Simultaneous Sequestration of Humic Acid-Complexed Pb(II), Zn(II), Cd(II), and As(V) by Sulfidated Zero-Valent Iron: Performance and Stability of Sequestration Products

The performance and mechanism of sulfidated nano-zero-valent iron for the simultaneous stabilization of arsenic and cadmium

Co-contamination of soil and groundwater with arsenic (As) and cadmium (Cd) is widespread. Sulfidized Nanoscale Zero-Valent Iron (S-nZVI) is effective in removing As and Cd from contaminated environments. However, the mechanisms governing As and Cd removal from systems containing both species are still unclear. This study investigated the effectiveness of S-nZVI in the simultaneous removal of Cd(II) and As(III) from contaminated solutions and their interaction mechanisms. Adsorption experiments were conducted under aerobic conditions to investigate the effect of Cd(II) and As(III) on their co-immobilisation at different As(III) and Cd(II) concentrations. S-nZVI was characterised before and after the reaction to elucidate the mechanism of its simultaneous immobilisation of As(III) and Cd(II). Batch experiments revealed that the presence of Cd(II) and As(III) together considerably promotes the passivation of S-nZVI. The adsorption of Cd(II) at Cd:As = 1:3 was 198.37 mg/g, which was 27.6 % higher than that in Cd(II)-only systems, and the adsorption of As(III) at As:Cd = 1:3 was 204.05 mg/g, which was 175 % higher than that in As(III)-only systems. The results of X-ray photoelectron spectroscopy and Fourier-transform infrared spectroscopy indicated that the removal of Cd(II) and As(III) by S-nZVI involves electrostatic adsorption, complexation and oxidation reactions, amongst which electrostatic adsorption and ternary-complex generation are responsible for the synergistic effect. As and Cd ions can form two types of surface complexes with FeOH or FeS on the outer layer of S-nZVI: anionic bridging to form Fe-As-Cd and cationic bridging to form Fe-Cd-As. This investigation elucidates the synergistic action of Cd(II) and As(III) during their removal using S-nZVI. Thus, S-nZVI is a promising material for the combined removal of Cd(II) and As(III), which can mitigate environmental pollution.

Trade-off between sulfidated zero-valent iron reactivity and air stability: Regulation of iron sulfides by ammonium dihydrogen phosphate

The reactivity and stability of zero-valent iron (ZVI) and sulfidated zero-valent iron (S-ZVI) are inherently contradictory. Iron sulfides (FeSX) on the S-ZVI surface play multiple roles, including electrostatic adsorption and catalyzing reduction. We proposed to balance the reactivity and air stability of S-ZVI by regulating FeSX. Benefiting from the superior coordination and accelerate electron transport capabilities of phosphate, herein, eco-friendly ammonium dihydrogen phosphate (ADP) was employed to synthesize N, P, and S-incorporated ZVI (NPS-ZVI) and regulate the FeSX. Raman, FTIR, XPS, and density functional theory (DFT) calculations were combined to reveal that HPO42- acts as the main P species on the Fe surface. The superior reactivity of NPS-ZVI was quantified by kobs, kSA, and kM of Cr(VI), which were 210.77, 27.44, and 211.17-fold than ZVI, respectively. NPS-ZVI demonstrated excellent reusability, with no risk of secondary pollution. Critically, NPS-ZVI could effectively maintain FeSX stability under the combination of diffusion limitation and surface protection mechanisms of ADP. The superior reactivity of NPS-ZVI was attributed to the fact that ADP maintains FeSX stability and accelerates electron transport. This study provides a novel strategy in balancing the reactivity and air stability of S-ZVI and offers theoretical support for material modification.

