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The small sized powdered ferric oxy-hydroxide, termed Dust Ferric Hydroxide (DFH), was applied in batch adsorption experiments to remove arsenic species from water. The DFH was characterized in terms of zero point charge, zeta potential, surface charge density, particle size and moisture content. Batch adsorption isotherm experiments indicated that the Freundlich model described the isothermal adsorption behavior of arsenic species notably well. The results indicated that the adsorption capacity of DFH in deionized ultrapure water, applying a residual equilibrium concentration of 10 µg/L at the equilibrium pH value of 7.9 ± 0.1, with a contact time of 96 h (i.e., Q10), was 6.9 and 3.5 µg/mg for As(V) and As(III), respectively, whereas the measured adsorption capacity of the conventionally used Granular Ferric Hydroxide (GFH), under similar conditions, was found to be 2.1 and 1.4 µg/mg for As(V) and As(III), respectively. Furthermore, the adsorption of arsenic species onto DFH in a Hamburg tap water matrix, as well as in an NSF challenge water matrix, was found to be significantly lower. The lowest recorded adsorption capacity at the same equilibrium concentration was 3.2 µg As(V)/mg and 1.1 µg As(III)/mg for the NSF water. Batch adsorption kinetics experiments were also conducted to study the impact of a water matrix on the behavior of removal kinetics for As(V) and As(III) species by DFH, and the respective data were best fitted to the second order kinetic model. The outcomes of this study confirm that the small sized iron oxide-based material, being a by-product of the production process of GFH adsorbent, has significant potential to be used for the adsorptive removal of arsenic species from water, especially when this material can be combined with the subsequent application of low-pressure membrane filtration/separation in a hybrid water treatment process.

Arsenic is among the major drinking water contaminants affecting populations in many countries because it causes serious health problems on long-term exposure. Two low-cost micro-sized iron oxyhydroxide-based adsorbents (which are by-products of the industrial production process of granular adsorbents), namely, micro granular ferric hydroxide (μGFH) and micro tetravalent manganese feroxyhyte (μTMF), were applied in batch adsorption kinetic tests and submerged microfiltration membrane adsorption hybrid system (SMAHS) to remove pentavalent arsenic (As(V)) from modeled drinking water. The adsorbents media were characterized in terms of iron content, BET surface area, pore volume, and particle size. The results of adsorption kinetics show that initial adsorption rate of As(V) by μTMF is faster than μGFH. The SMAHS results revealed that hydraulic residence time of As(V) in the slurry reactor plays a critical role. At longer residence time, the achieved adsorption capacities at As(V) permeate concentration of 10 μg/L (WHO guideline value) are 0.95 and 1.04 μg/mg for μGFH and μTMF, respectively. At shorter residence time of ~ 3 h, μTMF was able to treat 1.4 times more volumes of arsenic-polluted water than μGFH under the optimized experimental conditions due to its fast kinetic behavior. The outcomes of this study confirm that micro-sized iron oyxhydroxides, by-products of conventional adsorbent production processes, can successfully be employed in the proposed hybrid water treatment system to achieve drinking water guideline value for arsenic, without considerable fouling of the porous membrane. Graphical abstract

Red mud is a highly alkaline industrial solid waste generated by the alumina industry. Its stacking and disposal not only occupy land resources, but also cause environmental pollution. It is of great significance to separate and purify target compounds from complex mixtures of red mud based on their resource utilization properties. Ferrous oxalate and ferric hydroxide were prepared by oriented conversion based on redox reaction and chemical precipitation in this research. The products prepared were characterized by XRD, SEM and IR under different conditions (iron powder, ascorbic acid and photocatalysis). The results show that the two-stage acid leaching process can improve the leaching rate of iron from red mud significantly, and the experimental of oriented conversion of ferrous oxalate and ferric hydroxide from red mud with reproducibility. The sample prepared with ascorbic acid as reducing agent has the best effect. The development of this experiment provides a new idea for the directional classification and purification of ferrous oxalate and iron hydroxide from red mud.

