Conventional Water Treatment Processes Research Articles

Occurrence, efficiency of treatment processes, source apportionment and human health risk assessment of pharmaceuticals and xenoestrogen compounds in tap water from some Ghanaian communities

The occurrence of pharmaceuticals and xenoestrogen compounds (PXCs) in drinking water presents a dire human health risk challenge. The problem stems from the high anthropogenic pollution load on source water and the inefficiencies of the conventional water treatment plants in treating PXCs. This study assessed the PXCs levels and the consequential health risks of exposure to tap water from selected Ghanaian communities as well as that of raw water samples from the respective treatment plants. Thus the PXCs treatment efficiency of two drinking water treatment plants in the metropolises studied was also assessed. The study also conducted source apportionment of the PXCs in the tap water. Twenty six (26) tap and raw water samples from communities in the Cape Coast and Sekondi-Takoradi metropolises were extracted using SPE cartridges and analysed for PXCs using Ultra-fast-HPLC-UV instrument. Elevated levels of PXCs up to 24.79 and 22.02 μg/L were respectively recorded in raw and tap water samples from the metropolises. Consequently, elevated non-cancer health risk (HI > 1) to residential adults were found for tap water samples from Cape Coast metropolis and also for some samples from Sekondi-Takoradi metropolis. Again, elevated cumulative oral cancer risks >10−5 and dermal cancer risk up to 4 × 10−5 were recorded. The source apportionment revealed three significant sources of PXCs in tap water samples studied. The results revealed the inefficiency of the treatment plants in removing PXCs from the raw water during treatments. The situation thus requires urgent attention to ameliorate it, safeguarding public health. It is recommended that the conventional water treatment process employed be augmented with advanced treatment technologies to improve their efficacy in PXCs treatment.

The effectiveness and feasibility of ball-milled powdered activated carbon (BPAC) for removal of organic pesticides in conventional drinking water treatment process

In the conventional drinking water treatment process (CDWTP), powdered activated carbon (PAC) is commonly used for removing organic pesticides, or other organic contaminants. However, the hydraulic retention time (HRT) in CDWTP is insufficient for fulfilling PAC adsorption equilibrium to realize its full capacity. This study examined the adsorption kinetics, adsorption thermal dynamics, and removal efficiency for six organic pesticides using the ball-milled PAC (BPAC) with varying particle sizes in CDWTP. Based on the experiments with the pesticides of atrazine, diazinon, dimethoate, fenitrothion, isoproturon and thiometon, the results indicated that as the particle size reduced from around 38 μm for the commercial PAC to 1 μm for the BPAC, the adsorption rates for hydrophobic pesticides increased up to twentyfold. Diffusional adsorption from the bulk solution to the external PAC surface is the most likely predominant mechanism. This could allow a sufficient pesticides’ adsorption within the limited HRT and to achieve a great depth removal of these toxic compounds. However, the addition of BPAC with a diameter of 1 μm was observed to significantly increase residual particles in treated water after the conventional treatment process. With a further systematic evaluation of both adsorption rate and particle penetration, a particle size of around 6 μm BPAC was considered a practical compromise between the adsorption rate and particle penetration for real application. Results from five surface waters of different water quality indicated that, compared to commercial PAC, application of 6 μm BPAC could achieve up to a 75% reduction in adsorbent dosage while maintaining around the same pesticide removal efficiencies. Additionally, thermodynamic analyses suggest that adsorption of these pesticides could be enthalpically or entropically driven depending on the degree of pesticide hydrophobicity.

