Photocatalytic membrane reactors for produced water treatment and reuse: Fundamentals, affecting factors, rational design, and evaluation metrics

How can heteroatoms boost the performance of photoactive nanomaterials for wastewater purification?

Photocatalysis, as an alternative for treating persistent water pollutants, holds immense promise. However, limitations hinder sustained treatment and recycling under varying light conditions. This comprehensive review delves into the novel paradigm of metal and non-metal doping to overcome these challenges. It begins by discussing the fundamental principles of photocatalysis and its inherent limitations. Understanding these constraints is crucial for developing effective strategies. Band gap narrowing by metal and non-metal doping modifies the band gap, enabling visible-light absorption. Impurity energy levels and oxygen vacancies influenced the doping energy levels and surface defects. Interfacial electron transfer and charge carrier recombination are the most important factors that impact overall efficiency. The comparative analysis of nanomaterials are reviewed on various, including nanometal oxides, nanocarbon materials, and advanced two-dimensional structures. The synthesis process are narratively presented, emphasizing production yields, selectivity, and efficiency. The review has potential applications in the environment for efficient pollutant removal and water purification, economic cost-effective and scalable production and technological advancement catalyst design, in spite of its challenges in material stability, synthesis methods and optimizing band gaps. The novelty of the review paper is on the proposal of a new paradigm of heterojunctions of doped metal and non-metal photocatalysts to promise highly efficient water treatment. This review bridges the gap between fundamental research and practical applications, offering insights into tailored nano photocatalysts.

Non-targeted analysis and toxicity prediction for evaluation of photocatalytic membrane distillation removing organic contaminants from hypersaline oil and gas field-produced water

Membrane distillation (MD) has received ample recognition for treating complex wastewater, including hypersaline oil and gas (O&G) produced water (PW). Rigorous water quality assessment is critical in evaluating PW treatment because PW consists of numerous contaminants beyond the targets listed in general discharge and reuse standards. This study evaluated a novel photocatalytic membrane distillation (PMD) process, with and without a UV light source, against a standard vacuum membrane distillation (VMD) process for treating PW, utilizing targeted analyses and a non-targeted chemical identification workflow coupled with toxicity predictions. PMD with UV light resulted in better removals of dissolved organic carbon, ammoniacal nitrogen, and conductivity. Targeted organic analyses identified only trace amounts of acetone and 2-butanone in distillates. According to non-targeted analysis, the number of suspects reduced from 65 in feed to 25-30 across all distillate samples. Certain physicochemical properties of compounds influenced contaminant rejection in different MD configurations. According to preliminary toxicity predictions, VMD, PMD with and without UV distillate samples, respectively contained 21, 22, and 23 suspects associated with critical toxicity concerns. Overall, non-targeted analysis together with toxicity prediction provides a competent supportive tool to assess treatment efficiency and potential impacts on public health and the environment during PW reuse.

Occurrence, seasonal variations, and fate of household and personal care chemicals in a wastewater treatment plant with Bacillus bioreactor process

Household and personal care chemicals (HPCCs) constitute a significant component of everyday products, with their global usage on the rise. HPCCs are eventually discharged into municipal wastewater treatment plants (WWTPs). However, the behaviors of HPCCs inside the Bacillus Bioreactor (BBR) process, including their prevalence, fate, and elimination mechanisms, remain underexplored. Addressing this gap, our study delves into samples collected from a BBR process at a significant WWTP in the northeast of China. Our results spotlight the dominance of linear alkylbenzene sulfonates (LASs) in the influent with concentrations ranging between 238 and 789 μg/L, much higher than the other HPCC concentrations, and remained dominant in the subsequent treatment units. After treatment using the BBR process, the concentrations of HPCCs in the effluent were diminished. Examination of different treatment units underscores the grit chamber removed over 60% of higher-concentration HPCCs, while the performance of the (RBC) tank needs to be improved. Except for the ultraviolet radiation (UV)-filters, seasonal variations exert minimal impact on the concentrations and removal efficiencies of other HPCCs in the BBR process. According to the mass balance analysis, the important mechanisms for HPCC removal were biodegradation and sludge adsorption. Also, the octocrylene (OCT) concerns raised by the environmental risk assessment of the HPCCs residuals in the final effluent, indicate a moderate risk to the surrounding aquatic environment (0.1 < RQ < 1), whereas other HPCCs have a lower risk level (RQ < 0.1). Overall, the research offers new perspectives on the fate and elimination mechanisms of HPCCs throughout the BBR process.

