Phosphate removal from industrial wastewater through in-situ Fe2+ oxidation induced homogenous precipitation: Different oxidation approaches at wide-ranged pH

Manipulating electrochemical phosphate recovery from acidic wastewater for synthesizing LiFePO4/C cathode material

Phosphorus (P) recovery from wastewater offers a sustainable solution for mitigating pollution and securing resources for applications like lithium-ion batteries, where ferric phosphate is a valuable precursor. This study evaluates iron electrolysis for P removal and recovery from acidic wastewater with high phosphate concentrations and medium Ca²⁺ levels. The results suggested that effective P removal and high-purity iron phosphate production can be achieved by varying initial pH, current density, and oxidation conditions. Importantly, slow Fe release rates (0.02-0.04 mmol L⁻¹ min⁻¹) favored ferric phosphate formation (71%-77% removal), while faster rates (0.16-0.46 mmol L⁻¹ min⁻¹) predominantly produced vivianite (∼ 65% removal). In addition, air flush can enhance dissolved oxygen flux, achieving 89% P removal under rapid Fe release but with mixed products. H₂O₂ addition improved in situ Fe(II) oxidation, achieving 92% P removal and purer ferric phosphate. Compared to chemical precipitation, which required pH adjustment and suffered from Ca co-precipitation, iron electrolysis produced purer ferric phosphate directly, without pH pre-adjustment. The recovered ferric phosphate showed excellent potential as a precursor for high-performance LiFePO₄/C cathode material. These findings position iron electrolysis as a promising approach for sustainable P recovery and resource valorization from wastewater.

Ternary removal of Zn, Ni, and Mn from metal industry wastewater using soybean hulls as adsorbents

With the growth of the metalworking industry, effective control of wastewater with phosphate has become a global concern. This study took advantage of the abundant supply of natural soybean hulls as an adsorbent for the direct treatment of wastewater, aiming to remove Ni, Zn, and Mn from real wastewater produced during the phosphating stage of the metalworking industry to address this issue. Soybean hulls presented a specific surface area of 0.31 m2 g-1, average diameter of 0.2705 mm, and a pH value for PCZ of 6.43 at 25 °C. Real wastewater was acidic (pH 3.68) with COD of 1270 mg L-1 and highly concentrated in Ni, Mn, and Zn (343.45 mg L-1, 818.6 mg L-1, and 953.85 mg L-1, respectively). It was observed that the process depended on the adsorbent dosage, which can be linked to the low surface area of the material. The optimized pH value was found to be the natural pH of the effluent, which varied between 3 and 4. The average removal rates were 24.5% for Ni, 28.6% for Zn, and 16.5% for Mn, corresponding to the respective removal of 84.15, 135.07, and 272.80 mg L-1 in a ternary system. The maximum adsorption capacities were observed at 50 °C, estimated as 3.125 mg g-1 for Ni, 14.128 mg g-1 for Zn, and 7.8 mg g-1 for Mn. When evaluating the process kinetics, it was observed that adsorption capacity increased significantly during the initial 60 min, followed by a slower rate until saturation. The pseudo-first-order model provided the best fit for Ni adsorption, while Zn and Mn demonstrated the best fit with the pseudo-second-order model. This trend possibly occurred due to the different initial concentrations of each metal, which has shown to be a key factor in mass-driven adsorption mechanisms. Thus, using raw soybean hulls can be considered a viable alternative for coupling adsorption as a low-cost step to other treatment methods for metalworking wastewater.

Valence fixable ferrozine gel rod combined with smartphone for facile determination of redox-active Fe2+ in environmental water