Rapid Oxidative Dissolution of Zerovalent Iron Induced by Sulfite for Efficient Removal of Arsenate and Arsenite: Selective Formation of Scorodite

In this study, we proposed a moderate oxidation strategy for accelerating the oxidative dissolution of zerovalent iron (ZVI) using sulfite (S(IV)), thereby improving the removal of As(V) and As(III). Results revealed that, in the presence of 2.0 mM S(IV), both As(V) and As(III) were selectively converted into scorodite at pH0 3.0-7.0, while As(III) oxidation and As(V) immobilization were impressed over pH0 8.0-10.0. Batch experiments, radical quenching experiments, and electron spin resonance (ESR) measurements demonstrated that ZVI initially boosted S(IV) activation to generate SO4•-, •OH, and protons, and in turn, ZVI was further oxidized more intensely by these radicals than by oxygen. Concurrently, substantial protons derived from S(IV) oxidation neutralized hydroxyls produced by ZVI oxidation, maintaining an acidic environment conducive to the generation of scorodite rather than iron (hydr)oxides. Characterizations of X-ray diffraction (XRD), Raman, attenuated total reflectance-Fourier transform infrared (ATR-FTIR), X-ray photoelectron spectroscopy (XPS), X-ray absorption fine structure (XAFS), field emission scanning electron microscopy (FESEM), and high-resolution transmission electron microscopy (HRTEM) confirmed that scorodite was formed in situ and then exfoliated from the surface of ZVI, and approximately 75% of ZVI could still be recovered, which contributed to efficient As removal in successive runs and real As-polluted wastewater. The application of S(IV) achieved a balance among ZVI reactivity improvement, As(V)/As(III) removal, and raw material consumption, making it a promising approach for addressing arsenic contamination in wastewater treatment.

The critical role of organic matter for cadmium-lead interactions in soil: Mechanisms and risks

Understanding the interaction mechanisms between complex heavy metals and soil components is a prerequisite for effectively forecasting the mobility and availability of contaminants in soils. Soil organic matter (SOM), with its diverse functional groups, has long been a focal point of research interest. In this study, four soils with manipulated levels of SOM, cadmium (Cd) and lead (Pb) were subjected to a 90-day incubation experiment. The competitive interactions between Cd and Pb in soils were investigated using Fourier transform infrared spectrometer (FTIR), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and X-ray adsorption near-edge structure (XANES) analysis. Our results indicate that Pb competed with Cd for adsorption sites on the surface of SOM, particularly on carboxyl and hydroxyl functional groups. Approximately 22.6 % of Cd adsorption sites on humus were occupied by Pb. The use of sequentially extracted exchangeable heavy metals as indicators for environment risk assessments, considering variations in soil physico-chemical properties and synergistic or antagonistic effects between contaminants, provides a better estimation of metal bioavailability and its potential impacts. Integrating comprehensive contamination characterization of heavy metal interactions with the soil organic phase is an important advancement to assess the environmental risks of heavy metal dynamics in soil compared to individual contamination assessments.

Application of hybrid multi-criteria decision-making approach to analyze wastewater microalgae culture systems for bioenergy production

Bioenergy generation from microalgae can significantly contribute to climate mitigation and renewable energy production. In this regard, several multi-criteria decision-making method were employed to prioritize appropriate microalgae culture system for bioenergy production. Entropy weight, Criteria Importance Through Intercriteria Correlation (CRITIC) and Technique for Order of Preference by Similarity to Ideal Solution (TOPSIS) were the employed MCDA method. Fourteen microalgae culture systems were selected as a case study, which contain teen monoculture and four dual-culture. Initially, through ans in-depth review of the literature and expert views, four categories total eight indicators were selected as the evaluation indices of the study, namely 1) Proliferation: Half growth cycle and Max growth rate,2) Biomass output: Bio-crude yield and Lipid yield, 3) Nutrient utilization: residual concentration of total Nitrogen and total Phosphorus, and, 4) Stability: coefficient of variation of Bio-crude yield and Lipid yield. The result indicated that "Pediastrum sp. & Micractinium sp." was identified as the most bioenergy potential microalgae culture system, and the evaluation results of entropy weight method and CRITIC method are similar. It is pertinent to note that 1)the entropy weight method exhibits lower sample size requirements, 2) the critic method excels when dealing with larger sample sizes, and 3) the TOPSIS method necessitates the incorporation of appropriate weighting methods to ensure credible results. In the application stage, the key indicators related to cost can be further included in the evaluation indices.

