Search

Driven by the electric-vehicle revolution, a sharp increase in lithium (Li) demand as a result of the need to produce Li-ion batteries is expected in coming years. To enable a sustainable Li supply, there is an urgent need to develop cost-effective and environmentally friendly methods to extract Li from a variety of sources including Li-rich salt-lake brines, seawater, and wastewaters. While the prevalent lime soda evaporation method is suitable for the mass extraction of Li from brine sources with low Mg/Li ratios, it is time-consuming (1 year) and typically exhibits low Li recovery. Electrochemically-based methods have emerged as promising processes to recover Li given their ease of management, limited requirement for additional chemicals, minimal waste production, and high selectivity towards Li. This state-of-the-art review provides a comprehensive overview of current advances in two key electrochemical Li recovery technologies (electrosorption and electrodialysis) with particular attention given to advances in understanding of mechanism, materials, operational modes, and system configurations. We highlight the most pressing challenges these technologies encounter including (i) limited electrode capacity, poor electrode stability and co-insertion of impurity cations in the electrosorption process, and (ii) limited Li selectivity of available ion exchange membranes, ion leakage and membrane scaling in the electrodialysis process. We then systematically describe potentially effective strategies to overcome these challenges and, further, provide future perspectives, particularly with respect to the translation of innovation at bench-scale to industrial application.

A novel design for a flow-electrode capacitive deionization (FCDI) system consisting of tubular electrodes in a shell and tube heat exchanger configuration is proposed. Each electrode consists of a metallic mesh current collector along the inner circumference of a tubular ion-exchange membrane. This tubular FCDI design is suitable for scale-up as it consists of easily manufactured components which can be assembled in an array. An apparatus with 4 tubular electrodes with a large effective area (202.3 cm2) was constructed and shown to provide a high net salt (NaCl) removal rate (0.15 mg s-1 at 1.2 V applied voltage and ∼2000 mg L-1 influent total dissolved solids concentration). A computational fluid dynamics (CFD) model incorporating ion migration and transport mechanisms was developed to simulate the ion concentration and electrical potential profiles in the water channel. The results of CFD modelling highlighted the need to maximize regions of both high potential gradient and high hydraulic flow in order to achieve optimal salt removal. In brief, this study presents a new design approach for FCDI scale-up and provides a computational tool for optimization of this design and future innovative FCDI designs.

While flow anodic oxidation systems can efficiently generate hydroxyl radicals (·OH) and significantly enhance direct electron transfer (DET) processes that result in the oxidation of target contaminants via the charge percolating network of flow anode particles, challenges remain in constructing a flow anodic oxidation system that can be operated continuously with stable performance. Here we incorporate an ultrafiltration (UF) membrane module into the flow anodic oxidation system and achieve the continuous defluorination of perfluorooctanoic acid (PFOA) for 12 days with high efficiency (94.1%) and reasonable energy consumption (38.1 Wh mg

It is critical to both effectively remove and recover phosphate (P) from wastewater given the wide-ranging environmental (i.e., preventing eutrophication and restoring water quality) and economic (i.e., overcoming P resource scarcity) benefits. More recently, considerable academic effort has been devoted towards harvesting P as vivianite, which can be used as a potential slow-release fertilizer and possible reagent for the manufacture of lithium iron phosphate (LiFePO4), the precursor in fabricating Li-ion secondary batteries. In this study, we propose an innovative P recovery process, in which P is first preconcentrated via a flow-electrode capacitive deionization (FCDI) device followed by immobilization as vivianite crystals in a fluidized bed crystallization (FBC) column. The effects of different operational parameters on FCDI P preconcentration performance and energy consumption are investigated. Results show that 63% of P can be removed and concentrated in the flow-electrode chamber with a reasonable energy requirement under optimal operating conditions. The FBC system resulted in immobilization of ~80% of P as triangular or quadrangular pellets, which were verified to be high-purity vivianite crystals by scanning electron microscopy with energy dispersive X-ray spectroscopy (SEM-EDX) and extended X-ray absorption fine structure (EXAFS) spectroscopy. This study provides a pathway for efficient recovery of P as a value-added product (i.e., vivianite) from P-rich wastewaters.

The recovery of phosphorus (P) from wastewaters is a worthy goal considering the potential environmental and economic benefits. Flow-electrode capacitive deionization (FCDI), which employs flowable carbon electrodes instead of the static electrodes used in conventional CDI, has been demonstrated to be a promising P recovery technology. FCDI outperforms CDI and other competitive technologies in a number of aspects including (i) large salt adsorption capacity and (ii) extremely high water recovery rate. In this study, magnetic (Fe3O4 impregnated) activated carbon particles were prepared and applied as FCDI electrodes. The magnetic carbon electrodes were found to have a strong affinity towards P, facilitating the selective adsorption of P to the magnetic particles through a ligand exhange mechanism. Continuous operation of the FCDI system could be achieved with only three minutes required to separate the electrode particles from the brine stream on application of an external magnetic field. A P-rich stream was produced on regeneration of the exhausted magnetic electrodes using alkali solution. We envision that the use of magnetic carbon enhanced flow-electrodes will pave the way for efficient operation of FCDI as well as the preferential recovery of P.