Applying Recurrent Neural Networks and Blocked Cross-Validation to Model Conventional Drinking Water Treatment Processes

The jar test is the current standard method for predicting the performance of a conventional drinking water treatment (DWT) process and optimizing the coagulant dose. This test is time-consuming and requires human intervention, meaning it is infeasible for making continuous process predictions. As a potential alternative, we developed a machine learning (ML) model from historical DWT plant data that can operate continuously using real-time sensor data without human intervention for predicting clarified water turbidity 15 min in advance. We evaluated three types of models: multilayer perceptron (MLP), the long short-term memory (LSTM) recurrent neural network (RNN), and the gated recurrent unit (GRU) RNN. We also employed two training methodologies: the commonly used holdout method and the theoretically correct blocked cross-validation (BCV) method. We found that the RNN with GRU was the best model type overall and achieved a mean absolute error on an independent production set of as low as 0.044 NTU. We further found that models trained using BCV typically achieve errors equal to or lower than their counterparts trained using holdout. These results suggest that RNNs trained using BCV are superior for the development of ML models for DWT processes compared to those reported in earlier literature.

Is monochloramine pre-oxidation a viable strategy for enhancing the treatment efficiency of algae-laden water with conventional drinking water treatment process?

Algal blooms worldwide pose many challenges to drinking water production. Pre-oxidation with NaClO, KMnO4, or ozone is commonly used to enhance algal removal in conventional drinking water treatment processes. However, these currently utilized oxidation methods often result in significant algal cell lysis or impede the operation of the subsequent units. Higher algal removal with pre-chlorination in algal solutions prepared with natural water, compared to those prepared with ultrapure water, has been observed. In the present studies, preliminary findings indicate that ammonium in natural water alters chlorine species to NH2Cl, leading to improved treatment efficiency. NH2Cl with 1.5–3.0 mg∙L−1 as Cl2 with an oxidation time of 3–7 h significantly enhancing algal removal by coagulation. The selective oxidation of surface-absorbed organic matter (S-AOM) by NH2Cl, followed by the subsequent peeling off of this material from the algal surface, leading to an increase in zeta potential from −20.2 mV to −3.8 mV, constitutes the primary mechanism of enhanced algal removal through coagulation. These peeled S-AOM retained their large molecular weight and acted as polymer aids. Compared with NaClO and KMnO4, NH2Cl displays the best performance in improving algal removal, avoiding cell lysis, and decreasing the potential for nitrogenous disinfection byproducts formation under the reaction conditions used in this study. Notably, in major Chinese cities, water purification plants commonly rely on suburban lakes or reservoirs as water sources, necessitating the transportation of raw water over long distances for times up to several hours. These conditions favor the implementation of NH2Cl pre-oxidation. The collective results indicate the potential of NH2Cl oxidation as a viable pretreatment strategy for algal contamination during water treatment processes.

Development of an adsorption-enhanced heterogeneous Fenton process for abatement of natural organic matter by kaolin-supported phytogenic nanoscale zero-valent iron

Abstract Natural organic matter (NOM) serves as a precursor for the formation of carcinogenic disinfection by-products when not adequately removed by conventional water treatment processes. The degradation of NOM by heterogeneous Fenton processes is particularly attractive because it results in prospects of zero sludge discharge. This work reports on NOM degradation and adsorption via the heterogeneous Fenton process using phytogenic zero-valent iron nanoparticles (nZVI) synthesised from green tea extract (Camellia sinensis) and supported on kaolin (fK-nZVI). The synthesised material was characterised using scanning electron microscopy (SEM), Fourier-transform infrared spectroscopy (FTIR) and Brunauer-Emmett-Teller (BET) analysis. Morphological analysis of SEM images and BET values revealed an increase in the surface area from 11.64 m2 · g−1 for kaolin to 27.07 m2 · g−1 for the kaolin-supported nZVI (K-nZVI), thus presenting an ideal modification for effective adsorption. Fenton process parameters were optimised, that is, H2O2 concentration (5 mM) and pH (4.5). At equilibrium, the adsorption by the fK-nZVI system was 0.127 mg · g−1, a value higher than reported in other adsorption systems at equivalent adsorbent dosage and NOM concentration. The promising results obtained in this study indicate heterogeneous Fenton degradation and adsorption can be a viable and effective method for NOM removal from aqueous media.

