Chlorate induced false reduction in chemical oxygen demand (COD) based on standard dichromate method: Countermeasure and mechanism

Combined pollution of heavy metals and polycyclic aromatic hydrocarbons in non-ferrous metal smelting wastewater treatment plant: Distribution profiles, removal efficiency, and ecological risks to receiving river

Combined pollution status of heavy metals (HMs) and polycyclic aromatic hydrocarbons (PAHs) from non-ferrous metal smelting (NFMS) industry is crucial but has not been explored. Herein, the co-distribution of HMs and PAHs in a NFMS wastewater treatment plant and the impacts on the receiving river were investigated. Cu, As, and Ni were found to be the characteristic HMs, while Acenaphthylene was the characteristic PAHs in the NFMS wastewater. The removal of HMs and PAHs in wastewater showed a strong positive correlation (R2 > 0.84, p < 0.05) with removal efficiency of 90.7 % and 94.1 %, respectively. It was estimated 547.5 kg HMs and 13.3 kg PAHs were discharged into the receiving river annually. The average concentration of HMs and PAHs in downstream was respective 1.6 and 2.7 times higher than that in upstream, and the sites near discharge outlet had significant spatial autocorrelation (p < 0.05), suggesting the discharge of NFMS wastewater had significantly influenced the receiving river. Aquatic organisms were posed to moderate chronic ecological risk (RQC > 0.1) and surrounding residents were posed to probable carcinogenic risk (TCR > 10-5). This work provides new insights into understanding the combined pollution and corresponding ecological risks from key industrial sectors globally.

New insight of electrogenerated H2O2 into oxychlorides inhibition and decontamination promotion: From radical to nonradical pathway during anodic oxidation of high Cl--laden wastewater process

Anodic oxidation (AO) has been extensively hailed as a robust and promising technology for pollutant degradation, but the parasitic formation of oxychlorides (ClOx-) would induce a seriously over-evaluated electrochemical COD removal performance and dramatical biotoxicity increasement of the AO-treated Cl--laden effluents. Herein, we shed new light on the roles of H2O2 high-efficiently electrogenerated in three-dimensional (3D) reactor in inhibiting ClOx- production and promoting pollutant degradation, which has been overlooked in previous literature. Total yield of ClOx- in phenol simulated wastewater containing 30 mM Cl- was dropped from 25 mM and 24.3 mM to only 0.26 mM and 0.23 mM within 120 min after treating by 3D H2O2-involing systems with Ti/Ru-IrO2 and BDD anode, respectively. Meanwhile, the COD removal of 3D Ti/Ru-IrO2-based system was increased by 57 % (85 % removal at 0.011 kWh g-1 COD), comparable to that of 3D BDD-based system (90 % removal at 0.008 kWh g-1 COD), the energy consumption of which were far less than those of conventional 2D and 3D electro-Fenton systems (0.08-0.2 kWh g-1 COD). During degradation process of Cl--bearing phenol by 3D AO-H2O2 systems, the anodically produced species (Cl•, Cl2•-, ClO-) were rapidly quenched by the in-situ electrogenerated H2O2 and then successfully transformed into 1O2. The radical pathway of reaction between H2O2 and Cl•/Cl2•- had a more obviously thermodynamical advantage (∆G = 11.5 kJ mol-1) than nonradical pathway between H2O2 and ClO- (∆G = 171 kJ mol-1) based on DFT analysis. And the steady-state concentration of 1O2 was 8.8 × 10-9 M and 4.2 × 10-10 M in 3D Ti/Ru-IrO2 and BDD-based system, respectively, which collectively took responsibility for the termination of ClOx- production and promotion of organic pollutant degradation. This work provides a technical feasibility in the practical utilization of AO technology to wastewater treatment without toxic oxychloride by-products.

Combination of electro-oxidation and biological processes for lindane landfill leachate treatment: simultaneous degradation of contaminants and biological reduction of electro-generated chloride-derived by-products

Real lindane landfill leachate (HCH-LL) is characterised by high chlorinated organic compounds concentrations (primarily hexachlorocyclohexane (HCH) isomers and degradation products generated during more than 40 years of ageing), posing environmental and human health risks. In this work, the co-treatment of real HCH-LL (pre-treated via electro-oxidation (EO)) and urban wastewater using an activated sludge process operated in an anoxic/oxic sequencing batch (A/O-SBR) mode was investigated. EO tests were conducted employing either a boron-doped diamond (BDD) anode or a dimensionally stable anode (DSA), resulting in effective HCH isomers removal (>93 % after 20 Ah/L). Chloride-derived by-products (CDBPs) such as free chlorine (up to 828 mg Cl2/L), chlorate (up to 972 mg/L) (formed with EO (DSA)) and perchlorate (up to 1830 mg/L) (with EO (BDD)) persisted after the treatment. EO (DSA) resulted in inhibitory effects (up to 100 % respiration inhibition) on the biological process. Conversely, EO (BDD) negligibly affected biological respiration (up to 20 % less than without pre-treatment), while perchlorate bio-reduction by A/O-SBR was poor (28 %). Acetate addition in pre-treated HCH-LL for perchlorate bio-reduction allowed to achieve simultaneous contaminants removal (> 99 %) and CDBPs reduction (up to 100 %). Biodegradation and bio-adsorption tests without pre-treatment showed partial HCH isomers removal (about 40 %) and poor bio-adsorption.

