Mechanistic insight into pH-dependent adsorption and coprecipitation of chelated heavy metals by in-situ formed iron (oxy)hydroxides

Simultaneous Cr(III)-EDTA decomplexation and Cr(III) sequestration by catalytic ozonation with sulfidated zero-valent iron: Kinetics and removal mechanism

Efficient removal of recalcitrant Cr(III)-organic complexes is always challenged by slow decomplexation process and the possible accumulation of highly toxic Cr(VI). Herein, a sulfidated zero-valent iron coupled ozonation strategy (S-ZVI/O3) was proposed to achieve efficient decomplexation and simultaneous abatement of Cr(III)-EDTA without Cr(VI) accumulation. Results revealed that S-ZVI could catalyze O3 to enhance removal of Cr(III)-EDTA and total organic carbon. Moreover, 88.3 % of total Cr was sequestrated by S-ZVI/O3 and the corresponding kinetic constant was 3.1 times higher than that of ZVI/O3. Mechanistically, electron paramagnetic resonance and probing tests verified HO• was the dominant reactive oxidation species for decomplexation of Cr(III)-EDTA in both S-ZVI/O3 and ZVI/O3. More attractively, it was found that structural Fe(II) was the major O3 activator in S-ZVI/O3, whereas dissolved Fe2+ accounted for O3 catalyzation in ZVI/O3. The X-ray absorption spectroscopy analysis revealed that sulfidation treatment could enhance corrosion of ZVI with the formation of sufficient Fe(II) species. The in-situ formed Fe(II) could not only transform undesired Cr(VI) back to Cr(III) but also co-precipitate with Cr(III) to form solid Fe-Cr hydroxides. Besides Cr(III)-EDTA, S-ZVI/O3 is also applicable to other EDTA complexed heavy metals. This work would provide a new method for the heavy metal complexes removal from water.

The dual selective adsorption mechanism on low-concentration Cu(II): Structural confinement and bridging effect

In this study, sulfur was introduced into the graphene oxide-based aerogel system based on the theory of bridging effect, and an oxygen-sulfur synergistic system was established to realize the dual-mechanism selective adsorption to low-concentration Cu(II) with slit structure and targeted binding sites. Based on the difference of chemical properties between organic acids and surfactants, the surface of graphene oxide (GO) was functionalized to realize the regulation of the order of its lamellar structure and the construction of carbon defects. On this basis, sulfur source modified GO-based aerogel was created to accomplish selective adsorption to low-concentration Cu(II) by combining the self-accumulation of montmorillonite and GO with the cross-linking mechanism of Ca(II) and sodium alginate. Based on the density functional theory, the formation process of radicals on the material's surface both prior to and following the adsorption to Cu(II) was simulated, and the effective improvement on the catalytic ability of the material after loading Cu(II) was verified. This means that using Cu(II) saturated adsorbent as a photocatalyst to degrade organic pollutants, is a promising reuse strategy for hazardous waste. The above research provides a new research idea for the subsequent removal of low-concentration metal ions and the potential application of hazardous wastes.

Leachability Characteristic of Heavy Metals and Associated Health Risk Study in Typical tin Mine Tailings

In this study, the leachability characteristics of heavy metals (Pb, Cu, Ni, As, and Cd) in tailing-contaminated soils were analyzed to assess contamination levels and health risks. The distribution and sources of heavy metals were also examined. Our findings revealed that, although currently the exposed tailings themselves posed minimal risk of leaching, the soils in the mine vicinity were significantly contaminated with Pb, Cu, As, and Cd as the result of nearly 100 years of mining history. With increasing soil depth (0-4, 4-6, 6-10 m), the concentrations of these heavy metals (excluding Ni) exhibited an initial rise followed by a sharp decline. Furthermore, APCS-MLR (absolute principal component score-multiple linear regression) model indicated that Cd, Pb, Cu, and As were primarily influenced by mining activities, whereas Ni was predominantly determined by the soil parent material. Health risk assessments highlighted that As was a major contributor to carcinogenic and non-carcinogenic risks, and children were at higher risk than adults.