A comparison of typical ultrafiltration processes in drinking water treatment: Implications for fouling control and disinfection performance

Despite the rapid proliferation of ultrafiltration (UF) membranes in drinking water treatment, the long-term operational performance of various hybrid UF processes remains inadequately understood. Herein, three typically applied hybrid UF processes were directly compared on a pilot scale, focusing on effluent quality, fouling control and subsequent disinfection performance. Water quality analysis revealed that the direct UF process exhibited significant removal of high molecular weight and hydrophobicity fractions, while the integration of UF with conventional water treatment processes further enhanced the removal of medium molecular weight and transition fractions, as well as fluorescent organic matter. The augmentation of pretreatment processes resulted in the hybrid UF process featuring enhanced anti-fouling properties, characterized by decreased reversible and irreversible resistances under the constant flux condition. This improvement was attributed to reduced interaction between the membrane and foulants. Furthermore, the subsequent disinfection performance of hybrid UF processes was evaluated using chlorine and chlorine dioxide as disinfectants. The chlorine decay rate and disinfection by-product formation indicated that chlorine dioxide was more suitable for the UF process, whereas chlorine was better for the hybrid UF process with low organic matter content in the effluent. These comprehensive comparisons offer valuable insights for making informed decisions in adopting suitable hybrid UF processes for practical water treatment applications.

Remediation of groundwater pollution using photocatalytic membrane reactors

Recalcitrant pollutants in groundwater and industrial wastewater streams are very dangerous for the environment and human health. The conventional water treatment processes are not capable to degrade/removing them due to their very high chemical stability. The photocatalytic membrane reactors represent a possible route for overcoming the limits present in the conventional ones. In this study, photocatalyst loaded mixed matrix membranes (PMMM) were prepared using non solvent induced phase inversion process and titanium dioxide as a photocatalyst. SEM and EDX analyses, contact angle, and tensile strength measurements were used for characterizing the membranes. Their catalytic activity was assessed using the methyl orange as a recalcitrant pollutant. The photoactivity of the PMM membranes increased with the TiO2 content. The membrane loaded with the 2.5 wt % of titanium dioxide achieved better degradation and retained high degradation efficiency after 4 cycles, indicating the stability of the membrane.

Removal of Algae and Algal Toxins from a Drinking Water Source Using a Two-Stage Polymeric Ultrafiltration Membrane Process.

The release of algal toxins in algae-containing water sources poses a serious threat to drinking water safety and human health. The conventional water treatment processes of water plants have a limited ability to remove algae and algal toxins, especially algal toxins with a molecular weight (MW) of less than 1000 Da. To eliminate algal pollution from a water source, a two-stage ultrafiltration (UF) process with a large polysulfone hollow fiber membrane with a MW cut-off of 200 kDa and a small aromatic polyamide roll membrane with a MW cut-off of 1 kDa were applied after a traditional sand filter in a water treatment plant. UF operation conditions, including the operating time, pressure, and membrane flux, were investigated. With an operating pressure of 0.05-0.08 MPa, the polysulfone hollow fiber membrane removed algae effectively, as the influent algal cell concentration ranged from 1-30 cells/mL but exhibited a limited removal of algal toxins. With an operating pressure of 0.3-0.4 MPa, the elimination of microcystins (MCs) reached 96.3% with the aromatic polyamide roll membrane. The operating pressure, membrane flux, and operating time were selected as the experimental factors, and the effects on the UF efficiency to remove algal toxins and biodegradable dissolved organic carbon were investigated by the response surface methodology. The model showed that the order of influence on the membrane operating efficiency was operating pressure > membrane flux > running time. The optimal UF operating conditions were an operating pressure of 0.3 MPa, a membrane flux of 17.5 L/(m2·h), and a running time of 80 min.

