bioremediation study involved treatability tests which included slurry, microbial growth and column tests. The reverse osmosis study involved membrane fouling and leachate pre-treatment experiments. The bioremediation study stimulated the indigenous microorganisms by the addition of nutrients and carbon sources. The soil slurry and microbial growth tests determined the combination of nitrogen and phosphorus required to produce higher C02 evolution as an assessment of microbial activity. It was found in the column tests that the addition of a carbon source and appiopriate nutrient combinations resulted in a significant reduction of sulphate in both the leachate and the soil matrix. Furthermore, this was also accompanied by an increase in the microbial population in the soil matrix. It was also considered that- assimilatory sulphate reduction by microorganisms had taken place since H2S production could not be detected in the open system of the column. However, the high pH of the soil that was higher than 8 possibly caused H2S production undetected in this study. Zinc, manganesea nd copper,i n contrastw ere not reducedi n the soil matrix. Only arsenic showed significant reduction in the soil columns. Heavy metals were precipitateda nd were still presenti n high concentrationsin the leachatea nd would require further treatmenti n the liquid phase.T his was demonstratedb y the study of the use of a LPROM (Low PressureR everseO smosisM embrane)t o treat leachate from the contaminated soil. The reverse osmosis study showed that zinc and arsenic could be reduced by up to 86% and 97% respectively. Sulphate was also satisfactorily reduced up to 99%. However, the study on membrane fouling confirmed that the sulphate concentration was the main effect of fouling. Ferric chloride, aluminium sulphate, barium chloride and polyelectrolyte Zetag 92 were used for coagulation-flocculation in the pretreatment of the leachate. The study revealed that the sulphate concentration could only be reduced at the most by 43% using a FeC13, BaC12 and Zetag 92 combination. FeC13 showed better floc characteristics than alum whereas BaC12 improved sulphate removal but increased the turbidity in the supernatants. However, the use of BaC12 would significantly increase the cost of pretreatment. The study recommended a further investigation into the use of a range of readily available carbon, nitrogen and phosphorous sources in the soil column or at pilot-scale for designing a full-scale bioremediation system. Meanwhile, an investigation into other leachate pretreatment methods such as continuous microfiltration or anti-scalant addition was also suggested.