Frequent occurrence of harmful algal blooms (HABs) and red tides in freshwater and seawater poses serious threats to water treatment and drives the application of membrane-based technologies in algal separation. Despite the high removal efficiency of algal cells and their metabolites (e.g. organic matter and toxins) by membranes, the generation of concentrate and waste streams presents a major challenge. In this paper, we review the scenarios under which membrane-based processes are integrated with algal separation, with particular attention given to (i) drinking water production and desalination at low algal concentrations and (ii) cyanobacteria-laden water treatment/desalination. The concentrate and waste streams from backwashing and membrane cleaning in each scenario are characterised with this information facilitating a better understanding of the transport of algal cells and metabolites in membrane processes. Current strategies and gaps in managing concentrate and waste streams are identified with guidance and perspectives for future studies discussed in an Eisenhower framework.

Iron salts are routinely dosed in wastewater treatment as a means of achieving effluent phosphorous concentration goals. The iron oxides that result from addition of iron salts partake in various reactions, including reductive dissolution and phosphate adsorption. The reactivity of these oxides is controlled by the conditions of formation and the processes, such as aggregation, that lead to a reduction in accessible surface sites following formation. The presence of organic compounds is expected to significantly impact these processes in a number of ways. In this study, amorphous ferric oxide (AFO) reactivity and aging was investigated following the addition of ferric iron (Fe(III)) to three solution systems: two synthetic buffered systems, either containing no organic or containing alginate, and a supernatant system containing soluble microbial products (SMPs) sourced from a membrane bioreactor (MBR). Reactivity of the Fe(III) phases in these systems at various times (1-60 min) following Fe(III) addition was quantified by determining the rate constants for ascorbate-mediated reductive dissolution over short (5 min) and long (60 min) dissolution periods and for a range (0.5-10 mM) of ascorbate concentrations. AFO particle size was monitored using dynamic light scattering during the aging and dissolution periods. In the presence of alginate, AFO particles appeared to be stabilized against aggregation. However, aging in the alginate system was remarkably similar to the inorganic system where aging is associated with aggregation. An aging mechanism involving restructuring within the alginate-AFO assemblage was proposed. In the presence of SMPs, a greater diversity of Fe(III) phases was evident with both a small labile pool of organically complexed Fe(III) and a polydisperse population of stabilized AFO particles present. The prevalence of low molecular weight organic molecules facilitated stabilization of the Fe(III) oxyhydroxides formed but subsequent aging observed in the alginate system did not occur. The reactivity of the Fe(III) in the supernatant system was maintained with little loss in reactivity over at least 24 h. The capacity of SMPs to maintain high reactivity of AFO has important implications in a reactor where Fe(III) phases encounter alternating redox conditions due to sludge recirculation, creating a cycle of reductive dissolution, oxidation and precipitation.

Flow electrode capacitive deionization (FCDI) is a promising configuration for capacitive deionization due to its capability of continuous operation and achieving a relatively large salinity reduction. Due to the complexity of the multi-phase flow involved in FCDI, modeling FCDI system performance has been a challenge with no predictive FCDI model thus far developed. In this study, we developed an equivalent film-electrode (EFE) model for FCDI in which the flow electrodes are approximated as moving film electrodes that behave in a manner similar to conveyor belts. The EFE-FCDI model is validated using results from a series of FCDI experiments and then applied to elucidate the spatial variations of the key properties of the FCDI system and to resolve the contributions of different aspects of the system to energy consumption. The impact of activated carbon loading in the flow electrode and the feed and effluent target concentrations on the overall FCDI performance are also discussed based on model simulation. In summary, the EFE-FCDI model enhances our understanding of the system-level behavior of FCDI systems and can be employed for optimizing FCDI design and operation.

Capacitive de-ionization (CDI) systems are well-known for their low energy consumption making them suitable for applications powered by renewable energy. In this study, CDI technology is, for the first time, integrated with a suitably-scaled, stand-alone, renewable power system comprising photovoltaic panels and battery storage. Guidelines for designing and sizing such power systems are proposed including determining electrode charging current, PV panels and battery capacity. A 1 kW pilot plant was designed, constructed and operated to verify the proposed guidelines. Using the pilot plant, the total energy consumption of the system has been evaluated with different electrode charging currents and influent flow rates and the relationship between these parameters analyzed. This analysis has enabled the development of practical design guidelines for bulk water treatment with MCDI electrodes. The results of this study show that use of photovoltaic-powered MCDI water treatment, particularly when combined with energy recovery, is competitive against more mature water-treatment technologies for particular applications and at particular locations.