Efficiency of coagulation-flocculation and chlorination processes in the removal of endosulfan from water sources

ABSTRACT In this study, the efficiency of conventional drinking water treatment processes (coagulation-flocculation and chlorination) for endosulfan removal and metabolite formation were investigated. Taguchi experimental design and analysis of variance (ANOVA) were applied using Minitab program. The highest removal efficiency was obtained as 65% at 6 mg/L initial endosulfan concentration, 30 mg/L aluminium sulfate dose and pH 7, in the coagulation-flocculation experiments. In chlorination experiments, it was determined that the variables of initial endosulfan concentration, chlorine dose, pH and contact time contributed 55.84%, 21.25%, 5.74% and 3.48% to the endosulfan removal, respectively, according to the ANOVA results. The presence of endosulfan metabolites like endosulfan ether, sulfate, and lactone were also detected and measured. P values were obtained only in endosulfan sulfate formation as a result of chlorination, lower than 0.05 for initial endosulfan concentration, chlorine dose and time parameters; apart from this, P values were not statistically significant in all other experiments.

Triboelectric-photocatalytic coupling enhanced photogenerated electrons and holes utilization for efficient algal inactivation

Algal blooms have become a challenging problem in conventional water treatment processes due to numerous negative effects on human health and ecosystems. Photocatalysis has turned out to be a promising solution with the environmental and economic benefits beheld, but the rapid recombination of photogenerated electron and hole inhibited the application in practice. This study proposed a rotating triboelectric nanogenerator (R-TENG) as a self-powered bias for photocatalysis to enhance the algal removal efficiencies. R-TENG harvest waste water wave energy from coagulation tank to convert it into stable DC voltage (31 V) and DC current (27 μA). TiO2 nanotubes (TNT) serving as the anode in the photocatalysis with TENG system achieved complete inactivation of algae (>97 %) after 7 h of reaction, which was increased by 13.9 % compared with photocatalysis. This enhanced performance was attributed to the improvement of active species generation (e.g., •O2-, •OH and h+). During algal inactivation, the active species destroyed photosynthetic activity (decline of Chlorophyll a), antioxidant system (decrease of superoxide dismutase, catalase and malondialdehyde) and membrane in algal cells. Given the ubiquitous existence of algal cells in surface water, this work offers a novel method to develop high-performance electro-assisted photocatalysis for algal removal.

Research Progress on Removing Antibiotic Resistance Genes in Constructed Wetlands

Recently, the issue of environmental pollution and emerging antibiotic resistance genes (ARGs) has gradually gained attention. Antibiotic resistant bacteria (ARB) can be effectively reduced via the conventional water treatment processes. Although the ARB are completely inactivated during the disinfection process, the free ARGs can be incorporated into other microorganisms through transformation or transduction, allowing the ARGs to spread and propagate. Therefore, ARGs in wastewater must be handled by a specific process. It has been demonstrated in several studies that treatment using constructed wetland is an effective, economical, and environmentally friendly method of removing antibiotics and resistance genes from wastewater. Here, the research progress on the removal effect of constructed wetland water treatment systems on ARGs at home and abroad was reviewed. The results revealed that the removal efficiency of ARGs in subsurface flow constructed wetland was higher than that in surface flow-constructed wetlands. The composite-constructed wetland had significantly improved removal efficiency of ARGs compared to that in the ordinarily constructed wetland; however, the parameter setting of the composite process still requires further research. Several studies have reported that the removal efficiency of ARGs using constructed wetlands varies depending on the type of constructed wetland enhancement, plant, temperature, pH, and other factors. The results of the current study revealed that cross-mixing was the best way to combine plants, whereas the selection of plant species has not yet shown a clear dominant species. Temperature and pH affected the removal of ARGs by altering the microbial community in constructed wetlands. Although longer hydraulic residence time could increase the removal efficiency of ARGs, it also increased the enrichment risk of ARGs. The selection of constructed wetland substrate type should focus on fillers with a high specific surface area; the flow direction of the up-flow type was generally more efficient than the down-flow type in removing ARGs. In conclusion, the various factors (such as, the constructed wetland type, substrate type, hydraulic retention time, ambient temperature, and plant species) need to be integrated into the design of the constructed wetland system parameters to achieve the most effective treatment effect. The application of constructed wetlands in removing ARGs from the environment has broad prospects but also faces challenges.