Ferrous ion (Fe2+) can indicate the redox situation of water and also plays an important role in maintaining the ecological balance of water bodies. However, due to the redox-active property of Fe2+, it is still a huge challenge to sensitively and accurately determine Fe2+ especially in interstitial water. Herein, we prepared a ferrozine gel rod for valence fixation during sampling and subsequent smartphone-based detection of Fe2+. The electrode potential of the redox pair can be varied through the formation of Fe2+-ligand complexes, and when Ecomplex was higher than [Formula: see text] , the oxidation of Fe2+ by O2 was hindered, thus achieving the valence fixation of Fe2+. Six ligands were screened, and it was found that ferrozine could effectively increase the redox potential after complexing with Fe2+, and also exhibits an obvious color change while fixing the valence of Fe2+. To facilitate Fe2+ detection, a cross-linked porous polymer gel rod prepared by acrylamide and sodium alginate was used to encapsulate the ferrozine molecules. The ferrozine gel rod enabled fixation the valence of Fe2+ longer than 30 days, and the resulted purple-red color was pictured and analyzed by a smartphone. Ultimately, the developed ferrozine gel rod sensing system was able to achieve sensitive and linear detection of Fe2+ in the range of 1-200 μM with the limit of detection as low as 0.33 μM, and it also exhibited excellent selectivity and anti-interference ability. The accuracy and reliability of the method was verified by the determination of Fe2+ in spiked water samples and certified standard reference water samples. Finally, the ferrozine gel rod sensing system was successfully applied to in-situ detection of Fe2+ in interstitial water, overlying water and upper water of lake and river. This facile system that enabled valence fixation and fast detection is promising for detection of Fe2+ in environmental waters.

Superior performance of oxygen vacancy-enriched Cu-Co3O4/urushiol-rGO/peroxymonosulfate for hypophosphite and phosphite removal by enhancing singlet oxygen

The treatment of wastewater containing hypophosphite [P(I)] and phosphite [P(III)] is challenged by limitations of traditional Fenton oxidation such as low efficiency, secondary pollution and high costs. This study introduced a facile solvent-thermal method to synthesize Cu-Co3O4 nanoparticles uniformly loaded on graphene (Cu-Co3O4/U-rGO) through the reduction and coordination effects of urushiol (U). As prepared Cu-Co3O4/U-rGO exhibited excellent activity in activating peroxymonosulfate (PMS) for the oxidation of P(I)/P(III) to phosphate [P(V)] (0.229 min-1), along with high stability and reusability (91.5 % after 6 cycles), low metal leaching rate (Co: 0.2 mg/L, Cu: 0.05 mg/L), insensitivity to common anions in water and a wide pH range (3-11). The activation mechanism involved the synergistic effects from both urushiol and graphene, which promoted redox of Cu+/Cu2+ and Co2+/Co3+ and induced abundant oxygen vacancies for PMS activation to produce singlet oxygen. Furthermore, the Cu-Co3O4/U-rGO/PMS was also excellent in the oxidative removal of organic phosphorus. This study is expected to advance strategies for the treatment of P(I)/P(III)-rich wastewater and provide new insights for the development of low-cost, highly efficient heterogeneous catalysts with abundant oxygen vacancies.

Impact of sand media continuous drying and rewetting cyclic on nutrients transformation performance from reclaimed wastewater effluent at soil aquifer treatment

Reusing reclaimed wastewater became a practical resource for water utilization in groundwater recharge and irrigation activities. However, the quality of reclaimed wastewater needs improvement to meet the environmental regulations and reduce contamination risks. A laboratory-scale study simulated a soil aquifer treatment (SAT) system, exploring the synergistic effects of wet and dry cycles alongside key physicochemical parameters on pollutant removal efficiency using a glass column filled with quartz sand as the filtration medium. The investigation focused on the cyclic wetting and drying phases to unravel their impact on removing NH4+, NO3-, and PO43-. The synthetic wastewater introduced into the system exhibited varying pollutant concentrations during wet and dry periods, influenced by dynamic soil water content (WC%), pH, dissolved oxygen (DO), and oxidation-reduction potential (ORP). The high removal rates of 93% for PO43- and 43% for Total N2 demonstrate the system's capability to reduce concentrations significantly under dynamic alternating between wet and dry conditions. Results unveiled that the wet period consistently yielded higher removal rates for N2 species. Interestingly, for PO43-, the dry periods demonstrated a higher removal efficiency. Moreover, the study identified an average NO3- production during the experimental phases as a byproduct of nitrification. The average NO3- production in wet periods was 2.5 mg/L, whereas it slightly decreased to 2.2 mg/L in dry periods. These findings underscore the nuanced influence of wet and dry conditions on specific pollutants within SAT systems. Applying the logistic regression model and principal component analysis demonstrated the statistical significance of WC, pH, DO, and ORP in predicting wet/dry conditions, providing quantitative insights into their influential roles on the nutrient dynamic concentrations. This study contributes valuable data to our understanding of SAT systems, offering practical implications for designing and implementing sustainable wastewater treatment practices and pollution management across diverse environmental contexts.

