Effect of phosphate ions on the formation of iron oxide/hydroxide as a stabilizer

Iron-containing nanominerals for sustainable phosphate management: A comprehensive review and future perspectives

Adsorption, which is a quick and effective method for phosphate management, can effectively address the crisis of phosphorus mineral resources and control eutrophication. Phosphate management systems typically use iron-containing nanominerals (ICNs) with large surface areas and high activity, as well as modified ICNs (mICNs). This paper comprehensively reviews phosphate management by ICNs and mICNs in different water environments. mICNs have a higher affinity for phosphates than ICNs. Phosphate adsorption on ICNs and mICNs occurs through mechanisms such as surface complexation, surface precipitation, electrostatic ligand exchange, and electrostatic attraction. Ionic strength influences phosphate adsorption by changing the surface potential and isoelectric point of ICNs and mICNs. Anions exhibit inhibitory effects on ICNs and mICNs in phosphate adsorption, while cations display a promoting effect. More importantly, high concentrations and molecular weights of natural organic matter can inhibit phosphate adsorption by ICNs and mICNs. Sodium hydroxide has high regeneration capability for ICNs and mICNs. Compared to ICNs with high crystallinity, those with low crystallinity are less likely to desorb. ICNs and mICNs can effectively manage municipal wastewater, eutrophic seawater, and eutrophic lakes. Adsorption of ICNs and mICNs saturated with phosphate can be used as fertilizers in agricultural production. Notably, mICNs and ICNs have positive and negative effects on microorganisms and aquatic organisms in soil. Finally, this study introduces the following: trends and prospects of machine learning-guided mICN design, novel methods for modified ICNs, mICN regeneration, development of mICNs with high adsorption capacity and selectivity for phosphate, investigation of competing ions in different water environments by mICNs, and trends and prospects of in-depth research on the adsorption mechanism of phosphate by weakly crystalline ferrihydrite. This comprehensive review can provide novel insights into the research on high-performance mICNs for phosphate management in the future.

Nano-size plastics inhibited Cr(VI) species transformation during facilitated transport of green synthesized nano-iron in the presence of oxyanions

Remediating hexavalent chromium (Cr(VI))-contaminated soils using green synthesized nano-iron (g-nZVI), which merits high reactivity, low cost, and environmental friendliness, has attracted significant attention. However, the broad existence of nano plastics (NPs) could adsorb Cr(VI) and subsequently influence in situ remediation of Cr(VI)-contaminated soil by g-nZVI. To clarify this issue and improve the remediation efficiency, we investigated the co-transport between Cr(VI) and g-nZVI coexisting with sulfonyl-amino-modified nano plastics (SANPs) in water-saturated sand media in the presence of oxyanions (i.e., phosphate and sulfate) at environmentally relevant conditions. This study found that SANPs inhibited the Cr(VI) reduction to Cr(III) (i.e., Cr₂O₃) by g-nZVI, attributed to nZVI-SANPs hetero-aggregates and Cr(VI) adsorption on SANPs. Notably, "nZVI-[SANPs•••Cr(III)]" agglomerate happened via complexation of [-NH₃•••Cr(III)] between Cr(III) from Cr(VI) reduced by g-nZVI and amino group on SANPs. Further, the co-presence of phosphate (stronger adsorption on SANPs than g-nZVI) remarkably suppressed Cr(VI) reduction. Then, it promoted the co-transport of Cr(VI) with nZVI-SANPs hetero-aggregates, which could potentially threaten underground water. Fundamentally, sulfate would instead concentrate on SANPs, hardly impacting the reactions between Cr(VI) and g-nZVI. Overall, our findings provide crucial insights into understanding the Cr(VI) species transformation during co-transport with g-nZVI in ubiquitous complexed soil environments (i.e., containing oxyanions) contaminated by SANPs.

Bipolar Membranes Containing Iron-Based Catalysts for Efficient Water-Splitting Electrodialysis

Water-splitting electrodialysis (WSED) process using bipolar membranes (BPMs) is attracting attention as an eco-friendly and efficient electro-membrane process that can produce acids and bases from salt solutions. BPMs are a key component of the WSED process and should satisfy the requirements of high water-splitting capability, physicochemical stability, low membrane cost, etc. The water-splitting performance of BPMs can be determined by the catalytic materials introduced at the bipolar junction. Therefore, in this study, several kinds of iron metal compounds (i.e., Fe(OH)₃, Fe(OH)₃@Fe₃O₄, Fe(OH)₂EDTA, and Fe₃O₄@ZIF-8) were prepared and the catalytic activities for water-splitting reactions in BPMs were systematically analyzed. In addition, the pore-filling method was applied to fabricate low-cost/high-performance BPMs, and the 50 μm-thick BPMs prepared on the basis of PE porous support showed several times superior toughness compared to Fumatech FBM membrane. Through various electrochemical analyses, it was proven that Fe(OH)₂EDTA has the highest catalytic activity for water-splitting reactions and the best physical and electrochemical stabilities among the considered metal compounds. This is the result of stable complex formation between Fe and EDTA ligand, increase in hydrophilicity, and catalytic water-splitting reactions by weak acid and base groups included in EDTA as well as iron hydroxide. It was also confirmed that the hydrophilicity of the catalyst materials introduced to the bipolar junction plays a critical role in the water-splitting reactions of BPM.