Electrochemical Transformation of Trace Organic Contaminants in Latrine Wastewater

Abstract

Solar-powered electrochemical systems have shown promise for onsite wastewater treatment in regions where basic infrastructure for conventional wastewater treatment is not available. To assess the applicability of these systems for trace organic contaminant treatment, test compound electrolysis rate constants were measured in authentic latrine wastewater using mixed-metal oxide anodes coupled with stainless steel cathodes. Complete removal of ranitidine and cimetidine was achieved within 30 min of electrolysis at an applied potential of 3.5 V (0.7 A L −1 ). Removal of acetaminophen, ciprofloxacin, trimethoprim, propranolol, and carbamazepine (>80%) was achieved within 3 h of electrolysis. Oxidation of ranitidine, cimetidine, and ciprofloxacin was primarily attributed to reaction with NH 2 Cl. Transformation of trimethoprim, propranolol, and carbamazepine was attributed to direct electron transfer and to reactions with surface-bound reactive chlorine species. Relative contributions of aqueous phase •OH, •Cl, •Cl 2 − , HOCl/OCl − , and Cl 2 were determined to be negligible based on measured second-order reaction rate constants, probe compound reaction rates, and experiments in buffered Cl − solutions. Electrical energy per order of removal (E EO ) increased with increasing applied potentials and current densities. Test compound removal was most efficient at elevated Cl − concentrations present when treated wastewater is recycled for use as flushing water (i.e., ∼ 75 mM Cl − ; E EO = 0.2−6.9 kWh log −1 m −3 ). Identified halogenated and oxygenated electrolysis products typically underwent further transformations to unidentifiable products within the 3 h treatment cycle. Identifiable halogenated byproduct formation and accumulation was minimized during electrolysis of wastewater containing 75 mM Cl − .

Toxic Byproduct Formation during Electrochemical Treatment of Latrine Wastewater

“…5 Further, disinfection and color removal are completed prior to ammonium removal. 3,19 COD removal is typically more energy efficient before the breakpoint, 19 although achieving complete COD removal may require pretreatment or longer treatment times (Table SI 3).…”

“…Higher chloride concentrations may enhance electrolysis efficiency due to increased reactive chlorine species concentrations 5 but may also be expected to increase chlorinated byproduct formation.…”

“…Bromate also may be produced on BDD anodes, 20 although formation will be limited by the low bromide concentrations typical of latrine wastewater (i.e., ∼5 μM; maximum of ∼60 times WHO and EPA guidelines). 5 …”

“…8 Electrochemical bromate production 20 is limited by the low bromide concentrations typical of latrine wastewater. 5 …”

Nature, Mechanisms and Reactivity of Electrogenerated Reactive Species at Thin‐Film Boron‐Doped Diamond (BDD) Electrodes During Electrochemical Wastewater Treatment

“…[2,13,17] This behavior is attributed to the formation of highly refractory organochlorinated byproducts which are very difficult to mineralize. [70] Another drawback of the presence of Cl À ions in wastewater during electrooxidation is the high risk of production of highly toxic chlorinated amines, [111,112] trihalomethanes (THMs) and haloacetic acids (HAAs) [102,[113][114][115][116][117] as well as hazardous perchlorate from the oxidation of organic pollutants by electrogenerated RCS. [70,88] However, at higher Cl À concentrations, lower mineralization rate is always observed due to the formation of larger quantity of recalcitrant organochlorinated byproducts that are highly resistant to the oxidation by electrogenerated BDD( * OH).…”