Room-temperature synthesis of a Zr-UiO-66 metal-organic framework via mechanochemical pretreatment for the rapid removal of EDTA-chelated copper from water

Treatment of heavy metal pollution in complexed states within water bodies presents significant challenges in the current water treatment field. Adsorption as a means for the removal of heavy metals is characterized by its simplicity of operation, stable effluent, and minimal equipment requirements. Metal-organic frameworks (MOFs) as adsorbents hold significant interest for applications in water treatment. In this study, we investigated a green synthesis approach for the ball-milling pretreated synthesis of UiO-66(Zr) at room temperature, abbreviated as UiO-66(Zr)-rm. Besides having the same thermal stability and crystal structure as the product from microwave-assisted synthesis (UiO-66(Zr)-mw), the resulting UiO-66(Zr)-rm features smaller particle size and superior mesoporous structure. The adsorption efficiency and mechanism for removing EDTA-chelated copper (EDTA-CuII), a complexed heavy metal in water, were extensively analyzed. UiO-66(Zr)-rm presented a maximum adsorption capacity over EDTA-CuII of 43 mg g-1 and a much higher adsorption rate (0.16 g (mg h)-1) than UiO-66(Zr)-mw (0.06 g (mg h)-1). Hierarchically mesostructured defects allow the sorbate to have more effective diffusion in a shorter time to achieve faster adsorption kinetics. Benefiting from the mild synthesis conditions and nontoxic solvents, UiO-66(Zr) has the potential to be produced at a scaled-up level, thereby exhibiting excellent adsorption performance for the removal of complexed heavy metals in the future.

Effects of nitrogen regulation on heavy metal phytoextraction efficiency (Leucaena leucocephala): Application of a nitrogen fertilizer and a fungal agent

Lead (Pb) and cadmium (Cd) have been identified as the primary contaminants in soil, posing potential health threats. This study aimed to examine the effects of applying a nitrogen fertilizer and a fungal agent Trichoderma harzianum J2 (nitrogen alone, fungi alone, and combined use) on the phytoremediation of soils co-contaminated with Pb and Cd. The growth of Leucaena leucocephala was monitored in the seedling, differentiation, and maturity stages to fully comprehend the remediation mechanisms. In the maturity stage, the biomass of L. leucocephala significantly increased by 18% and 29% under nitrogen-alone (NCK+) and fungal agent-alone treatments (J2), respectively, compared with the control in contaminated soil (CK+). The remediation factors of Pb and Cd with NCK+ treatment significantly increased by 50% and 125%, respectively, while those with J2 treatment increased by 73% and 145%, respectively. The partial least squares path model suggested that the nitrogen-related soil properties were prominent factors affecting phytoextraction compared with biotic factors (microbial diversity and plant growth). This model explained 2.56 of the variation in Cd concentration under J2 treatment, and 2.97 and 2.82 of the variation in Pb concentration under NCK+ and J2 treatments, respectively. The redundancy analysis showed that the samples under NCK+ and J2 treatments were clustered similarly in all growth stages. Also, Chytridiomycota, Mucoromucota, and Ciliophora were the key bioindicators for coping with heavy metals. Overall, a similar remediation mechanism allowed T. harzianum J2 to replace the nitrogen fertilizer to avoid secondary pollution. In addition, their combined use further increased the remediation efficiency.

Immobilization mechanisms of Sr(II), Ni(II), and Cd(II) on glomalin-related soil protein in mangrove sediments at the microscopic scale

