Studies on electrochemical treatment of wastewater contaminated with organotin compounds

Different anode materials were tested to evaluate their suitability to eliminate organotin compounds electrochemically from shipyard process waters. The capacity of two types of anode materials was investigated: niobium coated with boron-doped diamond (BDD) and titanium coated with iridium dioxide, (Ti/IrO2). The aim of this work was to characterize the performance of the process using both anode materials at different current densities, and also to evaluate the generation of by-products. A further objective of this work was to evaluate if operating at low potentials with BDD anodes (to avoid the generation of elemental oxygen) the consumption of energy for degradation of pollutants could be minimized. The processes were tested on synthetic and real shipyard water containing approximately 25,000 ng L−1 of tributyltin (TBT) (as Sn) and 5000 ng L−1 dibutyltin. The range of current densities was between 6 and 70 mA cm−2. The results showed that electrochemical treatment is suitable to eliminate organotins down to very low concentrations following a stepwise debutylation mechanism. Both anode materials exhibited a similar performance with energy consumption in the range of 7-10 kWh m−3 in order to decrease organotins down to 100 ng L−1 (as Sn). For the water composition tested, BDD did not outperform Ti/IrO2 as initially expected, most probably because organotins were not only oxidized by OH, but also by active chlorine compounds generated by the oxidation of chloride present in the wastewater (1.6 g L−1, Cl−) with both anode materials. It was also found that the residual oxidants remaining in the treated effluent had to be eliminated if the water is to be discharged safely in the aquatic environment.