Research on the use of nanoparticles for pollutant adsorption has received increasing attention. However, there are problems with the recovery and persistence of nanoparticles in pollutant removal processes. Herein, ferric hydroxide-bacterial cellulose (BC) nanocomposites with high porosity were synthesized via in situ mineralization and employed to efficiently remove glyphosate from wastewater. The prepared BC@Fe(OH)3 nanocomposites were comprehensively characterized using scanning electron microscopy, Fourier transform infrared spectroscopy, and X-ray photoelectron spectroscopy, which indicated that the Fe(OH)3 nanoparticles were well positioned on the surface of BC, and the specific surface area of the BC@Fe(OH)3 nanocomposites reached 179.14 m2 g−1, with a pore volume and average pore diameter (0.766 m2 g−1 and 21.6 nm) much larger than those of pristine BC (0.412 m2 g−1 and 12.6 nm) and unsupported Fe(OH)3 (0.016 m2 g−1 and 20.7 nm). Batch adsorption experiments revealed that the synthesized BC@Fe(OH)3 nanocomposites had better adsorption performance than unsupported Fe(OH)3 and pristine BC; the maximum glyphosate adsorption capacity was 180.48 mg g−1 according to the fitting results of the Langmuir adsorption isotherm. We also investigated the kinetics and adsorption mechanism of glyphosate on BC@Fe(OH)3. The results showed that the adsorption of glyphosate involved multiple physical and chemical processes, such as electrostatic interactions, ligand exchange, hydrogen bond formation, and pore diffusion. Moreover, this material showed a high reuse rate and maintained approximately 50% of its adsorption capacity after four consecutive adsorption–desorption cycles. Thus, BC@Fe(OH)3 nanocomposites are expected to be promising, eco-friendly adsorbents for effectively removing glyphosate from wastewater.

Iron cation impurities reportedly enhance the oxygen evolution reaction (OER) activity of Ni-based catalysts, and the enhancement of OER activity by Fe cations has been extensively studied. Meanwhile, Fe salts, such as iron hydroxide and iron oxyhydroxide, in the electrolyte improve the OER performance, but the distinct roles of Fe cations and Fe salts have not been fully clarified or differentiated. In this study, NiO electrodes were synthesized, and their OER performance was evaluated in KOH electrolytes containing goethite (α-FeOOH). Unlike Fe cations, which enhance the performance via incorporation into the NiO structure, α-FeOOH boosts OER activity by adsorbing onto the electrode surface. Surface analysis revealed trace amounts of α-FeOOH on the NiO surface, indicating that physical contact alone enables α-FeOOH to adsorb onto NiO. Moreover, interactions between α-FeOOH and NiO were observed, suggesting their potential role in OER activity enhancement. These findings suggest that Fe salts in the electrolyte influence OER performance and should be considered in the development of OER electrodes.

To study the removal of nitrogen and phosphorus from low C/N ratio rural domestic sewage by Fe–C mixed fillers, in this study, a laboratory-scale iron-carbon microelectronics system (IC-ME) and an activated carbon system (AC) were established to purify rural domestic sewage with a C/N ratio of 1.9–4.4. The results show that the removal rates of NO3−-N, total nitrogen (TN), and total phosphorus (TP) of the IC-ME system are 89.25%, 80.64%, and 92.2%, respectively. During the hydraulic retention time (HRT) of 36 h, when the C/N ratio is 1.9. They are much higher than those of the AC system (NO3−-N: 31.09%; TN: 64.15%; TP: 26.34%). All the indicators reached the first class B standard of “Pollutant Discharge Standard of Urban Sewage Treatment Plant” (GB18918-2002) and the first-level discharge standard of Guangxi’s “Water Pollutants Discharge Standard for Rural Domestic Sewage Treatment Facilities” (DB45/2413–2021). Micro-electrolysis can provide electrons for denitrification, further facilitating the process. In addition, the effective phosphorus removal is caused primarily by the corrosion of the iron anodes, which produces Fe2+ and Fe3+ ions. These ions then react with PO43− to form phosphate precipitates, and at the same time, create Fe(OH)3/Fe(OH)2 colloids with OH− in the water, which can adsorb and flocculate organic phosphorus and PO43−. Based on high-throughput sequencing studies, the microbial abundance of Bacteroidetes, Chloroflexi, and Firmicutes is much higher in the IC-ME system than in the AC system. Overall, the IC-ME process provides a new strategy for treating domestic wastewater in rural areas with low C/N ratios.

