Decentralized approach toward organic pollutants removal using UV radiation in combination with H2O2-based electrochemical water technologies.

The current literature lacks a comprehensive discussion on the trade-off between pollutant degradation/mineralization and treatment time costs in utilizing UV light in combination with H2O2-based electrochemical advanced oxidation processes (EAOPs). The present study sheds light on the benefits of using the photoelectro-Fenton (PEF) process with UVA or UVC for methylparaben (MetP) degradation in real drinking water. Although light boosts the photodegradation of refractory Fe(III) complexes and the photolysis of H2O2 (with UVC only), the energy-intensive nature of light-based treatments is acknowledged. To help tackle the high energy consumption issue, a novel approach was employed: partial application of UVA or UVC light after a predetermined electro-Fenton electrolysis time. The proposed treatment approach yielded satisfactory comparable results to those obtained from the application of PEF/UVA or PEF/UVC in terms of total organic carbon removal (ca. 100%), with notably lower energy consumption (ca. 50%). The study delves into the combined method's feasibility, analyzing pollutant degradation/mineralization process and overall energy consumption. The research identifies possible degradation routes based on intermediate detection and radical quenching experiments. Finally, toxicological assessments evaluate the toxicity levels of MetP and its intermediates. The findings of this study bring meaningful contributions to the fore and point to the highly promising potential of the proposed approach, in terms of sustainability and cost-effectiveness, when applied for decentralized water treatment.

Enabling circular economy by N-recovery: Electrocatalytic reduction of nitrate with cobalt hydroxide nanocomposites on copper foam treating low conductivity groundwater effluents.

Fertilizers play a vital role in the food-energy-water nexus. The traditional method of artificial nitrogen fixation to produce ammonia is a high-energy intensive centralized process that has caused an imbalance of the N-cycle due to the release of N-species to water. Electrocatalytic nitrate reduction (ENR) to ammonia is a promising N-resource recovery alternative that can enable the circular reuse of ammonia in decentralized settings. However, the primary challenge is identifying selective and affordable electrocatalysts. Identifying electrodes that rely on something other than platinum-group metals is required to surpass barriers associated with using expensive and endangered elements. In this study, an earth-abundant bimetallic catalyst, Cu/Co(OH)x, prepared and optimized by electrodeposition, demonstrates superior ammonia production. Under environmentally relevant conditions of 30 mg NO3--N L-1, Cu/Co(OH)x showed higher ammonia production than pristine Cu foam with 0.7 and 0.3 mmol NH3 gcat-1 h-1, respectively. The experimental evaluation demonstrated direct reduction and catalytic hydrogenation mechanisms in Cu/Co(OH)x sites. Leaching analyses suggest that Cu/Co(OH)x has outstanding stability with negligible metal concentration below the maximum contaminant level for both Cu and Co. These results provide a framework for using earth-abundant materials in ENR with comparable efficiency and energy consumption to platinum-group materials.

Overcoming barriers for nitrate electrochemical reduction: By-passing water hardness.

Water matrix composition impacts water treatment performance. However, matrix composition impacts have rarely been studied for electrochemical water treatment processes, and the correlation between the composition and the treatment efficiency is lacking. This work evaluated the electrochemical reduction of nitrate (ERN) using different complex water matrices: groundwater, brackish water, and reverse osmosis (RO) concentrate/brine. The ERN was conducted using a tin (Sn) cathode because of the high selectivity towards nitrogen evolution reported for Sn electrocatalysts. The co-existence of calcium (Ca2+), magnesium (Mg2+), and carbonate (CO32-) ions in water caused a 4-fold decrease in the nitrate conversion into innocuous nitrogen gas due to inorganic scaling formation on the cathode surface. XRF and XRD analysis of fouled catalyst surfaces detected brucite (Mg(OH)2), calcite (CaCO3), and dolomite (CaMg(CO3)2) mineral scales formed on the cathode surface. Surface scaling created a physical barrier on the electrode that decreased the ERN efficiency. Identifying these main sources of ERN inhibition was key to devising potential fouling mitigation strategies. For this reason, the chemical softening pre-treatment of a real brackish water was conducted and this significantly increased nitrate conversion and faradaic efficiency during subsequent ERN treatment, leading to a lower electric energy consumption per order. Understanding the ionic foulant composition responsible for influencing electrochemically-driven technologies are the first steps that must be taken to move towards niche applications such as decentralized ERN. Thus, we propose either direct ERN implementation in regions facing high nitrate levels in soft waters, or a hybrid softening/nitrate removal system for those regions where high nitrate and high-water hardness appear simultaneously.

Highly porous seeding-free boron-doped ultrananocrystalline diamond used as high-performance anode for electrochemical removal of carbaryl from water.