Electrochemical Removal of Metal-Organic Complexes in Metal Plating Wastewater: A Comparative Study of Cu-EDTA and Ni-EDTA Removal Mechanisms

Cu and Ni complexes with ethylenediaminetetraacetic acid (Cu/Ni-EDTA), which are commonly present in metal plating industry wastewaters, pose a serious threat to both the environment and human health due to their high toxicity and low biodegradability. In this study, the treatment of solutions containing either or both Cu-EDTA and Ni-EDTA using an electrochemical process is investigated under both oxidizing and reducing electrolysis conditions. Our results indicate that Cu-EDTA is decomplexed as a result of the cathodic reduction of Cu(II) with subsequent electrodeposition of Cu(0) at the cathode when the cathode potential is more negative than the reduction potential of Cu-EDTA to Cu(0). In contrast, the very negative reduction potential of Ni-EDTA to Ni(0) renders the direct reduction of EDTA-complexed Ni(II) at the cathode unimportant. The removal of Ni during the electrolysis process mainly occurs via anodic oxidation of EDTA in Ni-EDTA, with the resulting formation of low-molecular-weight organic acids and the release of Ni2+, which is subsequently deposited as Ni0 on the cathode. A kinetic model incorporating the key reactions occurring in the electrolysis process has been developed, which satisfactorily describes EDTA, Cu, Ni, and TOC removal. Overall, this study improves our understanding of the mechanism of removal of heavy metals from solution during the electrochemical advanced oxidation of metal plating wastewaters.

Synthesis of co-pyrolyzed biochar using red mud and peanut shell for removing phosphate from pickling wastewater: Performance and mechanism

Iron (Fe)/iron oxide-modified biochar has practicable adsorption capability for phosphorus (P), but it is expensive. In this study, we synthesized novel low-cost and eco-friendly adsorbents co-pyrolyzed biochars using Fe-rich red mud (RM) and peanut shell (PS) wastes via a one-step pyrolysis process for removing P from pickling wastewater. The preparation conditions (heating rate, pyrolysis temperature, and feedstock ratio) and P adsorption behaviors were systematically investigated. In addition, a series of characterization and approximate site energy distribution (ASED) analyses were conducted to understand the P adsorption mechanisms. The magnetic biochar (BR7P3) with m (RM):m (PS) of 7:3 prepared at 900°C and 10 °C/min had a high surface area (164.43 m2/g) and different abundant ions (including Fe3+, and Al3+). In addition, BR7P3 exhibited the best P removal capability (142.6 mg/g). The Fe2O3 from RM was successfully reduced to Fe0, which was easily oxidized as Fe3+ to precipitate with H2PO4-. The electrostatic effect, Fe-O-P bonding, and surface precipitation were the main mechanisms of P removal. ASED analyses revealed that high distribution frequency and solution temperature led to a high P adsorption rate of the adsorbent. Therefore, this study provides new insight into the waste-to-wealth strategy by transforming PS and RM into mineral-biomass biochar with excellent P adsorption capability and environmental adaptability.

Phosphate removal using dolomite modified with ultrasound: mathematical and experimental analysis

We studied the dolomite modified using an ultrasound bath and its application in phosphate removal. The modification was applied to improve the physicochemical properties of the dolomite and then to enhance its suitability as an adsorbent solid. The settings for analyzing the adsorbent modification were bath temperature and sonication time. The modified dolomite was characterized by electron microscopy, N₂ adsorption/desorption, pore size, and X-ray diffraction. To grasp the pollutant's adsorption mechanism more precisely, we used experimental research and mathematical model analysis. Design of Experiments was conducted to determine the ideal circumstances. In addition, the Bayesian method of Markov Chain Monte Carlo was used to estimate the isotherm and kinetic model parameters. A thermodynamic study was done to investigate the adsorption mechanism. Results show that the surface area of the modified dolomite was greater, enhancing its adsorption properties. To remove more than 90% of the phosphate, the optimal operational parameters for the adsorption were pH 9, 1.77 g of adsorbent mass, and 55 minutes of contact time. The pseudo-first-order, Redlich-Peterson and Sips models presented a good fit to the experimental data. Thermodynamics suggested a spontaneous and endothermic process. The mechanism suggested that physisorption and chemisorption could be involved in phosphate removal.