Improved boosting and self-attention RBF networks for COD prediction based on UV-vis

Chemical Oxygen Demand (COD) is crucial for assessing water quality. Compared to traditional chemical detection methods, UV-vis spectroscopy for measuring COD offers advantages such as speed, reduced consumption of materials, and no secondary pollution. Considering the impact of suspended particles in water, this paper proposes an optimized boosting model based on a combination strategy for turbidity compensation, using absorption spectra obtained from reservoir water samples via UV-vis. A self-attention mechanism is introduced into the radial basis function (RBF) network, resulting in a COD detection model based on the saRBF framework. This model facilitates comprehensive optimization of the entire process, from turbidity compensation of the original absorption spectrum to the subsequent COD prediction. Experimental results show that the proposed COD measurement model achieves a coefficient of determination of 0.9267, a root mean square error of 1.2669, and a mean absolute error of 1.0097, outperforming other COD measurement models. This work provides a new approach for turbidity compensation and COD detection research.

Simultaneous Determination of Chemical Oxygen Demand, Total Nitrogen, Ammonia, and Phosphate in Surface Water Based on a Multielectrode System

A technique for monitoring chemical oxygen demand (COD), total nitrogen (TN), ammonia (N-NH4), and phosphate (P-PO4) in surface water with a targeted signal multielectrode system (Cu, Ir, Rh, Co(OH)2, and Zr(OH)4 electrodes) is proposed for the first time. Each water quality index is specifically detected by at least two electrodes with distinct selectivity sensing mechanisms. Cyclic voltammetry and electrochemical impedance measurements are employed for multidimensional signal acquisition, complemented by normalization and Least Absolute Shrinkage and Selection Operator (LASSO) for principal feature extraction and dimension reduction. Multiple linear regression (MLR), partial least-squares (PLS), and eXtreme Gradient Boosting (XGBoost) were employed to evaluate the established prediction model. The precisions of the multielectrode system are ±10%/±5 ppm of COD, ±10%/±0.2 ppm of TN, ±5%/±0.1 ppm of N-NH4, and ±5%/±0.01 ppm of P-PO4. The analysis time of the multielectrode system is reduced from hours to minutes compared with traditional analysis, without any sample pretreatment, facilitating continuous online monitoring in the field. The developed multielectrode system offers a feasible strategy for online in situ monitoring of surface water quality.

Differentiating the reaction mechanism of three-dimensionally electrocatalytic system packed with different particle electrodes: Electro-oxidation versus electro-fenton

Recently, there are still some controversial mechanisms of the 3D electrocatalytic oxidation system, which would probably confound its industrial application. From the conventional viewpoint, the Ti₄O₇ material may be the desired particle electrodes in the 3D system since its high oxygen evolution potential favors the production of ^•OH via H₂O splitting reaction at the anode side of Ti₄O₇ particle electrodes. In fact, the incorporation of Ti₄O₇ particles showed phenol degradation of 88% and COD removal of 51% within 120 min, under the optimum conditions at energy consumption of 0.668 kWh g^-1 COD, the performance of which was much lower than those in many previous literatures. In contrast, the prepared carbon black-polytetrafluoroethylene composite (CB-PTFE) particles with abundant oxygen-containing functional groups could yield considerable amounts of H₂O₂ (200 mg L^-1) in the 3D reactor and achieved a complete degradation of phenol and COD removal of 80% in the presence of Fe²⁺, accompanying a low energy consumption of only 0.080 kWh g^-1 COD. It was estimated that only 20% of Ti₄O₇ particles near the anode attained the potential over 2.73 V/SCE at 30 mA cm^-2 based on the potential test and simulation, responsible for the low yield of ^•OH via the H₂O splitting on Ti₄O₇ (1.74 × 10^-14 M), and the main role of Ti₄O₇ particle electrodes in phenol degradation was through direct oxidation. For the CB-PTFE-based 3D system, current density of 10 mA cm^-2 was sufficient for all the CB-PTFE particles to attain cathodic potential of -0.67 V/SCE, conducive to the high yield of H₂O₂ and ^•OH (9.11 × 10^-14 M) in the presence of Fe²⁺, and the ^•OH-mediated indirect oxidation was mainly responsible for the phenol degradation. Generally, this study can provide a deep insight into the 3D electrocatalytic oxidation technology and help to develop the high-efficiency and cost-efficient 3D technologies for industrial application.