An electrical- and chemical-free approach using microfilter and Ag-based catalysts for emergency drinking water treatment

In emergency cases such as disasters, supplying enough safe drinking water is one of the most urgent needs for human life. This research aimed to develop a chemical- and electricity-free drinking water treatment system based on microfiltration and catalysts of Ag and Ag-TiO2-SiO2 coated on glass spheres in both dark and sunlight conditions. The system was applied to treat raw water samples taken from five rivers in Vietnam, simulated floodwater samples with high turbidity and E. coli concentration, and real flooding water samples. The results showed the filtration unit fed with simulated floodwater generated lower fluxes and shorter working durations before chemical cleaning than that fed with normal river water. However, filtrate quality was not significantly different for these five water samples with very low values of turbidity (0-0.66 NTU), total coliform (5-19 CFU (100 mL)-1), and E. coli (0-17 CFU (100 mL)-1). The catalyst units using glass spheres coated with either Ag in dark and sunlight or Ag-TiO2-SiO2 under sunlight can completely remove E. coli and coliform as well as additionally mitigate natural organic matters. This study hence suggests the combination of microfiltration and Ag-based catalytic treatment for safe drinking water supply in emergencies, especially for flooding conditions.

New directions on membranes for removal and degradation of emerging pollutants in aqueous systems

Emerging pollutants (EPs) refer to a group of non-regulated chemical or biological substances that have been recently introduced or detected in the environment. These pollutants tend to exhibit resistance to conventional treatment methods and can persist in the environment for prolonged periods, posing potential adverse effects on ecosystems and human health. As we enter a new era of managing these pollutants, membrane-based technologies hold significant promise in mitigating impact of EPs on the environment and safeguarding human health due to their high selectivity, efficiency, cost-effectiveness and capability for simultaneous separation and degradation. Moreover, these technologies continue to evolve rapidly with the development of new membrane materials and functionalities, advanced treatment strategies, and analyses for effectively treating EPs of more recent concerns. The objective of this review is to present the latest directions and advancements in membrane-based technologies for addressing EPs. By highlighting the progress in this field, we aim to share valuable perspectives with researchers and contribute to the development of future directions in sustainable treatments for EPs.

Tetracycline removal from wastewater via g-C3N4 loaded RSM-CCD-optimised hybrid photocatalytic membrane reactor

In this study, a split-type photocatalytic membrane reactor (PMR), incorporating suspended graphitic carbon nitride (g-C3N4) as photocatalyst and a layered polymeric composite (using polyamide, polyethersulfone and polysulfone polymers) as a membrane was fabricated to remove tetracycline (TC) from aqueous solutions as the world's second most used and discharged antibiotic in wastewater. The photocatalyst was synthesised from melamine by ultrasonic-assisted thermal polymerisation method and, along with the membrane, was characterised using various methods, including Brunauer-Emmett-Teller analysis (BET), Fourier-transform infrared spectroscopy (FTIR), X-ray diffraction analysis (XRD), Field emission scanning electron microscopy (FESEM), and Ultraviolet-visible spectroscopy (UV-Vis). The PMR process was optimised, using Design-Expert software for tetracycline removal in terms of UV irradiation time, pH, photocatalyst loading, tetracycline concentration, and membrane separation iteration. It was revealed that a membrane-integrated reactor as a sustainable system could effectively produce clean water by simultaneous removal of tetracycline and photocatalyst from aqueous solution. The maximum removal of 94.8% was obtained at the tetracycline concentration of 22.16 ppm, pH of 9.78 with 0.56 g/L of photocatalyst in the irradiation time of 113.77 min after six times of passing membrane. The PMR system showed reasonable reusability by about a 25.8% drop in TC removal efficiency after seven cycles at optimal conditions. The outcomes demonstrate the promising performance of the proposed PMR system in tetracycline removal from water and suggest that it can be scaled as an effective approach for a sustainable supply of antibiotic-free clean water.

