First Essays for Cadmium Wastewater Elimination by Sulphate-Reducing Bacteria

Abstract

Sulphate-reducing bacteria (SRB) are anaerobic bacteria that couple the oxidation of simple organic compounds to the reduction of sulphate to sulphide. SRB have become important components of bioremediation systems. In this work, a natural SRB sludge was collected in the coastal mud flat of Colindres (Cantabria, Spain) and the removal of soluble cadmium as cadmium sulphide from a synthetic feeding was studied. Discontinuous experiments were performed at room temperature, using four 600 ml reactors. In stage 1, a 250 mL biomass solution (18.16 g/L of volatile matter) was introduced in each reactor and was fed with a solution of cadmium sulphate, sodium sulphate and sucrose as carbon and energy source. The Chemical Oxygen Demand (COD) to SO4-2 ratio (this SO4-2 coming from Na2SO4) was 10 in all the stages of the four reactors, and the different cadmium content tested in each reactor varied from 4 to 12 mg/L. Stages 2, 3, 4 and 5 were performed with the same biomass but with higher initial cadmium amount, from 16 to 300 mg/L, in order to determine the amount of metal causing inhibition of the bacterial activity. Soluble cadmium was analyzed by Atomic Absorption Spectrometry. Additionally in stage 4, the evolution of pH and COD was studied. Results indicated that inhibition of biomass was not found up to a Cd concentration of 300 mg/L. Continuous experiments were performed in an UASB reactor of 1 litter of useful capacity, being the hydraulic residence time 0.833 days and 720 mg of COD/day the organic matter in the influent. After a period of acclimation, biomass was fed with increasing amounts of soluble Cadmium and the specific rate of cadmium elimination was determined.