Enzymatic recovery of platinum (IV) from industrial wastewater using a biosulphidogenic hydrogenase

Abstract

It has been established that dissolved heavy metals escaping into the environment pose a serious health hazard. As a result, there is an urgent need for controlling metal emissions into the environment. The aims of this study were to purify and biochemically characterise hydrogenase(s) from sulphate reducing consortium (SRB) and investigate the potential of the purified enzyme(s) in the recovery of platinum from wastewaters. A hydrogenase from sulphate reducing consortium was purified by a combination of PEG 20000 concentration, ion exchange (Toyopearl-Super Q 650 S) and size exclusion (Sephacryl S-200) chromatographies. SDS-PAGE analysis revealed a distinct protein band with a molecular mass of 58 kDa. The investigation of enzymatic platinum (IV) reduction in vitro, showed highest hydrogen-dependent platinum (IV) reducing activity in the presence of hydrogenase and its physiological electron carrier, cytochrome c3. When the purified hydrogenase enzyme (with and without cytochrome c3) was used with the industrial effluent, containing 7.9 mg.l-1 platinum, only 10 – 15% recovery was noted pointing to a suppression of enzyme activity due to the low pH (0.38) of the effluent. Bioremediation studies on industrial effluent using resting SRB cells showed a 34% platinum removal from the effluent while growing SRB cells, within a sulphidogenic reactor, gave a platinum removal of 78%, with the pH of the system fluctuating at around 5.6. Evidence of sulphate reduction and sulphide generation were not observed during this treatment process suggesting that platinum sulphide was not formed and supporting the argument that the increased amount (78%) of platinum removal from the industrial wastewater by the growing SRB cells was due to more hydrogenase/cytochrome c3 enzyme complex being available.