Photoelectrocatalytic modification of nanofiltration membranes with SrF2/Ti3C2Tx to simultaneously enhance heavy metal ions rejection and permeability

Heavy metal pollution can significantly harm water systems and human health. Combining photoelectrocatalytic (PEC) and nanofiltration (NF) membrane separation technologies can effectively remove heavy metal ions from wastewater. In this study, a water bath method was used to form SrF2/Ti3C2Tx (ST) nanoparticles on the surface of polyvinylidene fluoride (PVDF) membranes and an additional polyamide (PA) functional layer was formed at the interface by crosslinking. ST@PA composite NF membranes (STPP) with good photocatalytic performance were obtained. The separation and catalytic properties of the STPP membranes were controlled by the ST content, which modifies the surface structure and properties of the membranes. The membrane with optimal ST crosslinking exhibited a water contact angle of 50.8°, pure water flux of 24.6 L·m-2·h-1·bar-1, and rejection rates of Mn2+, Ni2+, Cu2+, and Zn2+ of 98.8%, 95.3%, 95.7%, and 97.3%, respectively, under PEC-assisted separation with visible light illumination from a Xe lamp (300 W) and an applied voltage (2 V). The STPP membranes showed improved rejection rates of heavy metal ions under PEC-assisted operation. The mechanism for the improved membrane performance under PEC conditions was preliminarily clarified considering the relationship between the photocatalytic and filtration properties of STPP membranes along with the influence of light irradiation and an external voltage on the heavy metal ions. The generation of electrons, holes, superoxide radicals, and hydroxyl radicals during membrane operation enhances the rejection rates of heavy metal ions. Based on these results, STPP membranes are considered a promising technology for industrial applications in heavy metal removal.

Comparison of photocatalysis and photolysis of 2,2,4,4-tetrabromodiphenyl ether (BDE-47): Operational parameters, kinetic studies, and data validation using three modern machine learning models

Polybrominated diphenyl ethers (PBDEs) are halogenated organic compounds that are among the major pollutants of water, and there is an urgent need for their removal. This work compared the application of two techniques, i.e., photocatalytic reaction (PCR) and photolysis (PL), for 2,2,4,4- tetrabromodiphenyl ether (BDE-47) degradation. Although a limited degradation of BDE-47 was observed by photolysis (LED/N₂), photocatalytic oxidation by using TiO₂/LED/N₂ proved to be effective in the degradation of BDE-47. The use of a photocatalyst enhanced the extent of BDE-47 degradation by around 10% at optimum conditions in anaerobic systems. Experimental results were systematically validated through modeling with three new and powerful Machine Learning (ML) approaches, including Gradient Boosted Decision Tree (GBDT), Artificial Neural Network (ANN), and Symbolic Regression (SBR). Four statistical criteria (Coefficient of Determination (R²), Root Mean Square Error (RMSE), Average Relative Error (ARER), and Absolute Error (ABER)) were calculated for model validation. Among the applied models, the developed GBDT was the desirable model for predicting the remaining concentration (Ce) of BDE-47 for both processes. Total Organic Carbon (TOC) and Chemical Oxygen Demand (COD) results confirmed that BDE-47 mineralization required additional time than its degradation in both PCR and PL systems. The kinetic study demonstrated that BDE-47 degradation for both processes followed the pseudo-first-order form of the Langmuir-Hinshelwood (L-H) model. More importantly, the calculated electrical energy consumption of photolysis was shown to be ten percent higher than that for photocatalysis, possibly due to the higher irradiation time required in direct photolysis, which in turn increases electricity consumption. This study is useful in proposing a feasible and promising treatment process for the degradation of BDE-47.

Application of Turbiscan Stability Index for the Preparation of Alumina Photocatalytic Membranes for Dye Removal

In this work, for the first time, the stability of the TiO₂ suspensions used for the photocatalytic membrane preparation was studied by considering the Turbiscan Stability Index (TSI). The use of a stable suspension during the membrane preparation (by the dip-coating method) permitted a better dispersion of the TiO₂ nanoparticles into the membrane structure due to a reduction of agglomerates formation. The dip-coating was performed on the macroporous structure (external surface) of the Al₂O₃ membrane to avoid large reduction of the permeability. In addition, the reduction of the suspension infiltration along the membrane's cross-section allowed us to preserve the separative layer of the modified membrane. The water flux was reduced by about 11% after the dip-coating. The photocatalytic performance of the prepared membranes was evaluated using the methyl orange as a model pollutant. The reusability of the photocatalytic membranes was also demonstrated.

