Bromide and iodide selectivity in membrane capacitive deionisation, and its potential application to reduce the formation of disinfection by-products in water treatment

Removal of iodide anions in water by silver nanoparticles supported on polystyrene anion exchanger

The removal of iodide (I^-) from source waters is an effective strategy to minimize the formation of iodinated disinfection by-products (DBPs), which are more toxic than their brominated and chlorinated analogues. In this work, a nanocomposite Ag-D201 was synthesized by multiple in situ reduction of Ag-complex in D201 polymer matrix, to achieve highly efficient removal of iodide from water. Scanning electron microscope /energy dispersive spectrometer characterization showed that uniform cubic silver nanoparticles (AgNPs) evenly dispersed in the D201 pores. The equilibrium isotherms data for iodide adsorption onto Ag-D201 was well fitted with Langmuir isotherm with the adsorption capacity of 533 mg/g at neutral pH. The adsorption capacity of Ag-D201 increased with the decrease of pH in acidic aqueous solution, and reached the maximum value of 802 mg/g at pH 2. This was attributed to the oxidization of I^-, by dissolved oxygen under the catalysis of AgNPs, to I₂ which was finally adsorbed as AgI₃. However, the aqueous solutions at pH 7 - 11 could hardly affect the iodide adsorption. The adsorption of I^- was barely affected by real water matrixes such as competitive anions (SO₄^2-, NO₃^-, HCO₃^-, Cl^-) and natural organic matter, of which interference of NOM was offset by the presence of Ca²⁺. The proposed synergistic mechanism for the excellent performance of iodide adsorption by the absorbent was ascribed to the Donnan membrane effect caused by the D201 resin, the chemisorption of I^- by AgNPs, and the catalytic effect of AgNPs.

A novel CNTs/QCS/BiOBr composite membrane with electron-ion transfer channel for Br- recovery in ESIX process

Based on the superior selectivity of bismuth oxybromide (BiOBr) for Br^-, the excellent electrical conductivity of carbon nanotubes (CNTs), and the ion exchange capacity of quaternized chitosan (QCS), a three-dimensional network composite membrane electrode CNTs/QCS/BiOBr was constructed, in which BiOBr served as the storage space for Br^-, CNTs provided the electron transfer pathway, and QCS cross-linked by glutaraldehyde (GA) was used for ion transfer. The CNTs/QCS/BiOBr composite membrane exhibits superior conductivity after the introduction of the polymer electrolyte, which is seven orders of magnitude higher than that of conventional ion-exchange membranes. Furthermore, the addition of the electroactive material BiOBr improved the adsorption capacity for Br^- by a factor of 2.7 in electrochemically switched ion exchange (ESIX) system. Meanwhile, the CNTs/QCS/BiOBr composite membrane displays excellent Br^- selectivity in mixed solutions of Br^-, Cl^-, SO₄^2- and NO₃^-. Therein, the covalent bond cross-linking within the CNTs/QCS/BiOBr composite membrane endows it great electrochemical stability. The synergistic adsorption mechanism of the CNTs/QCS/BiOBr composite membrane provides a new direction for achieving more efficient ion separation.

Emerging periodate-based oxidation technologies for water decontamination: A state-of-the-art mechanistic review and future perspectives

With the ever-increasing severity of the ongoing water crisis, it is of great significance to develop efficient, eco-friendly water treatment technologies. As an emerging oxidant in the advanced oxidation processes (AOPs), periodate (PI) has received worldwide attention owing to the advantages of superior stability, susceptible activation capability, and high efficiency for decontamination. This is the first review that conducts a comprehensive analysis of the mechanism, pollutant transformation pathway, toxicity evolution, barriers, and future directions of PI-based AOPs based on the scientific information and experimental data reported in recent years. The pollutant elimination in PI-based AOPs was mainly attributed to the in situ generate reactive oxygen species (e.g., ^•OH, O(³P), ¹O₂, and O₂^•-), reactive iodine species (e.g., IO₃^• and IO₄^•), and high-valent metal-oxo species with exceptionally high reactivity. These reactive species were derived from the PI activated by the external energy, metal activators, alkaline, freezing, hydroxylamine, H₂O_2, etc. It is noteworthy that direct electron transport could also dominate the decontamination in carbon-based catalyst/PI systems. Furthermore, PI was transformed to iodate (IO₃^-) stoichiometrically via an oxygen-atom transfer process in most PI-based AOPs systems. However, the production of I₂, I^-, and HOI was sometimes inevitable. Furthermore, the transformation pathway of typical micropollutants was clarified, and the in silico QSAR-based prediction results indicated that most transformation products retained biodegradation recalcitrance and multi-endpoint toxicity. The barriers faced by the PI-based AOPs were also clarified with potential solutions. Finally, future perspectives and research directions are highlighted based on the current state of PI-based AOPs. This review enhances our in-depth understanding of PI-based AOPs for pollutant elimination and identifies future research needs to focus on the reduction of toxic byproducts.