The role of interface interaction between iron/sulfate-reducing bacteria (ISRB) and goethite in sulfur (S) redox cycling couple with Cd immobilization

Microbial sulfate reduction leads to the formation of various chalcophile trace metal sulfides, thereby immobilizing chalcophile trace metals in sediments. Iron/sulfate-reducing bacteria (ISRB) are ubiquitous in soils and sediments, its ability to reduce Fe(III) (oxyhydr)oxides and biogeochemical significance have attracted much attention. This research investigated the effect of the goethite and ISRB induced S cycle on cadmium mobility. The experiment demonstrated that the removal of Cd(II) in coexistence of ISRB19 and goethite was more efficiently than their individual components. Combined with X-ray absorption spectroscopy (XAS), transmission electron microscopy (TEM), Raman spectra and X-ray photoelectron spectroscopy (XPS), conclusions can be drawn that goethite enhanced Cd(II) retention by ISRB, which was attributed to the formation of metabolism product during interaction between ISRB19 (Enterobacter chengduensis) and goethite. Our results revealed the interaction of goethite and ISRB in S cycling under anaerobic conditions with its implications for Cd(II) remediation.

Artificial intelligence-driven assessment of critical inputs for lead adsorption by agro-food wastes in wastewater treatment

Due to environmental concerns and economic value, the adsorption process using agricultural wastes is one of the promising methods to remove lead (Pb) from contaminated water. The relationships between agricultural waste properties, adsorption conditions, and the maximum Pb adsorption capacity of selected adsorbents have not been adequately explored. A thorough understanding of these interactions is crucial for optimizing adsorption processes and enhancing the efficiency of agricultural wastes as sustainable adsorbents. To assess Pb adsorption by agricultural wastes and identify the key influencing factors, three artificial intelligence techniques, namely Extreme Learning Machine (ELM), Adaptive Nuro-Fuzzy Inference Systems (ANFIS), and Group Method of Data Handling (GMDH) have been employed in this study. Seven input variables, namely time, ratio, initial ion concentration, type of adsorbents from agricultural wastes, pH, temperature, and agitation speed, from 771 data points were used as inputs for model development, while the quantity of Pb adsorbed was chosen as target parameter. To identify the best input combinations with one to seven variables, 127 models were defined and analyzed using ELM integrated with the cross-validation technique. The results highlighted that the initial ion concentration is the most critical factor in enhancing heavy metal adsorption, and temperature is the least important factor. The top models, utilizing one to seven input variable(s), were then modeled with ANFIS and GMDH. Subsequently, all three models were compared. The GMDH model with four input variables (initial ion concentration, type of adsorbent, time, and agitation speed) demonstrated the highest performance in terms of accuracy and simplicity.

Selective adsorption of Cu(II) on amino-modified alginate-based aerogel: As a catalyst for the degradation of organic contaminant

In this study, amino-modified graphene oxide(NGO) was prepared by introducing amino functional groups. Based on the cross-linking between Ca(II) and sodium alginate (SA), associated with dense slit-like pore resulted from the nano-sheet accumulation of NGO and montmorillonite (MMT), composite aerogels (NGM) with stable pore structure were constructed, thus it realized the selective recovery of hydrated copper ions in complex wastewater systems. Raman analysis and density functional theory calculation confirmed the construction of amino-modified defect GO and significantly improved its chemical reactivity, which laid the foundation for the construction of slit pore structure of NGM (SEM can confirm). At the same time, it proposed that the good selective adsorption of Cu(II) on NGM was related to the synergism of strong electrostatic force, ion exchange and complexation based on the characterizations of FT-IR and XPS. In order to realize the value-added utilization of NGM aerogel (NGMC) after adsorbing Cu(II), NGMC was used as a catalyst to degrade organic pollutants in wastewater. Systematic experiments shown that NGMC can degrade organic pollutants with a degradation efficiency >80 %. In summary, NGM had a broad application prospect for selective recovery of Cu(II) from complex wastewater systems without second pollution.