Glomalin-related soil protein (GRSP) is a significant component in the sequestration of heavy metal in soils, but its mechanisms for metal adsorption are poorly known. This study combined spectroscopic data with molecular docking simulations to reveal metal adsorption onto GRSP's surface functional groups at the molecular level. The EXAFS combined with FTIR and XPS analyses indicated that the adsorption of Cd(II), Sr(II), and Ni(II) by GRSP occurred mainly through the coordination of -OH and -COOH groups with the metal. The -COOH and -OH groups bound to the metal as electron donors and the electron density of the oxygen atom decreased, suggesting that electrostatic attraction might be involved in the adsorption process. Two-dimensional correlation spectroscopy revealed that preferential adsorption occurred on GRSP for the metal in sequential order of -COOH groups followed by -OH groups. The presence of the Ni-C shell in the Ni EXAFS spectrum suggested that Ni formed organometallic complexes with the GRSP surface. However, Sr-C and Cd-C were absent in the second shell of the Sr and Cd spectra, which was attributed to the adsorption of Sr and Cd ions with large hydration ion radius by GRSP to form outer-sphere complexes. Through molecular docking simulations, negatively charged residues such as ASP151 and ASP472 in GRSP were found to provide electrostatic attraction and ligand combination for the metal adsorption, which was consistent with the spectroscopic analyses. Overall, these findings provided new insights into the interaction mechanisms between GRSP and metals, which will help deepen our understanding of the ecological functions of GRSP in metal sequestration.

A primary battery for efficient cadmium contamination remediation and electricity generation

In this work, two kinds of primary batteries, both of which included a Zn anode, C rod cathode, copper wire and electrolyte composed of Cd2+-contaminated water or soil, were constructed in the first attempt to both remove Cd2+ and generate electricity. Unlike traditional technologies such as electrokinetic remediation with high energy consumption, this technology could realize Cd2+ migration to aggregation and solidification and generate energy at the same time through simultaneous galvanic reactions. The passive surface of Zn and C was proven via electrochemical measurements to be porous to maintain the relatively active galvanic reactions for continuous Cd2+ precipitation. Cd2+ RE (removal efficiency) and electricity generation were investigated under different conditions, based on which two empirical models were established to predict them successfully. In soil, KCl was added to desorb Cd2+ from soil colloids to promote Cd2+ removal. These systems were also proven to remove Cd2+ efficiently when their effects on plants, zebrafish, and the soil bacterial community were tested. LEDs could be lit for days by utilizing the electricity produced herein. This work provides a novel, green, and low-cost route to remediate Cd2+ contamination and generate electricity simultaneously, which is of extensive practical significance in the environmental and energy fields.

Mechanism and ecological environmental risk assessment of peroxymonosulfate for the treatment of heavy metals in soil

Oxidation technologies based on peroxymonosulfate (PMS) have been effectively used for the remediation of soil organic pollutants due to their high efficiency. However, the effects of advanced PMS-based oxidation technologies on other soil pollutants, such as heavy metals, remain unknown. In this study, changes in the form of heavy metals in soil after using PMS and the risk of pollution to the ecological environment were investigated. Furthermore, two risk assessment methods, the mung bean germination toxicity test and groundwater leaching soil column test, were employed to evaluate the soil before and after PMS treatment. The results showed that PMS has a strong ability to degrade complex compounds, enabling the transformation of heavy metals, such as Cd, Pb, and Zn, from stable to active states in the soil. The risk assessments showed that PMS treatment activated heavy metals in the soil, which delayed the growth of plants, increased heavy metal content in plant tissues and the risk of groundwater pollution. These findings provide a new perspective for understanding the effects of PMS on soil, thus facilitating the sustained and reliable development of future research in the field of advanced oxidation applied to soil treatment.

Highly selective detection and removal of mercury ions in the aquatic environment based on magnetic ZIF-71 multifunctional composites with sufficient chlorine functional groups

The development of both detection and removal technologies for heavy metal ions is of great importance. Most of the existing adsorbents that contain oxygen, nitrogen or sulfur functional groups can remove heavy metals, but achieving both selective detection and removal of a single metal ion is difficult because they bind to a wide range of heavy metal ions. Herein, we selected zeolite imidazolium hydrochloride framework-71 (ZIF-71) with sufficient chlorine functional groups to fabricate magnetic ZIF-71 multifunctional composites (M-ZIF-71). M-ZIF-71 had a large specific surface area, excellent water stability, and good magnetic properties, which made M-ZIF-71 conducive to the separation and recovery of adsorbents and the assembly of electrodes. M-ZIF-71 exhibited high selectivity, wide linear range (1-500 μg/L), and low detection limit (0.32 μg/L) for electrochemical detection of mercury ions (Hg2+). Meanwhile, M-ZIF-71 demonstrated rapid Hg2+ adsorption with a high capacity of 571.2 mg/g and excellent recyclability. The high selectivity for Hg2+ was attributed to the powerful affinity of highly electronegative chlorine and Hg2+. Moreover, XPS spectra demonstrated the interaction between chlorine and Hg2+. This work provides a new inspiration for applications in the targeted monitoring and removal of heavy metal pollution.