An overview of wastewater treatment using combined heterogeneous photocatalysis and membrane distillation

The need for efficient remediation solutions to wastewater has risen due to the concerning prevalence of toxic organic pollutants. It is possible for the linked photocatalysis-membrane separation system to concurrently achieve the photoreaction of pollutants and their removal from wastewater in order to accomplish the goal of completely purifying the wastewater. This investigation's objective is to provide analytical overview of the photocatalytic and membrane coupling process, photocatalytic membrane reactors, and the potential applications of these technologies in the treatment of wastewater for the persistent organic matter removal. In the review, an examination of photocatalytic and membrane processes to remove organic compounds from wastewater is presented. Based on the literature analysis, it was observed that the application of photocatalytic membrane reactors is significantly influenced by a wide variety of factors. Some of these factors include pollutant concentration, dissolved oxygen, aeration, pH, and hydraulic retention time. Light intensity is another factor that has a significant influence. It was also revealed how distillation membranes work when integrated with photocatalytic process. This brief overview will help researchers understand how successful coupled photocatalytic and membrane distillation are in the treatment of wastewater containing organic pollutants.

TiO2 based Photocatalysis membranes: An efficient strategy for pharmaceutical mineralization

Among the various emerging contaminants, pharmaceuticals (PhACs) seem to have adverse effects on the quality of water. Even the smallest concentration of PhACs in ground water and drinking water is harmful to humans and aquatic species. Among all the deaths reported due to COVID-19, the mortality rate was higher for those patients who consumed antibiotics. Consequently, PhAC in water is a serious concern and their removal needs immediate attention. This study has focused on the PhACs' degradation by collaborating photocatalysis with membrane filtration. TiO₂-based photocatalytic membrane is an innovative strategy which demonstrates mineralization of PhACs as a safer option. To highlight the same, an emphasis on the preparation and reinforcing properties of TiO₂-based nanomembranes has been elaborated in this review. Further, mineralization of antibiotics or cytostatic compounds and their degradation mechanisms is also highlighted using TiO₂ assisted membrane photocatalysis. Experimental reactor configurations have been discussed for commercial implementation of photoreactors for PhAC degradation anchored photocatalytic nanomembranes. Challenges and future perspectives are emphasized in order to design a nanomembrane based prototype in future for wastewater management.

Analysis of Regulatory Framework for Produced Water Management and Reuse in Major Oil- and Gas-Producing Regions in the United States

The rapid development of unconventional oil and gas (O&G) extraction around the world produces a significant amount of wastewater that requires appropriate management and disposal. Produced water (PW) is primarily disposed of through saltwater disposal wells, and other reuse/disposal methods include using PW for hydraulic fracturing, enhanced oil recovery, well drilling, evaporation ponds or seepage pits within the O&G field, and transferring PW offsite for management or reuse. Currently, 1–2% of PW in the U.S. is used outside the O&G field after treatment. With the considerable interest in PW reuse to reduce environmental implications and alleviate regional water scarcity, it is imperative to analyze the current regulatory framework for PW management and reuse. In the U.S., PW is subject to a complex set of federal, state, and sometimes local regulations to address the wide range of PW management, construction, and operation practices. Under the supervision of the U.S. Environment Protection Agency (U.S. EPA), different states have their own regulatory agencies and requirements based on state-specific practices and laws. This study analyzed the regulatory framework in major O&G-producing regions surrounding the management of PW, including relevant laws and jurisdictional illustrations of water rules and responsibilities, water quality standards, and PW disposal and current/potential beneficial reuse up to early 2022. The selected eastern states (based on the 98th meridian designated by the U.S. EPA as a tool to separate discharge permitting) include the Appalachian Basin (Marcellus and Utica shale areas of Pennsylvania, Ohio, and West Virginia), Oklahoma, and Texas; and the western states include California, Colorado, New Mexico, and Wyoming. These regions represent different regulations; climates; water quantities; quality diversities; and geologic, geographic, and hydrologic conditions. This review is particularly focused on the water quality standards, reuse practices and scenarios, risks assessment, knowledge gaps, and research needs for the potential reuse of treated PW outside of O&G fields. Given the complexity surrounding PW regulations and rules, this study is intended as preliminary guidance for PW management, and for identifying the knowledge gaps and research needs to reduce the potential impacts of treated PW reuse on the environment and public health. The regulations and experiences learned from these case studies would significantly benefit other states and countries with O&G sources for the protection of their environment and public health.