Investigation of the migration of natural organic matter-iron-antimony nano-colloids in acid mine drainage

Colloids can potentially affect the efficacy of traditional acid mine drainage (AMD) treatment methods such as precipitation and filtration. However, it is unclear how colloids affect antimony (Sb) migration in AMD, especially when natural organic matter (NOM) is present. To conduct an in-depth investigation on the formation and migration behavior of NOM, iron (Fe), Sb and NOM-Fe-Sb colloids in AMD, experiments were performed under simulated AMD conditions. The results demonstrate significant variations in the formation of NOM-Fe-Sb colloids (1-3-450 nm) as the molar ratio of carbon to iron (C/Fe) increases within acidic conditions (pH = 3). Increasing the C/Fe molar ratio from 0.1 to 1.2 resulted in a decrease in colloid formation but an increase in particulate fraction. The distribution of colloidal Sb, Sb(III), and Fe(III) within the NOM-Fe-Sb colloids decreased from 68 % to 55 %, 72 % to 57 %, and 68 % to 55 %, respectively. Their distribution in the particulate fraction increased from 28 % to 42 %, 21 % to 34 %, and 8 % to 27 %. XRD, FTIR, and SEM-EDS analyses demonstrated that NOM facilitates the formation and crystallization of Fe3O4 and FeSbO4 crystalline phases. The formation of the colloids depended on pH. Our results indicate that NOM-Fe-Sb colloids can form when the pH ≤ 4, and the proportion of colloidal Sb fraction within the NOM-Fe-Sb colloids increased from 9 % to a maximum of 73 %. Column experiments show that the concentration of NOM-Fe-Sb colloids reaches its peak and remains stable at approximately 3.5 pore volumes (PVs), facilitating the migration of Sb in the porous media. At pH ≥ 5, stable NOM-Fe-Sb colloids do not form, and the proportion of colloidal Sb fraction decreases from 7 % to 0 %. This implies that as pH increases, the electrostatic repulsion between colloidal particles weakens, resulting in a reduction in the colloidal fraction and an increase in the particulate fraction. At higher pH values (pH ≥ 5), the repulsive forces between colloidal particles nearly disappear, promoting particle aggregation. The findings of this study provide important scientific evidence for understanding the migration behavior of NOM-Fe-Sb colloids in AMD. As the pH gradually shifts from acidic to near-neutral pH during the remediation process of AMD, these results could be applied to develop new strategies for this purpose.

Simultaneous chelated heavy metals removal and sludge recovery through titanium coagulation: From waste to resource

Green methods for chelated heavy metals treatment and recovery are essential for coordinated development of resources and environment. Herein, a simple and competent method, titanium salt (TiCl4) coagulation was developed to remove and recycle chelated heavy metals. Our results revealed that this method proved to be effective for metals-citrate [Cu(II), Ni(II), Zn(II) and Cr(VI)], achieving removal efficiencies of 95 %, 92 %, 99 %, and 99 % within 30 min, surpassing direct alkaline precipitation and well-used Fe(III) coagulation. Whereafter, the copper-containing sludge was successfully transformed into copper-doped titanium dioxide (TiO2) photocatalysts by facile calcination. Through comprehensively investigating physicochemical properties by a suite of characterization techniques, we confirmed that doping of Cu induced bandgap narrowing, high specific surface area as well as the formation of oxygen vacancy. Accordingly, the recycling photocatalysts showed remarkable enhanced photocatalytic performance than the pristine TiO2, achieving improvement in the degradation efficiency of 82 %, 61 % and 67 % for carbamazepine(CBZ), bisphenol A (BPA) and methyl orange (MO). In addition, both radical (OH and O2-) and non-radical (1O2 and h+) pathways synergistically contributed to the removal of organic pollutants during photocatalysis. Ultimately, based on economic feasibility assessment and life cycle assessment (LCA), the copper-containing titanium coagulation sludge reuse for photocatalyst could bring lower carbon emissions, reduced environmental risks and higher economic benefits. The elucidation of this study provides new insights into the removal and recycle of chelated heavy metals from wastewater by using an environment-friendly and cost-effective method.