Effect and mechanism of kaolinite loading amorphous zero-valent iron to stabilize cadmium in soil

Amorphousness effectively improves the electron transfer rate of zero-valent iron. In this study, a novel kaolinite loading amorphous zero-valent iron composite (K-AZVI) was prepared and applied to the remediation of soils with cadmium (Cd) pollution concentrations of 20, 50, and 100 mg/kg respectively. The results showed that the application of K-AZVI increased the pH and cation exchange capacity (CEC) of soil, and decreased the dissolved organic carbon (DOC) and organic matter (OM) of soil, thus indirectly promoting the adsorption of Cd in the soil. After 28 days of stabilization, the stabilizing efficiency of K-AZVI on the water-soluble Cd content in soil reached 98.72 %. Under the amendment of 0.25 %-1.0 % (w/w), the available Cd content in 20-100 mg/kg contaminated soil decreased by 46.47 %-62.23 %, 24.10 %-41.52 %, and 16.09 %-30.51 % respectively compared with CK. More importantly, the addition of K-AZVI promoted the transformation of 33.18 %-48.42 % exchangeable fraction (EXC) to 10.09 %-20.14 % residual fraction (RES), which increased the abundance and diversity of soil bacterial communities. Comprehensive risk assessment showed that adding 1.0 % K-AZVI provided the best remediation on contaminated soil. In addition, the results of scanning electron microscopy (SEM), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS) of K-AZVI before and after the reaction showed that the stabilization mechanism of K-AZVI to Cd in soil is mainly the stable metal species (Cd(OH)2, CdO and CdFe2O4) formed by the direct complexation and coprecipitation of a large number of iron oxides formed by the rapid corrosion of amorphous zero-valent iron (AZVI). Overall, this work provides a promising approach to the remediation of Cd-contaminated soil using K-AZVI composites.

Immobilization of zinc and cadmium by biochar-based sulfidated nanoscale zero-valent iron in a co-contaminated soil: Performance, mechanism, and microbial response

Mining and smelting of mineral resources causes excessive accumulation of potentially toxic metals (PTMs) in surrounding soils. Here, biochar-based sulfidated nanoscale zero-valent iron (SNZVI/BC) was designed via a one-step liquid phase reduction method to immobilize cadmium (Cd) and zinc (Zn) in a copolluted arable soil. A 60 d soil incubation experiment revealed that Cd and Zn immobilization efficiency by 6 % SNZVI/BC (25.2-26.2 %) was higher than those by individual SNZVI (13.9-18.0 %) or biochar (14.0-19.3 %) based on the changes in diethylene triamine pentaacetic acid (DTPA)-extractable PTM concentrations in soils, exhibiting a synergistic effect. Cd2+ or Zn2+ replaced isomorphously Fe2+ in amorphous ferrous sulfide, as revealed by XRD, XPS, and high-resolution TEM-EDS, forming metal sulfide precipitates and thus immobilizing PTMs. PTM immobilization was further enhanced by adsorption by biochar and oxidation products (Fe2O3 and Fe3O4) of SNZVI via precipitation and surface complexation. SNZVI/BC also increased the concentration of dissolved organic carbon and soil pH, thus stimulating the abundances of beneficial bacteria, i.e., Bacilli, Clostridia, and Desulfuromonadia. These functional bacteria further facilitated microbial Fe(III) reduction, production of ammonium and available potassium, and immobilization of PTMs in soils. The predicted function of the soil microbial community was improved after supplementation with SNZVI/BC. Overall, SNZVI/BC could be a promising functional material that not only immobilized PTMs but also enhanced available nutrients in cocontaminated soils.

