Heterojunction Ni-Sb-SnO2 Anode for Oxidant Generation and Electrochemical Water Treatment
Authors: Zhang Yi , Yang Yang , Hoffmann Michael R. 
Published: December 2020

Clean water supply is critical for public health and food production. Currently, about 2.7 billion people around the world lack access to clean water for daily activities. Inadequate sanitation and resulting water-borne diseases lead to millions of deaths every year. While centralized wastewater treatment facilities are not practical in many parts of the developing world, small-scale decentralized treatment represents an attractive alternative that can provide necessary water treatment for reuse. Among various techniques, electrochemical oxidation (EO) has been demonstrated to be particularly suitable for the purpose of decentralized onsite treatment.

Performance of EO process depends greatly on the in-situ generation of reactive species, which is determined by the nature of anode materials. Desirable anodes should be efficient in generating reactive species and inert in producing oxygen (O2). Common electrodes possessing these features, such as boron-doped diamond (BDD), antimony-doped tin oxide (Sb-SnO2), and lead oxide (PbO2), have been studied extensively. Among them, Sb-SnO2 is most applicable for onsite treatment purpose since it has lower manufacture cost comparing to BDD and less toxic byproduct generation comparing to PbO2. A modification of Sb-SnO2 by adding nickel as an additional dopant (Ni-Sb-SnO2) has been reported for its ozone generation capacity, which is desirable since O3 represents a powerful and more environmentally-friendly oxidant comparing to free chlorine due to cleaner transformation byproducts.

While many studies have tried to examine these two types of coatings separately and possible mechanisms for ozone generation have been proposed, none had studied the changes in reactive species generation or reaction kinetics in various electrolytes when ozone is present in an electrochemical system. In this study, a new double-layer anode (NAT/AT) that consists of Sb-SnO2 bottom layer (AT) coated on a titanium plate and Ni-Sb-SnO2 top layer (NAT) was designed and prepared. The Sb-SnO2 bottom layer is expected to act as ohmic contact to enhance electron transfer from the top layer to the titanium base, while O3 generation can be achieved by Ni-Sb-SnO2 top layer. Moreover, having Ni-Sb-SnO2 as the top layer offers the advantage in keeping the amount of Ni and Sb low to control leaching.

The double-layer NAT/AT was characterized and its performance in reactive species (free chlorine, ozone, hydroxy radicals) was tested against the respective single-layer AT and NAT electrodes. Kinetic modeling was invoked to elucidate the mechanisms behind reactive species generation and quantify contributions to target compound removal from each species. To evaluate its applicability in actual treatment, NAT/AT was further tested in treating real latrine wastewater, during which removal of emerging organic contaminants and pathogens was monitored.

Significant O3 production was detected at NAT/AT and NAT electrodes, confirming the role of nickel in O3 generation. Meanwhile, addition of nickel leads to lower chlorine evolution activity at NAT/AT and NAT comparing to AT, which likely results from a shift of reactive species. Generation of hydroxyl radicals (HO) was inspected using benzoic acid (BA) as a probe compound. Significant degradation of 1 mM BA in 30 mM NaClO4 was observed with all AT, NAT/AT, and NAT electrodes within 60-90 min electrolysis. Among the three, NAT/AT has the highest BA removal rate (NAT/AT > NAT > AT), which could be explained by transformation of aqueous O3 to HO as well as higher contribution from direct electron transfer.

When chloride is present (NaCl electrolyte used), we found that O3 generation was reduced since adsorbed chloride on the active sites could inhibit the recombination of adsorbed oxygen to give O3. With chloride, BA degradation was also slower due to the homogeneous consumption of HO by Cl- and electrogenerated free chlorine species. The results suggest that, unlike many commercially available anodes, addition of chloride is not required for electrochemical treatment processes using NAT/AT. For the above processes, rates for reactive species (O3, HO, Cl, Cl2) generation were estimated by fitting the kinetic model to experimental data. Target compound degradation in various scenarios could be successfully precited by the model.

In treating real human wastewater. Rapid COD removal was observed (around or below 100 mg/L) within 75 min, a lot shorter timescale comparing to commercially available electrodes. Significant removal of all spiked pharmaceutical compounds (~80%) was achieved in the same time period.

In our study, ozone chemistry at the microenvironment of electrode/electrolyte interface was investigated and important mechanistic insights were revealed. The double-layer designed NAT/AT anode has demonstrated high reactivity for O3 and HO generation as well as capability in wastewater treatment. While further testing and optimization are required, NAT/AT possesses great potential for application in decentralized wastewater treatment.

Figure 1