Enhanced Oxidation of Organic Compounds by the Ferrihydrite-Ferrate System: The Role of Intramolecular Electron Transfer and Intermediate Iron Species

Previous studies mostly held that the oxidation capacity of ferrate depends on the involvement of intermediate iron species (i.e., FeIV/FeV), however, the potential role of the metastable complex was disregarded in ferrate-based heterogeneous catalytic oxidation processes. Herein, we reported a complexation-mediated electron transfer mechanism in the ferrihydrite-ferrate system toward sulfamethoxazole (SMX) degradation. A synergy between intermediate FeIV/FeV oxidation and the intramolecular electron transfer step was proposed. Specifically, the conversion of phenyl methyl sulfoxide (PMSO) to methyl phenyl sulfone (PMSO2) suggested that FeIV/FeV was involved in the oxidation of SMX. Moreover, based on the in situ Raman test and chronopotentiometry analysis, the formation of the metastable complex of ferrihydrite/ferrate was found, which possesses higher oxidation potential than free ferrate and could achieve the preliminary oxidation of organics via the electron transfer step. In addition, the amino group of SMX could complex with ferrate, and the resulting metastable complex of ferrihydrite/ferrate would combine further with SMX molecules, leading to intramolecular electron transfer and SMX degradation. The ferrate loss experiments suggested that ferrihydrite could accelerate the decomposition of ferrate. Finally, the effects of pH value, anions, humic acid, and actual water on the degradation of SMX by ferrihydrite-ferrate were also revealed. Overall, ferrihydrite demonstrated high catalytic capacity, good reusability, and nontoxic performance for ferrate activation. The ferrihydrite-ferrate process may be a green and promising method for organic removal in wastewater treatment.

Adsorption of potentially harmful elements by metal-biochar prepared via Co-pyrolysis of coffee grounds and Nano Fe(III) oxides

Nano Fe(III) oxide (FO) was used as an amendment material in CO₂-assisted pyrolysis of spent coffee grounds (SCG) and its impacts on the syngas (H₂ & CO) generation and biochar adsorptive properties were investigated. Amendment of FO led to 153 and 682% increase of H₂ and CO in pyrolytic process of SCG, respectively, which is deemed to arise from enhanced thermal cracking of hydrocarbons and oxygen transfer reaction mediated by FO. Incorporation of FO successfully created porous structure in the produced biochar. The adsorption tests revealed that the biochar exhibited bi-functional capability to remove both positively charged Cd(II) and Ni(II), and negatively charged Sb(V). The adsorption of Cd(II) and Ni(II) was hardly deteriorated in the multiple adsorption cycles, and the adsorption of Sb(V) was further enhanced through formation of surface ternary complexes. The overall results demonstrated nano Fe(III) oxide is a promising amendment material in CO₂-assisted pyrolysis of lignocellulosic biomass for enhancing syngas generation and producing functional biochar.

Preparation of a novel hydrogel of sodium alginate using rural waste bone meal for efficient adsorption of heavy metals cadmium ion