Cations facilitate sulfidation of zero-valent iron by elemental sulfur: Mechanism and dechlorination application

The solid-solid reaction of microscale zero-valent iron (mZVI) with elemental sulfur (S0) in water can form sulfidated mZVI (S-mZVI) with high reactivity and selectivity. However, the inherent passivation layer of mZVI hinders the sulfidation. In this study, we demonstrate that ionic solutions of Me-chloride (Me: Mg2+, Ca2+, K+, Na+ and Fe2+) can accelerate the sulfidation of mZVI by S0. The S0 with S/Fe molar ratio of 0.1 was fully reacted with mZVI in all solutions to form unevenly distributed FeS species on S-mZVIs as confirmed by SEM-EDX and XANES characterization. The cations depassivated the mZVI surface by driving the proton release from the surface site (FeOH) and resulting in localized acidification. The probe reaction test (tetrachloride dechlorination) and open circuit potential (EOCP) measurement demonstrated that Mg2+ was most efficient in depassivating the mZVI and therefore promoting sulfidation. The decrease of surface proton for hydrogenolysis on the S-mZVI synthesized in MgCl2 solution also inhibited the formation of cis-1,2-dichloroethylene by 14-79% compared to other S-mZVIs during trichloroethylene dechlorination. In addition, the synthesized S-mZVIs exhibited the highest reduction capacity reported so far. These findings provide a theoretical basis for the facile on-site sulfidation of mZVI by S0 with cation-rich natural waters for sustainable remediation of contaminated sites.

Chemical Bond Bridging across Two Domains: Generation of Fe(II) and In Situ Formation of FeSx on Zerovalent Iron

Sulfidation of zerovalent iron (SZVI) can strengthen the decontamination ability by promoting the electron transfer from inner Fe⁰ to external pollutants by iron sulfide (FeS_x). Although FeS_x forms easily, the mechanism for the FeS_x bonding on the ZVI surface through a liquid precipitation method is elusive. In this work, we demonstrate a key pathway for the sulfidation of ZVI, namely, the in situ formation of FeS_x on ZVI surface, which leads to chemical bonding across two domains: the pristine ZVI and the newly formed FeS_x phase. The two chemically bridged heterophases display superior activity in electron transportation compared to the physically coated SZVI, eventually bringing about the better performance in reducing Cr(VI) species. It is revealed that the formation of chemically bonded FeS_x requires balancing the rates for the two processes of Fe(II) release and sulfidation, which can be achieved by tuning the pH and S(-II) concentration. This study elucidates a mechanism for surface generation of FeS_x on ZVI, and it provides new perspectives to design high-quality SZVI for environmental applications.

Dual promoted ciprofloxacin degradation by Fe0/PS system with ascorbic acid and pre-magnetization

As a widely used and hard-to-degrade pharmaceuticals and personal care product (PPCP), ciprofloxacin (CIP) was frequently found in water environment and the detected concentration was gradually increased. Although zero-valent iron (ZVI) has been shown to be effective in destroying refractory organic pollutants, the practical application and sustained catalytic performance is not satisfactory. Herein, introduction of ascorbic acid (AA) and employment of pre-magnetized Fe⁰ was achieved to maintain a high-concentration of Fe²⁺ during persulfate (PS) activation. Pre-Fe⁰/PS/AA system presented the best performance for CIP degradation, achieving almost complete elimination of 5 mg/L CIP within 40 min in the reaction conditions of 0.2 g/L pre-Fe⁰，0.05 mM AA and 0.2 mM PS. The CIP degradation retarded as excess pre-Fe⁰ and AA were added, therefore, the optimum dosages of pre-Fe⁰ and AA were determined to be 0.2 g/L and 0.05 mM, respectively. The CIP degradation gradually decreased as the initial pH increased from 3.05 to 11.03. The presence of Cl^-, HCO₃^-, Al³⁺, Cu²⁺ and humic acid significantly influenced the performance of CIP removal, while Zn²⁺, Mg²⁺, Mn²⁺, and NO₃^- slightly affected the CIP degradation. Combined with the results of HPLC analysis and previous literature, several possible degradation pathways of CIP were proposed.