Retention and Inactivation of Quality Indicator Bacteria Using a Photocatalytic Membrane Reactor

The development of effective disinfection treatment processes is crucial to help the water industry cope with the inevitable challenges resulting from the increase in human population and climate change. Climate change leads to heavy rainfall, flooding and hot weather events that are associated with waterborne diseases. Developing effective treatment technologies will improve our resilience to cope with these events and our capacity to safeguard public health. A submerged hybrid reactor was used to test the efficiency of membrane filtration, direct photolysis (using ultraviolet-C low-pressure mercury lamps, as well as ultraviolet-C and ultraviolet-A light-emitting diodes panels) and the combination of both treatment processes (membrane filtration and photolysis) to retain and inactivate water quality indicator bacteria. The developed photocatalytic membranes effectively retained the target microorganisms that were then successfully inactivated by photolysis and advanced oxidation processes. The new hybrid reactor could be a promising approach to treat drinking water, recreational water and wastewater produced by different industries in small-scale systems. Furthermore, the results obtained with membranes coated with titanium dioxide and copper combined with ultraviolet-A light sources show that the process may be a promising approach to guarantee water disinfection using natural sunlight.

Characterization of produced water and surrounding surface water in the Permian Basin, the United States

A thorough understanding of produced water (PW) quality is critical to advance the knowledge and tools for effective PW management, treatment, risk assessment, and feasibility for beneficial reuse outside the oil and gas industry. This study provides the first step to better understand PW quality to develop beneficial reuse programs that are protective of human health and the environment. In total, 46 PW samples from unconventional operations in the Permian Basin and ten surface water samples from the Pecos River in New Mexico were collected for quantitative target analyses of more than 300 constituents. Water quality analyses of Pecos River samples could provide context and baseline information for the potential discharge and reuse of treated PW in this area. Temporal PW and river water quality changes were monitored for eight months in 2020. PW samples had total dissolved solids (TDS) concentrations ranging from 100,800-201,500 mg/L. Various mineral salts, metals, oil and grease, volatile and semi-volatile organic compounds, radionuclides, ammonia, hydraulic fracturing additives, and per- and polyfluoroalkyl substances were detected at different concentrations. Chemical characterization of organic compounds found in Pecos River water showed no evidence of PW origin. Isometric log-ratio Na-Cl-Br analysis showed the salinity in the Pecos River samples appeared to be linked to an increase in natural shallow brine inputs. This study outlines baseline analytical information to advance PW research by describing PW and surrounding surface water quality in the Permian Basin that will assist in determining management strategies, treatment methods, potential beneficial reuse applications, and potential environmental impacts specific to intended beneficial use of treated PW.

Functionalized Nanomembranes and Plasma Technologies for Produced Water Treatment: A Review

The treatment of produced water, associated with oil & gas production, is envisioned to gain more significant attention in the coming years due to increasing energy demand and growing interests to promote sustainable developments. This review presents innovative practical solutions for oil/water separation, desalination, and purification of polluted water sources using a combination of porous membranes and plasma treatment technologies. Both these technologies can be used to treat produced water separately, but their combination results in a significant synergistic impact. The membranes functionalized by plasma show a remarkable increase in their efficiency characterized by enhanced oil rejection capability and reusability, while plasma treatment of water combined with membranes and/or adsorbents could be used to soften water and achieve high purity.

A Review of Titanium Dioxide (TiO2)-Based Photocatalyst for Oilfield-Produced Water Treatment

Oilfield produced water (OPW) has become a primary environmental concern due to the high concentration of dissolved organic pollutants that lead to bioaccumulation with high toxicity, resistance to biodegradation, carcinogenicity, and the inhibition of reproduction, endocrine, and non-endocrine systems in aquatic biota. Photodegradation using photocatalysts has been considered as a promising technology to sustainably resolve OPW pollutants due to its benefits, including not requiring additional chemicals and producing a harmless compound as the result of pollutant photodegradation. Currently, titanium dioxide (TiO₂) has gained great attention as a promising photocatalyst due to its beneficial properties among the other photocatalysts, such as excellent optical and electronic properties, high chemical stability, low cost, non-toxicity, and eco-friendliness. However, the photoactivity of TiO₂ is still inhibited because it has a wide band gap and a low quantum field. Hence, the modification approaches for TiO₂ can improve its properties in terms of the photocatalytic ability, which would likely boost the charge carrier transfer, prevent the recombination of electrons and holes, and enhance the visible light response. In this review, we provide an overview of several routes for modifying TiO₂. The as-improved photocatalytic performance of the modified TiO₂ with regard to OPW treatment is reviewed. The stability of modified TiO₂ was also studied. The future perspective and challenges in developing the modification of TiO₂-based photocatalysts are explained.