Enhanced adsorption of phosphate from pickling wastewater by Fe-N co-pyrolysis biochar: Performance, mechanism and reusability

A one-step method of preparation using a novel nitrogen (N)-doped Fe-rich biochar (N₅-CB) resulted in a maximum adsorption capacity (314.52 mg/g) compared with Fe-rich biochar (CB, 104.044 mg/g). It can be used to adsorb phosphate (P) efficiently. Additionally, the adsorption kinetics, isotherms, and thermodynamics indicated that the adsorption of P onto N₅-CB was mainly mediated via multilayer coverage, endothermic, spontaneous, and physical mechanisms. The main adsorption mechanisms include Fe-P precipitation, FeOP bonding, and electronic effect. Further, the highly active Fe-N_x sites and graphitic N induced by N doping were the dominant driving force underlying enhanced P adsorption. Active Fe-N_x sites resulted in a positively-charged carbon structure and P absorption via electrostatic effect. Based on the simple method of pyrolysis, N₅-CB can be used in P removal from pickling wastewater with excellent adsorption capacity and remarkable recyclability.

Enhanced immobilization of phosphate by ferrihydrite during the photoreductive dissolution process

The surface interactions of iron (hydr)oxides with various environmental chemicals play a vital role in controlling their environmental transport and fate. As a bioessential element, phosphorus and its biogeochemical cycling are usually controlled by its adsorption on iron (hydr)oxides. Photoreductive dissolution of iron (hydr)oxides can change their surface structure and properties, but its influence on the adsorption of phosphate remains unknown. Here, an enhanced removal of phosphate during the photoreductive dissolution of ferrihydrite (Fh) was investigated. The K_d value of phosphate adsorption on Fh under light irradiation is evidently larger than that in the dark (21 vs 13 L/g). Zeta potential determination in combination with X-ray photoelectron spectroscopy analysis suggested that the released Fe²⁺ from Fh surface during photoreductive dissolution can be oxidized to Fe³⁺, which then co-adsorb with phosphate back to Fh surface, enhancing the immobilization of phosphate on Fh. In situ ATR-FTIR results disclosed that light irradiation could further facilitate the formation of ternary complexes and surface precipitation on Fh, even after the increment of phosphate adsorption becoming negligible in the dark, and the relative content of surface precipitation increased evidently. The desorption ratio of phosphate from the irradiated Fh sample was reduced, which should be attributed to a high content of surface precipitation that can tightly bind phosphate on Fh. The findings of this study highlight an important yet previously unappreciated pathway that light irradiation can enhance the immobilization of phosphate on iron (hydr)oxides.

Characteristics of the Properties of Absodan Plus Sorbent and Its Ability to Remove Phosphates and Chromates from Aqueous Solutions

The aim of the research was to characterize the parameters of the diatomite sorbent Absodan Plus as well as to assess its suitability for the adsorption of chromates and phosphates from acidic aqueous solutions simulating the conditions occurring in some types of industrial wastewater. The scope of the research includes XRD, SEM, BET, and PZC analyses, and 3D observation of commercial diatomite granules and batch tests to determine the constants of kinetics and the equilibrium of chromates and phosphates adsorption. Absodan Plus is a diatomite commercial material containing an amorphous phase (33%) and is also the crystalline phase of quartz, hematite, and grossite. The material is macro- and mesoporous and its specific surface area is about 30 m²/g. Its PZC is around pH = 5.5–6.0 and in an acidic environment is able to adsorb the anions. The saturation of the adsorbent surface with molecules of the adsorbed substance occurs after 2 h for chromates and 2.5 h for phosphates. The maximum adsorption capacity of Absodan Plus in terms of phosphorus and chromium amounts to 9.46 mg P/g and 39.1 mg Cr/g, respectively. As shown by XRD analysis, Absodan Plus contains an admixture of hematite, which can support the removal of chromium and phosphorus.