The control of the growth of hematite nanoparticles from iron chloride solutions under hydrothermal conditions in the presence of two different structure promoters has been studied using a range of both structural and spectroscopic techniques including the first report of photo induced force microscopy (PiFM) to map the topographic distribution of the structure-directing agents on the developing nanoparticles. We show that the shape of the nanoparticles can be controlled using the concentration of phosphate ions up to a limit determined to be ~6 × 10−3 mol. Akaganéite (β-FeOOH) is a major component of the nanoparticles formed in the absence of structure directors but only present in the very early stages (< 8 h) of particle growth when phosphate is present. The PiFM data suggest a correlation between the areas in which phosphate ions are adsorbed and areas where akaganéite persists on the surface. In contrast, goethite (α-FeOOH) is a directly observed precursor of the hematite nanorods when 1,2-diamino propane is present. The PiFM data shows goethite in the center of the developing particles consistent with a mechanism in which the iron hydroxide re-dissolves and precipitates at the nanorod ends as hematite.

The High-Density Sludge (HDS) process is widely used for the treatment of acid mine water as it produces a sludge of high density. The aim of this study was the development of a process where iron in mine water can be removed as magnetite, to assist with rapid settling of sludge. It was concluded that Fe2+ can be removed as Fe3O4 (magnetite) by forming Fe(OH)2 and Fe(OH)3 in the mole ratio of 1:2. Magnetite can form in the absence or presence of gypsum. The settling rate of magnetite-rich sludge is substantially faster than that of ferric hydroxide-rich sludge. It is recommended that further studies be carried out on the separation of magnetite gypsum through magnetic separation.

To reveal the influencing process and mechanism of seawater intrusion on groundwater Fe in coastal zones, the local groundwater in Buzhuang Town, together with those in the neighboring area where no seawater intruded, was sampled and comparatively analyzed, and the static simulation experiments were also performed in laboratory. The local groundwater has Fe levels of 6.09–196.96 μg/L, with an average of 73.38 μg/L, but groundwater Fe levels from the neighboring area are 1.3–17.7 times of those in local groundwater. Such facts indicate the groundwater Fe levels decreased due to seawater intrusion. The groundwater Fe levels are significantly negatively correlated with pH, significantly positively correlated with Ca2+, Mg2+, and positively correlated with SO42−. The simulation experiments indicate leached Fe increases with a greater mixture of seawater, increasing concentrations of NaCl and CaCl2, but decreases with increasing NaHCO3 concentrations. Fe(OH)2 and Fe(OH)3 minerals are super-saturated because of the high pH and high OH− concentration resulting from seawater intrusion. By this way, the dissolving ability of groundwater Fe is restricted. Therefore, pH is the key factor determining groundwater Fe levels in coastal zones. Another, the decreasing of Ca2+, Mg2+ in groundwater decreases Fe levels because of the co-precipitation and deactivation of groundwater Fe. Salt effect and NaHCO3 contribute less to groundwater Fe levels because of the restriction of maximum Fe solubility by high OH− and super-saturation of Fe-bearing minerals. The influencing model of groundwater Fe levels under the effect of seawater intrusion is forwarded.

In this study, molded fiber products (MFPs) were prepared from lignin compounded with Lewis acid-modified fibers using enzymatic hydrolysis lignin (EHL) as a bio-phenol. The fibers were modified and compounded entirely through hot-pressing. To improve the reactivity of enzymatic lignin, hydroxylated enzymatic hydrolysis lignin (HEHL) was prepared by hydroxylation modification of purified EHL with hydrogen peroxide (H2O2) and ferrous hydroxide (Fe(OH)3). HEHL was mixed uniformly with Lewis acid-modified fibers on a pressure machine and modified during the molding process. The purpose of Lewis acid degradation of hemicellulose-converted furfural with HEHL was to generate a resin structure to improve the mechanical properties of a MFPs. The microstructure of the MFP was shown to be generated by resin structure, and it was demonstrated that HEHL was compounded on Lewis acid-modified fibers during the molding process. The thermal stability of the MFP with composite HEHL did not change significantly owing to the addition of lignin and had higher tensile strength (46.28 MPa) and flexural strength (65.26 MPa) compared to uncompounded and modified MFP. The results of this study are expected to promote the application of high lignin content fibers in molded fibers.