Deficiency and excess of groundwater iodine and their health associations

More than two billion people worldwide have suffered thyroid disorders from either iodine deficiency or excess. By creating the national map of groundwater iodine throughout China, we reveal the spatial responses of diverse health risks to iodine in continental groundwater. Greater non-carcinogenic risks relevant to lower iodine more likely occur in the areas of higher altitude, while those associated with high groundwater iodine are concentrated in the areas suffered from transgressions enhanced by land over-use and intensive anthropogenic overexploitation. The potential roles of groundwater iodine species are also explored: iodide might be associated with subclinical hypothyroidism particularly in higher iodine regions, whereas iodate impacts on thyroid risks in presence of universal salt iodization exhibit high uncertainties in lower iodine regions. This implies that accurate iodine supply depending on spatial heterogeneity and dietary iodine structure optimization are highly needed to mitigate thyroid risks in iodine-deficient and -excess areas globally.

Removal of halides from drinking water: technological achievements in the past ten years and research needs

Disinfection is an essential process for drinking water supplies resulting in the formation of unintended disinfection by-products (DBPs), many of which are potentially toxic and are known as the possible or probable human carcinogens. As of now, 100⁺ DBPs were characterized while about 600⁺ others can be formed in the supply water. To protect the human health, many regulatory agencies have set the guideline values for several DBPs. Removal of halide ions and natural organic matter prior to disinfection is an important step to reduce DBPs, and the associated exposure and risks. To date, many publications have reported various methods for halide removal from drinking water. The most review about halide removal technologies, associated challenges, and future research needs was published in 2012. Since then, a number of studies have been published on different methods of halide removal techniques. This paper aims to review the state of research on halide removal techniques focusing on the development during the past 10 years (2012-2021). The techniques were clustered into six major groups: adsorption, ion exchange, coagulation, advanced oxidation, membrane separation, and combined techniques. The progress on these groups of technologies, their advantages, and limitations were examined, and the future research directions to produce the safe drinking water were identified.

Electrode for selective bromide removal in membrane capacitive deionisation

Due to the shortage of freshwater around the world, seawater is becoming an important water source. However, seawater contains a high concentration of bromide that can form harmful disinfection by-products during water disinfection. Therefore, the current seawater reverse osmosis (SWRO) has to adopt two-pass reverse osmosis (RO) configuration for effective bromide removal, increasing the overall desalination cost. In this study, a bromide selective composite electrode was developed for membrane capacitive deionisation (MCDI). The composite electrode was developed by coating a mixture of bromide selective resin and anion exchange polymer on the surface of the commercial activated carbon electrode, and its performance was compared to that of conventional carbon electrode. The results demonstrated that the composite electrode has ten times better bromide selectivity than the conventional carbon electrode. The study shows the potential application of MCDI for the selective removal of target ions from water sources and the potential for resource recovery through basic modification of commercial electrode.

Investigation of bromide removal and bromate minimization of membrane capacitive deionization for drinking water treatment

The ubiquitous bromide (Br^-) poses a challenge to current drinking water treatment schemes due to the formation of brominated disinfection by-products, especially bromate (BrO₃^-). A cost-effective and energy-efficient technology to remove Br^- before disinfection is highly desired. In this work, the application of membrane capacitive deionization (MCDI) for the removal of Br^- and BrO₃^- minimization for drinking water treatment was systematically investigated. Results showed that the removal of Br^- by MCDI followed the pseudo-second-order kinetics, in which kinetics was faster at lower Br^- concentration. Additionally, Br^- displayed a preferential electrosorption over Cl^- in MCDI despite the relatively smaller amounts. Due to high removal performance of Br^-, 99.49% of BrO₃^- minimization can be achieved. Moreover, the presence of humic acid (HA) had a negative effect on the removal of Br^- and BrO₃^- minimization. However, Br^- could be more preferentially removed than Cl^- in the presence of HA due to the weak interaction with HA. Finally, by treating an actual surface water sample, it was found that the removal rate of Br^- was 91.80%, and 83.97% of BrO₃^- minimization can be achieved. BrO₃^- concentration of effluent meets the control standard. Overall, these results prove the feasibility of MCDI for practical drinking water treatment.