Enhanced removal of P(V), Mo(VI) and W(VI) generated oxyanions using Fe-MOF as adsorbent from hydrometallurgical waste liquid: Exploring the influence of ionic polymerization

Environmentally hazardous P(V), Mo(VI) and W(VI) generated oxyanions exist widely in the waste liquid of nonferrous hydrometallurgy. In this work, Fe-MOF material was simply prepared via solvothermal synthesis and then used as an adsorbent to remove P(V), Mo(VI) and W(VI) oxyanions from hydrometallurgical waste liquid. Several important parameters, including solution pH, oxyanion concentration, contact time, adsorbent amount, temperature and coexistent heavy metal ions, were systematically investigated. The results demonstrate that adsorption process was almost pH-independent over a broad range of pH 3.0-10.0. The adsorption efficiency was strongly associated with the chemical species of oxyanions. The higher polymerisation degree of oxyanions was more favourable for removal efficiency. Additionally, the maximum removal efficiencies for P(V), Mo(VI) and W(VI) oxyanions under optimum conditions were approximately 100%. Furthermore, the adsorption kinetics and isotherms of oxyanions on the adsorbent separately belonged to the pseudo-second-order and Langmuir isotherm models. XPS analysis revealed that inner-sphere complexation played a dominant role in the adsorption removal process. Fe-MOFs with pH-independent properties, abundant binding sites and high stability are prospective adsorbents for treating waste liquids in the hydrometallurgical industry.

Phosphate removal from aqueous solution by electrochemical coupling siderite packed column

The use of iron species to remove PO₄^3- is widely used, and the fresh Fe³⁺ produced in situ demonstrate better effect on the removal of PO₄^3- in many researches. Therefore, in order to develop a simpler and more efficient method for PO₄^3- removal, we designed an easy operation by electrochemically dissolving siderite to produce fresh Fe³⁺ in situ for PO₄^3- removal from wastewater. Results showed that current intensity at 20 mA, initial pH at 6, initial PO₄^3- concentration at 1 mM and influent flow rate at 2.5 mL min^-1 were the best parameters for removing PO₄^3-, ensuring that the PO₄^3- concentration of effluent can be kept below 1 mg L^-1 through the electrochemical system. Different from other studies, a large amount of Fe²⁺ can be dissolved from natural minerals without adding H⁺ to the system and Fe³⁺ species are generated in situ from the oxidation of the Fe²⁺ without using a specific oxidizer. This electrochemical treatment method with siderite as a packed column can be used as a new method of high efficiency, simple operation and low-cost for treating eutrophic water bodies.

Simultaneous reduction and immobilization of Cr(VI) in seasonally frozen areas: Remediation mechanisms and the role of ageing

Among the toxic metals, hexavalent chromium [Cr(VI)] has attracted much attention due to its high mobility and toxicity, rendering considerable challenges for long-term remediation. In this study, the soil was collected from a dichromate contaminated industrial site in Liaoning Province, a seasonally frozen area in northern China, and subjected to frequent freeze-thaw cycles. Three additives, including (i) ferrous sulfate; (ii) calcium polysulfide; and (iii) combined biochar and calcium polysulfide were applied to reduce and immobilize Cr(VI) in the soils. The samples underwent 28 days of incubation followed by 16 freeze-thaw cycles. The toxicity characteristic leaching procedure (TCLP) and simulated acid rain leaching were adopted to test the remediation performances. It was observed that all three treatments can significantly reduce and immobilize Cr(VI) after short-term incubation, while biochar with abundant functional groups could adsorb and reduce Cr(VI) effectively. Notably, the concentration of Cr(VI) in TCLP leachates after incubation in combined treatment decreased by 67.87% and 37.27%, respectively, compared with the application of ferrous sulfate or calcium polysulfide alone. Freeze-thaw cycles induced the disintegration of soil particles and increased the risk of contaminant mobilization. Conversely, biochar particles has become finer and even produced nanoparticles with ageing, accompanied by the increase in oxygen-containing surface functional groups. Additionally, the specific surface area increased with the pyrolysis of biochar, which further enhanced the retention of soil colloidal particles and suppressed the migration of contaminants. Therefore, the cumulative release of Cr(VI) in the combined treatment (i.e., 10.97 ~ 32.97 mg/kg) was much lower than that of the other two treatments after freeze-thaw ageing. Overall, the combination of biochar and calcium polysulfide displayed advantages in the reduction and immobilization of Cr(VI), and offered a long-term, effective strategy for the remediation of Cr(VI) contaminated soils in cold regions.