Electrically conductive membrane distillation via an alternating current operation for zero liquid discharge

Membrane distillation (MD) shows promise for achieving high salinity treatment and zero liquid discharge (ZLD) compared to conventional water treatment processes due to its unique characteristics, including low energy consumption and high resulting water quality. However, performance degradation due to fouling and scaling under high recovery conditions remains a challenge, particularly considering the need to control both cations and anions for maximum scaling mitigation. Accordingly, in this study, alternating current (AC) operation for electrically conductive membrane distillation (ECMD) is newly proposed, based on its potential for controlling both cations and anions, in contrast to conventional direct current (DC) operation. Systematic experiments and theoretical analysis show that water recovery in ECMD can be increased by 27% through AC operation. The proposed modification and effective AC operation of ECMD increase the practicality of using MD in desalination for a high recovery rate, perhaps even for ZLD.

Impact of Drinking Water Treatment Process Using Life Cycle Assessment (LCA) to Minimize Environmental Impact Risk

The clean water treatment plant at DWTP is carried out with conventional treatment. The conventional water treatment process has an impact on the environment due to the clean water treatment process. Clean water treatment produces GWP and Eutrophication impacts derived from the use of chemicals and electricity. This study aims to analyze the factors causing environmental impacts arising from the clean water treatment process at the PDAM Water Treatment Plant (IPA) and determine policy priorities related to reducing environmental impacts and improving the quality of drinking water treatment at the Water Treatment Plant based on the results of LCA and AHP studies. The Life Cycle Assessment (LCA) method is a method of assessing the potential environmental impact and evaluating the environmental performance of a process on the product. Life Cycle Assessment (LCA) consists of four stages, namely determination of Goal and Scope, Life Cycle Inventory, Life Cycle Impact Assessment, and data interpretation. In research, the scope of LCA used is Gate to Gate, namely in the Coagulation, Flocculation, and Sedimentation process units. From the results of the LCA study, the potential impact of GWP of 12760.76 KgCO2-eq comes from flocculation which produces floc from the addition of chemicals. Meanwhile, the potential impact of eutrophication is 268.55 KgPO4-eq derived from mud generation. The results of the decision in reducing environmental impact at DWTP by optimizing chemicals as a top priority, then the second priority by recycling sludge, and finally making savings and energy alternatives are the last priority. Each respondent's score reached 55%, 23% and 22%. The results of the implementation of the program if implemented can reduce the impact of GWP 20079.17 kg CO2-eq and the impact of eutrophication can be reduced by 201.401 kg PO4-eq.mportant matters discussed in the paper that significantly contribute to the final result of the research may be noted here, but you have to consider, however, the limited space of the abstract.

Fluorescence-based indicators predict the performance of conventional drinking water treatment processes: Evaluation based on the changes in the compositions of dissolved organic matter

This study investigated the treatability of dissolved organic matter (DOM) by the selected lab-scale drinking water treatment processes using fluorescence excitation-emission matrix (EEM) analysis. The fluorescence ratio Peak 3/Peak 2 was established from well-defined fluorescence peak intensity of humic-like components (Ex/Em: 225 nm/425 nm) and protein-like components (Ex/Em: 230 nm/345 nm). Peak 3/Peak 2 predicted the aromatic characteristics of DOM and their origins in the different natural surface water feeding the different drinking water treatment plants. The drinking water treatment processes confirmed the treatability of DOM using Peak 3/Peak 2 and was well-confirmed by specific UV260 absorbance relative to dissolved organic carbon (DOC) (SUVA) and fluorescence-based indices. Peak 3/Peak 2 was demonstrated to have a strong correlation with SUVA and DOC removal for the water after treatment by coagulation, adsorption, and chlorination. Compared to the humification index and fluorescence index, Peak 3/Peak 2 is better for indicating the DOM composition in terms of treatability. These findings can broaden the use of fluorescence spectroscopy in water treatment applications, by developing the fluorescence ratio to evaluate the performance of drinking water treatment plants.