Adsorption has been an important method for removing heavy metals from industrial wastewater. However, there has been a lack of an environmentally friendly, low-cost, biodegradable and easily recyclable material. China produces bones are not fully utilized leads to a waste of resources Therefore, efficient application of bone meal (BM) for remediation of contaminants in water would provide a promising alternative for resource utilization of bones. In this paper, we use a combination of BM and sodium alginate (SA) to prepare a novel BM/SA/calcium ion (BM/SA/Ca²⁺) double cross-linked composite hydrogel (BMSAH). Enhance the mechanical structure of SA while making the BM easy to recycle and reuse. The morphology and structure of the BMSAH were characterized using FT-IR spectroscopy and SEM-EDS. suggesting that the BMSAH can provide a larger specific surface area and high number of adsorption sites. The effects of the solution pH, ionic strength and contact time on the adsorption capacity of the BMSAH were investigated in depth, Under different conditions, BMSAH has a strong adsorption capacity of >90 %. XPS and FT-IR analysis showed that Cd²⁺ was adsorbed mainly via coordination interactions and hydrogen bonds with the carboxyl groups and nitrogen atoms in the BMSAH. A pseudo-second-order kinetic model, particle diffusion model and Isothermal adsorption lines indicate that the surface of the BMSAH is non-uniform suggesting that the adsorption of heavy metal ions by the BMSAH involves a combination of surface adsorption and intraparticle diffusion mechanisms, which is an overall chemical-physical adsorption process. In addition, the adsorption capacity of BMSAH remained above 90 % after three desorption cycles. Our work provides a new method for the preparation of a low-cost, high mechanical performance, biodegradable and easily recyclable physical hydrogels used for the removal of heavy metal ions.

Influence of citrate/tartrate on chromite crystallization behavior and its potential environmental implications

The ferrite process has been developed to purify wastewater containing heavy metal ions and recycle valuable metals by forming chromium ferrite. However, organic matter has an important influence on the crystallization behavior and stability of chromite synthesized from chromium-containing wastewater. We focused on the influence and effect mechanism of two typical organic acid salts (citrate (CA) and tartrate (TA)) on the process of chromium mineralization. It was found that the presence of organic matter leads to the increase of the residual content of Cr in CA system (0.50 mmol/L) and TA system (0.61 mmol/L) in the solution, and the removal of chromium was mainly due to the surface adsorption of Fe(III) hydrolysate. The decreased crystallinity of mineralized products is ascribed to the completion of organic compounds with Fe(II) and Fe(III), which hinders the formation of ferrite precursors. There was bidentate and monodentate chelation between -COO- and metal ions in the CA system and TA system respectively, which resulted in a stronger affinity between CA and iron. This study provides the underlying mechanism for Cr(III) solid oxidation by the ferrite method in an organic matter environment and is of great significance to prevent and control chromium pollution in the environment.

Enhanced decomplexation of Cu-EDTA and simultaneous removal of Cu(II) by electron beam irradiation accompanied with autocatalytic fenton-like reaction: Synergistic performance and mechanism

Widely existing heavy metal complexes with high stability and poor biodegradability are intractable to be eliminated by conventional methods. In this study, electron beam (EB) irradiation characterized by rapidly producing strong oxidizing radicals was employed to effectively decompose Cu-ethylenediaminetetraacetic acid (Cu-EDTA) with almost complete elimination at 5 kGy. In terms of heavy metal removal, EB irradiation at relatively low doses was insufficient to remove copper ions, which was only 17.2% under 15 kGy. However, with the extra addition of 8 mM H₂O₂, such an irradiation dose could result in 99.0% copper ions removal. Mechanism analysis indicated that EB irradiation combined with spontaneously induced Fenton-like reactions were responsible for its excellent performance. The prime function of EB irradiation was to destroy the structure of Cu-EDTA with in-situ produced ·OH, and the subsequent released Cu-based intermediates could activate H₂O₂ to initiate autocatalytic chain reactions, correspondingly accelerating the degradation of complexes and the liberation of metal ions. Highly oxidative ·OH and O₂·^- were demonstrated as main active species acted on different positions of Cu-EDTA to realize gradual decarboxylation, synchronously generating low molecular weight compounds. XRD and XPS analysis showed that the released copper ions were mainly precipitated in the form of CuO, Cu(OH)₂ and Cu₂(OH)₂CO₃. In general, EB/H₂O₂ was an adoptable strategy for the disposal of such refractory heavy metal complexes.