Electrogenerated oxychlorides induced overlooked negative effects on electro-oxidation wastewater treatment in terms of over-evaluated COD removal efficiency and biotoxicity

The high-efficiency and environmentally-friendly electro-oxidation (EO) would lose its competitive edge because of the production of oxychloride by-products (ClO_x^-), which has not yet drawn significant attention in academic and engineering communities. In this study, the negative effects of the electrogenerated ClO_x^- were compared among four commonly used anode materials (BDD, Ti₄O₇, PbO₂ and Ru-IrO₂) in terms of ClO_x^- interference on the evaluation of electrochemical COD removal performance and biotoxicity. Apparently, the COD removal performance of various EO systems were highly enhanced with increasing current density in the presence of Cl^-, e.g., the amounts of COD removed by various EO systems from the phenol solution with an initial COD content of 280 mg L^-1 at 40 mA cm^-2 within 120 min decreased in the order: Ti₄O₇ of 265 mg L^-1 > BDD of 257 mg L^-1 > PbO₂ of 202 mg L^-1 > Ru-IrO₂ of 118 mg L^-1, which was different from the case with the absence of Cl^- (BDD of 200 mg L^-1 > Ti₄O₇ of 112 mg L^-1 > PbO₂ of ¹08 mg L^-1 > Ru-IrO₂ of 80 mg L^-1) and the results after removing ClO_x^- by anoxic sulfite-based method (BDD of 205 mg L^-1 > Ti₄O₇ of 160 mg L^-1 > PbO₂ of 153 mg L^-1 > Ru-IrO₂ of 99 mg L^-1). These results can be ascribed to the ClO_x^- interference on COD evaluation, the extent of which decreased in the order: ClO₃^- > ClO^- (where ClO₄^- cannot impact COD test). The highest overrated electrochemical COD removal performance of Ti₄O₇ may be associated with its relatively high production of ClO₃^- and the low mineralization extent. The chlorella inhibition ratio of ClO_x^- decreased in the order: ClO^- > ClO₃^- >> ClO₄^-, which accounted for the biotoxicity increasement of the treated water (PbO₂ 68%, Ti₄O₇ 56%^, BDD 53%, Ru-IrO₂ 25%). Generally, the inevitable problems of overrated electrochemical COD removal performance and biotoxicity increasement induced by ClO_x^- should deserve significant attention and effective countermeasures should be also developed when employing EO process for wastewater treatment.

Activated carbon enhanced traditional activated sludge process for chemical explosion accident wastewater treatment

With the development of industries, explosion accidents occur frequently during production, transportation, usage and storage of hazard chemicals. It remained challenging to efficiently treat the resultant wastewater. As an enhancement of traditional process, the activated carbon-activated sludge (AC-AS) process has a promising potential in treating wastewater with high concentrations of toxic compounds, chemical oxygen demand (COD) and ammonia nitrogen (NH₄⁺-N), etc. In this paper, activated carbon (AC), activated sludge (AS) and AC-AS were used to treat the wastewater produced from an explosion accident in the Xiangshui Chemical Industrial Park. The removal efficiency was assessed by the removal performances of COD, dissolved organic carbon (DOC), NH₄⁺-N, aniline and nitrobenzene. Increased removal efficiency and shortened treatment time were achieved in the AC-AS system. To achieve the same COD, DOC and aniline removal (90%), the AC-AS system saved 30, 38 and 58 h compared with the AS system, respectively. The enhancement mechanism of AC on the AS was explored by metagenomic analysis and three-dimensional excitation-emission-matrix spectra (3DEEMs). More organics, especially aromatic substances were removed in the AC-AS system. These results showed that the addition of AC promoted the microbial activity in pollutant degradation. Bacteria, such as Pyrinomonas, Acidobacteria and Nitrospira and genes, such as hao, pmoA-amoA, pmoB-amoB and pmoC-amoC, were found in the AC-AS reactor, which might have played important roles in the degradation of pollutants. To sum up, AC might have enhanced the growth of aerobic bacteria which further improved the removal efficiency via the combined effects of adsorption and biodegradation. The successful treatment of Xiangshui accident wastewater using the AC-AS demonstrated the potential universal characteristics of the process for the treatment of wastewater with high concentration of organic matter and toxicity. This study is expected to provide reference and guidance for the treatment of similar accident wastewaters.