Ammonia nitrogen removal from wastewaters has gained much attention in recent decades as a result of the environmental problems associated with discharge of excessive amounts of this critical nutrient including eutrophication of receiving waters, generation of offensive odours as a result of organism decay and complications associated with the disinfection of water supplies. While removal via biological processes represents the principal means by which a reduction in dissolved nitrogen concentrations is achieved, an electrochemical advanced oxidation process has been proposed as a potentially effective alternate means of removing ammonia from wastewaters with the removal associated with the in situ generation of oxidants (particularly active chlorine) at the anode. Here we describe the influence of key factors on the rate and extent of ammonia nitrogen removal in an electrochemical cell with a Ti/IrO2-RuO2 anode and Ti cathode. The rate of ammonia removal was found to be dependent on both current density and initial chloride concentration with ∼95% ammonia removed from a 20 mM Cl− solution within approximately 40 min at a current density of 3 mA cm−2, resulting in an energy consumption of 126 kWh kg−1 NH4+-N. Additionally, we show that by-products formation is effectively suppressed during the electrolysis process. A mechanistically-based kinetic model incorporating the key processes operating in the ammonia electro-oxidation process was developed with particular attention given to (i) anodic generation of active chlorine and other chloride-related by-products, (ii) active chlorine mediated ammonia oxidation. The results demonstrate that the electrochemical advanced oxidation process is a promising technology for treatment of ammonia-containing wastewaters with advantages including simplicity, safety and effectiveness.

Capacitive deionization (CDI) is considered to be one of the most promising technologies for the desalination of brackish water with low to medium salinity. In practical applications, Faradaic redox reactions occurring in CDI may have both negative and positive effects on CDI performance. In this review, we present an overview of the types and mechanisms of Faradaic reactions in CDI systems including anodic oxidation of carbon electrodes, cathodic reduction of oxygen and Faradaic ion storage and identify their apparent negative and positive effects on water desalination. A variety of strategies including development of novel electrode materials and use of alternative configurations and/or operational modes are proposed for the purpose of mitigation or elimination of the deterioration of electrodes and the formation of byproducts caused by undesired side Faradaic reactions. It is also recognized that Faradaic reactions facilitate a variety of exciting new applications including i) the incorporation of intercalation electrodes to enhance water desalination or to selectively separate certain ions through reversible Faradaic reactions and ii) the use of particular anodic oxidation and cathodic reduction reactions to realize functions such as water disinfection and contaminant removal.

While microalgae have been suggested as a promising substitute to conventional fossil fuels, their cost effective cultivation and harvesting constitutes a major challenge. In the work described here, a novel photosynthetic microbial fuel cell (PMFC) in which a stainless steel mesh with biofilm formed on it serves as both the cathode and filtration material has been developed. Results of this study reveal that, in addition to inducing oxygen reduction reactions under illumination, the biocathode is effective in preventing the washout of algae during continuous operation, resulting in retained biomass concentrations reaching 3.5–6.5 g L−1. The maximum output current density reached ∼200 mA m−2 under irradiation, which is comparable with recent PMFC studies. Microbial diversity analyses targeting 16S and 18S rRNA genes indicated that the eukaryotic species belonging to the genus Chlorella was able to sustain its community dominance (>96%) over other competing species over the course of the studies. In the absence of catalysts such as Pt, a consortium of photosynthetic organisms including plant growth-promoting bacteria such as Azospirillum and Rhizobium were overrepresented in the biofilm, with these organisms most likely contributing to cathodic electron transfer. Energy flow analysis suggested that the PMFC system held the potential to achieve theoretical energy balance in simultaneous algae production and wastewater treatment.

A comparative study of the ability of microparticulate zerovalent iron (mZVI) and nanoparticulate zerovalent iron (nZVI) to oxidize a target compound (in this study, 14C-labelled formate) under aerobic conditions has been conducted with specific consideration given to differences in reaction mechanisms. Results of Fe K-edge extended X-ray absorption fine structure (EXAFS) spectroscopy showed that mZVI underwent a slow transformation to ferrihydrite while nZVI, in contrast, rapidly transformed into lepidocrocite. The behavior of mZVI (compared to nZVI) could be attributed to either (i) a lower reactivity with oxygen and/or water, (ii) surface passivation by ferrihydrite resulting in reduced electron conductivity, and/or (iii) the relatively low concentration of Fe(II) which, in the case of nZVI, catalyzed the transformation of ferrihydrite to lepidocrocite. The influence of these structural transformations on contaminant removal was profound with the ferrihydrite that formed on mZVI inducing rapid adsorption of formate and moderating reactions of mZVI with oxygen and/or water. Although surface passivation of mZVI was significant, the effectiveness of the ensuing heterogeneous redox reactions in the mZVI/O2 system, as characterized by the molar ratio of oxidized formate to consumed Fe(0) (i.e., 13.7 ± 0.8 μM/M), was comparable to that for nZVI (16.5 ± 1.4 μM/M). The results of this study highlight the potential of mZVI for the oxidative degradation of target organics in preference to nZVI despite its lower intrinsic reactivity though some means (either natural or engineered) of inducing continual depassivation of the iron oxyhydroxide-coated mZVI would be required in order to maintain ongoing oxidant production.