Boron-doped diamond (BDD) electrodes are regarded as the most promising catalytic materials that are highly efficient and suitable for application in advanced electrochemical oxidation processes targeted at the removal of recalcitrant contaminants in different water matrices. Improving the synthesis of these electrodes through the enhancement of their morphology, structure and stability has become the goal of the material scientists. The present work reports the use of an ultranano-diamond electrode with a highly porous structure (B-UNCDWS/TDNT/Ti) for the treatment of water containing carbaryl. The application of the proposed electrode at current density of 75 mA cm-2 led to the complete removal of the pollutant (carbaryl) from the synthetic medium in 30 min of electrolysis with an electric energy per order of 4.01 kWh m-3 order-1. The results obtained from the time-course analysis of the carboxylic acids and nitrogen-based ions present in the solution showed that the concentrations of nitrogen-based ions were within the established maximum levels for human consumption. Under optimal operating conditions, the proposed electrode was successfully employed for the complete removal of carbaryl in real water. Thus, the findings of this study show that the unique, easy-to-prepare BDD-based electrode proposed in this study is a highly efficient tool which has excellent application potential for the removal of recalcitrant pollutants in water.

Treatment of an azo dye effluent by peroxi-coagulation and its comparison to traditional electrochemical advanced processes.

Peroxi-coagulation (PC) is an interesting new process that has not been widely studied in the literature. This work presents the application of this technology to treat an azo dye synthetic effluent, studying the effect of different parameters including initial pH, current density (j), initial dye concentration and supporting electrolyte. The two former variables significantly affected the colour removal of the wastewater, followed by the initial dye concentration and the kind of electrolyte, in a lesser extent. The optimum operating conditions achieved were initial pH of 3.0, j = 33.3 mA cm-2, 100 mg L-1 of methyl orange (MO) and Na2SO4 as supporting electrolyte. The performance of PC was also compared to other electrochemical advanced processes, under similar experimental conditions. Results indicate that the kinetic decay of the MO increases in the following order: electrocoagulation (EC) < electrochemical oxidation (EO) with electrogenerated H2O2 << PC < electro-Fenton (EF). This behaviour is given to the high oxidant character of the homogenous OH radicals generated by EF and PC approaches. The EO process with production of H2O2 (EO-H2O2) is limited by mass transport and the EC, as a separation method, takes longer times to achieve similar removal results. Energy requirements about 0.06 kWh gCOD-1, 0.09 kWh gCOD-1, 0.7 kWh gCOD-1 and 0.1 kWh gCOD-1 were achieved for PC, EF, EO-H2O2 and EC, respectively. Degradation intermediates were monitored and carboxylic acids were detected for PC and EF processes, being rapidly removed by the former technology. PC emerges as a promising and competitive alternative for wastewaters depollution, among other oxidative approaches.

Phenolic wastewaters depuration by electrochemical oxidation process using Ti/IrO2 anodes.

The electrochemical oxidation (EO) of phenolic wastewaters mimicking olive oil mill effluents was carried out in a batch stirring reactor using Ti/IrO2 anodes, varying the nature (NaCl and Na2SO4) and electrolyte concentration (1.8-20 g L-1), current density (57-119 mA cm-2) and initial pH (3.4-9). Phenolic content (TPh) and chemical oxygen demand (COD) removals were monitored as a function of applied charge and over time. The nature of the electrolyte greatly affected the efficiency of the system, followed by the influence of the current density. The NaCl concentration and the initial pH influenced the process in a lesser extent. The best operating conditions achieved were 10 g L-1 of NaCl, current density of 119 mA cm-2 and initial pH of 3.4. These parameters led to 100 and 84.8% of TPh and COD removal, respectively. Under these conditions, some morphological differences were observed by SEM on the surface of the anode after treatment. To study the potential toxicity of the synthetic effluent in neuronal activity, this mixture was applied to rat brain slices prior to and after EO. The results indicate that although the treated effluent causes a smaller depression of the neuronal reactive oxygen species (ROS) signal than the untreated one, it leads to a potentiation instead of recovery, upon washout. Furthermore, the purification of a real olive mill wastewater (OMW), with the organic load of the synthetic effluent, using the same optimised operating conditions, achieved total phenolic compounds abatement and 62.8% of COD removal.This study demonstrates the applicability of this EO as a pre-treatment process of a real effluent, in order to achieve the legal limit values to be discharged into natural streams regarding its organic load.

Treatment of a simulated phenolic effluent by heterogeneous catalytic ozonation using Pt/Al2O3.

Non-catalytic and catalytic ozonation over Pt/Al2O3 were considered in the treatment of a synthetic effluent composed of six phenolic acids usually present in olive mill wastewaters. In both processes the medium pH affected the rate of ozone decomposition and the formation of hydroxyl radicals. The optimum values were achieved for the catalytic system under pH 7 with 93.0 and 47.7%, respectively, of total phenol content and chemical oxygen demand (COD) removal, after 120 minutes of reaction. For pH 3, the catalytic ozonation followed a free radical pathway perceived by the presence of radical scavengers. No significant structural differences were observed between the fresh and used solid catalyst in X-ray diffraction analysis. Aluminium leaching behaviour was also evaluated at the end of each experiment. Moreover, a sequence of feed-batch trials involving the catalyst reutilization exhibited almost constant activity during the operation time. Eco-toxicological tests were performed for both processes, revealing that the treated effluent still presents some ecological impact, although it is lower than that for the raw wastewater.