Membrane modification in enhancement of virus removal: A critical review

Many waterborne diseases are related with viruses, and COVID-19 worldwide has raised the concern of virus security in water into the public horizon. Compared to other conventional water treatment processes, membrane technology can achieve satisfactory virus removal with fewer chemicals, and prevent the outbreaks of viruses to a maximal extent. Researchers developed new modification methods to improve membrane performance. This review focused on the membrane modifications that enhance the performance in virus removal. The characteristics of viruses and their removal by membrane filtration were briefly generalized, and membrane modifications were systematically discussed through different virus removal mechanisms, including size exclusion, hydrophilic and hydrophobic interactions, electronic interactions, and inactivation. Advanced functional materials for membrane modification were summarized based on their nature. Furthermore, it is suggested that membranes should be enhanced through different mechanisms mainly based on their ranks of pore size. The current review provided theoretical support regarding membrane modifications in the enhancement of virus removal and avenues for practical application.

Synthesis and application of manganese-doped zinc oxide as a potential adsorbent for removal of Congo red dye from wastewater

Synthetic dyes are considered toxic compounds and as such are not easily removed by conventional water treatment processes. This study demonstrated the synthesis of pure and manganese- (Mn), silver- (Ag), and iron- (Fe) doped zinc oxide (ZnO) nanoparticles via the wet chemical route. In particular, it investigated the batch adsorption studies and physiochemical properties of synthesized pure and doped ZnO materials for removing toxic congo red (CR) dye. X-ray diffraction (XRD), Raman spectroscopy, and energy-dispersive X-ray spectroscopy (EDS) confirmed the synthesis of the pure and doped ZnO materials. The batch adsorption investigation revealed adsorption efficiencies of 99.4% for CR dye at an optimal dose of 0.03 g/30 ml for Mn-doped ZnO at a solution pH of 2. The adsorption capacity of each of the synthesized materials was found to be in order Mn-doped ZnO (232.5 mg/g) > Ag-doped ZnO (222.2 mg/g) > pure ZnO (212.7 mg/g) > Fe-doped ZnO (208.3 mg/g). Both pseudo-second-order kinetics model and the Langmuir isotherm model accurately explained the adsorption behaviors of CR dye. As such, Van der Waal interactions, H-bonding, and electrostatic interaction were found to be the adsorption mechanisms responsible for dye removal. In addition, the desorption–regeneration investigation indicated the successful reuse of the exhausted Mn-doped ZnO material for five cycles of CR dye adsorption with an efficiency of 83.1%. Overall, this study has demonstrated that Mn-doped ZnO could be considered a viable adsorbent for the cleanup of dye-contaminated water.

Detoxication of organophosphate pesticides in source waters by incorporation of an alkaline hydrolysis procedure in conventional drinking water treatment processes

Organophosphorus pesticides (OPPs) in source waters are a potential threat to drinking water safety. Although these chemicals could be removed upon chlorination in conventional water treatment, the formation of toxic decomposition products, i.e., oxons, can increase the toxicity of finished water. To address this problem, this study proposed a simple and practical treatment method for detoxifying OPPs by incorporating an alkaline hydrolysis procedure after pre-chlorination in the conventional treatment process. The experimental results showed that pre-chlorination with a chlorine dose of 1.5 mg/L could transform 87 %–92 % of OPPs with initial concentrations <50 μg/L into oxons, resulting in a significant increase in toxicity. However, by adjusting solution pH to around 11 through addition of a base fostered hydrolysis reactions of the chlorinated by-products, reducing the toxicity of the treated solution to about 0 % after a 30-minute reaction. Furthermore, the present study demonstrated that, even at a higher OPPs concentration (e.g., 100 μg/L for individual OPPs), a complete detoxification of the treated water could be achieved via adjusting pre-chlorination dose or reaction time, or hydrolysis reaction time. The presence of the common inorganic ions and organic matter in the source water matrix has negligible effects on the degradation and detoxication of OPPs. This study thus demonstrated that a combined pre-chlorination and alkaline hydrolysis process could be an effective and efficient treatment protocol for both degradation and detoxication of OPPs in water treatment.