Humic substances derived from unconventional resources: extraction, properties, environmental impacts, and prospects

Humic substances comprise up to 70% of the total organic matter in soils, between 50 and 80% of the dissolved organic matter in water, and about 25% of dissolved organic matter in groundwater. Elucidation of the complex structure and properties of humic substances requires advanced analytical tools; however, they are of fundamental importance in medicine, agriculture, technology, and the environment, at large. Although they are naturally occurring, significant efforts are now being directed into their extraction owing to their relevance in improving soil properties and other environmental applications. In the present review, the different fractions of humic substances were elucidated, underlying the mechanisms by which they function in soils. Furthermore, the extraction processes of humic substances from various feedstock were illustrated, with the alkali extraction technique being the most widely used. In addition, the functional group and elemental composition of humic substances were discussed. The similarities and/or variations in the properties of humic substances as influenced by the source and origin of feedstock were highlighted. Finally, the environmental impacts of humic substances were discussed while highlighting prospects of humic acid production. This review offers enormous potential in identifying these knowledge gaps while recommending the need for inter- and multidisciplinary studies in making extensive efforts toward the sustainable production of humic substances.

Enhanced immobilization of lead, cadmium, and arsenic in smelter-contaminated soil by sulfidated zero-valent iron

Soils contaminated with multiple heavy metal(loid)s (HMs) such as lead (Pb), cadmium (Cd), and arsenic (As) are of great concern in many countries. In this study, taking three lead-zinc smelter soils, the performance of sulfidated zero-valent iron (S-ZVI) toward Pb, Cd, and As immobilization was systemically investigated. Results showed that more than 88% of water-extractable Pb and Cd could be immobilized and transformed into reducible, oxidizable, and/or reducible forms by S-ZVI within 3 h, whereas only 3-56% of them could be immobilized by unsulfidated ZVI even after 72 h. Meanwhile, the phytoavailability of the tested HMs could be effectively reduced by 79% after S-ZVI amendment. More importantly, anoxic/oxic incubation tests revealed that the dissolved concentrations of HMs were much lower in S-ZVI-treated soils than in the untreated or unmodified ZVI-treated soils. Speciation analysis further suggested that unmodified ZVI seemed to reduce the long-term soil stability by changing the residual HMs species to mild-acid soluble and/or reducible ones. In contrast, S-ZVI could effectively alleviate the remobilization of HMs under the changeover of soil redox environments. All these findings indicate that S-ZVI may be a promising amendment for the immobilization of Pb, Cd, and As in smelter-contaminated soil.

Constructing the active surface soil layer with ZVI-biochar amendment for simultaneous immobilization of As and Zn in both contaminated soil and groundwater: Continuous versus intermittent infiltration mode

In this study, the zero valent iron-biochar composite (ZVI-BC) was applied to construct an active surface soil layer for the simultaneous remediation of As-Zn contaminated soil and groundwater, focusing on the influence of the infiltration mode of pumped-up groundwater into soil. The active surface soil treated more contaminated groundwater for As (4.45-5.46 L kg^-1 soil) than that for Zn (2.52-3.13 L kg^-1 soil) under both continuous and intermittent infiltration modes, with about 98% As and 95% Zn removed from groundwater and retained in the soil. As(V) precipitated with Fe(III) due to ZVI oxidation, which was responsible for the As immobilization. The soil under the intermittent infiltration mode was enriched by the Sphingomonas with arsenate reductase gene, which promoted more reduction of As(V) into As(III) and facilitated coprecipitation of As(III) with Fe(III). The Mn oxide determined the sorption of Zn in the active soil layer, where the Hyphomicrobium, one type of manganese oxidizing bacteria, was much higher under the continuous infiltration mode, which accounted for the more Zn immobilization. After the remediation, both As and Zn immobilized in the active surface soil showed high stability, with the average downward migration rate of only 0.207-0.368 cm year^-1 within 20-year rainfall exposure. Our findings indicate that this active surface soil layer is applicable for simultaneous immobilization of As and Zn in both contaminated soil and groundwater, and the groundwater intermittent infiltration could be a better option considering the remediation effectiveness, the immobilization mechanism, the long-term stability, and the energetic efficiency.