Removal of Glyphosate from water through adsorption onto Goethite Nanoparticles

The adverse effects of prolonged exposure to Glyphosate for human, animal and aquatic life have gained significant attention in recent years. The application of Glyphosate over the past four decades has resulted in the detection of the organophosphorus herbicide in many waterways. Conventional water treatment processes are inefficient for removing this highly hydrophilic compound. However, adsorption processes have emerged as a potential process for removing organic contaminants from aqueous matrices. This study involved the synthesis of Goethite Nanoparticles via a co-precipitation reaction as an adsorbent material for Glyphosate. The Goethite Nanoparticles were characterized by X-Ray Diffraction, Fourier-Transform Infrared Spectroscopy and Scanning Electron Microscope images. The X-Ray Diffraction spectrum was in similar agreement to JCPDS file for Goethite (JCPDS 29-0713) with an average crystallite size determined by the Scherrer formula of 8.87 nm. Adsorption studies conducted in Glyphosate spiked solutions were utilized to determine the optimum operating conditions. Adsorption was optimum in slightly acidic conditions (pH 5) with an equilibrium time of 8 h. Adsorption kinetics and isotherms were applied to the adsorption data. The data best fit the pseudo-second-order kinetics equations indicating the presence of chemisorption processes. A Langmuir's Adsorption Capacity of 277.77 mg/g was calculated subsequently agreeing to experimentally derived values.

Degradation of Polystyrene Nanoplastics in UV/NaClO and UV/PMS Systems: Insights into Degradation Efficiency, Mechanism, and Toxicity Evaluation

Nanoplastics have gradually become a concern due to the wide use of plastics. Nanoplastics in aqueous phase can be exposed to users through water supply networks and cannot be efficiently removed by conventional water treatment processes. This work studied the degradation of polystyrene nanoplastics (PS-NP) by two commonly used advanced oxidation systems: UV/NaClO and UV/peroxymonosulfate (PMS). Results showed that almost no turbidity was detected in the PS-NP solution (5.00 mg/L) after treated by both UV/NaClO and UV/PMS for 360 min, suggesting the excellent turbidity removal ability. Yet, scanning electron microscope (SEM) and total organic carbon (TOC) removal tests demonstrated that PS-NP could not be completely degraded by UV/NaClO. The mineralization rate using UV/NaClO was only 7.00% even when the NaClO concentration increased to 5.00 mM, and many PS-NP particles could still be observed in SEM images. By contrast, the mineralization rate reached 63.90% in the UV/PMS system under the identical experimental conditions, and no spherical particles appeared in the SEM results. Density functional theory (DFT) calculations revealed that the different reaction sites and energy barriers of SO4•− and •Cl on PS-NP resulted in the differences in mineralization rates and degradation intermediates. The degradation pathway of PS-NP by UV/NaClO and UV/PMS was proposed accordingly. Additionally, the intermediates toxicity evaluation by a luminescent bacteria test showed that the inhibition rate in the UV/NaClO system (2.97%) was not markedly different from that in the control group without any treatment (1.98%); while that in UV/PMS system increased sharply to 98.19%. This work demonstrated that UV/PMS was more effective in PS-NP degradation than UV/NaClO, and the chemical risks of degradation intermediates were non-negligible.

Photo-Based Advanced Oxidation Processes for Zero Pollution: Where Are We Now?

The ambitious “zero pollution” goal entails the upgrade of conventional (waste)water treatment processes. Among various options, photo-based advanced oxidation processes (photo-AOPs) have shown promising capabilities in terms of eliminating contaminants of emerging concern. While photo-AOP applications exist worldwide, some key issues remain that hamper their wider practice and achieving the potential zero pollution goal. In this perspective, we briefly summarize the state of photo-AOPs, including photo-hydrogen peroxide (H2O2), -Fenton, -persulfate (S2O82- ), -ozone (O3 ), and -catalysis systems. We also discuss the major theoretical and technical knowledge gaps of photo-AOPs, which require further research to advance these treatments and to progress towards zero-toxic discharge for safe water reuse.