Thallium removal from wastewater using sulfidized zero-valent manganese: Effects of sulfidation method and liquid nitrogen pretreatment

Zero-valent manganese (ZVMn) possesses high reducibility in theory, while sulfide exhibits strong affinity towards a variety of heavy metals owing to the low solubility of metal sulfides. Yet the performance and mechanisms on using sulfidized zero-valent manganese (SZVMn) to remove thallium (Tl) from wastewater still remain unclear. In this study, the performance of Tl(I) removal using SZVMn synthesized by borohydrides reduction followed by sulfides modification, with and without liquid nitrogen treatment, was compared and the mechanism behind was investigated. The results show that at a S/Mn molar ratio of 1.0, liquid nitrogen modified SZVMn (LSZVMn) possessed more interior channels and pores than SZVMn, with 65.3% higher specific surface area and 73.7% higher porosity, leading to 6.4-8.1% improvement in adsorption of Tl(I) at pH 4-10. LSZVMn showed effectiveness and robustness in Tl(I) removal in the presence of co-existing ions up to 0.1 M. The adsorption of Tl(I) conformed to the pseudo-1st-order kinetic model, and followed the Langmuir isothermal model, with the maximum Tl adsorption capacity of 264.9 mg·g^-1 at 288 K. The mechanism of Tl(I) removal with SZVMn was found to include sulfidation-induced precipitation, manganese reduction, surface complexation, and electrostatic attraction. The liquid nitrogen pretreatment embrittled and cracked the outer shell of S/Mn compounds, resulted in a highly hierarchical structure, enhancing the manganese reduction and improving the Tl(I) removal. Based on the above results, the SZVMn and its liquid nitrogen-modified derivatives are novel and effective environmental materials for Tl(I) removal from wastewater, and the application of SZVMn to the removal of other pollutants merits investigation in future study.

Organolead Halide-Based Coordination Polymers: Intrinsic Stability and Photophysical Applications

ConspectusOrganolead halide-based photovoltaics are one of the state-of-the-art solar cell systems with efficiencies increasing to 25% over the past decade, ascribed to their high light-absorption coefficient, broad wavelength coverage, tunable band structure, and excellent carrier mobility. Indeed, these optical characteristics are highly demanding in photocatalysis and photoluminescence (PL), which also involve the solar energy utilization and charge transport. However, the vast majority of organolead halides are ionically bonded structures and susceptible to degradation upon high-polarity protic molecules (e.g., water (vapor) and alcohol), which are often inevitable in many photochemical applications. Encapsulation is a commonly used stabilization approach by coating protective layers, avoiding the direct contact between organolead halides and polar molecules. However, this may partially hinder the light penetration to the inner hybrid halide materials, and introduce new interface problems that are important in photocatalysis and luminescent sensing. Therefore, developing intrinsically stable organometal halide hybrids is a major target for their applications in optoelectronic applications.In this Account, recent research progress on the synthesis of organolead halide-based coordination polymers for a variety of photoactive applications is described. Herein, we propose a general strategy to advance the intrinsic stability of organometal halide crystalline materials by using coordinating anionic organic linkers, which occupy the excellent photophysical features analogous to those of perovskites. Unlike the organoammonium cations as for ionically bonded structures, the anionic structure-directing agents (e.g., organocarboxylates) render well-defined metal-carboxylate coordination motifs in extended architectures spanning from low-dimensional (0D, 1D) to high-dimensional cationic inorganic Pb-X-Pb (X = F^-/Cl^-/Br^-/I^-) sublattices. This family of organolead halide coordination polymers can endure chemically reactive environments over a wide range of pH and aqueous boiling condition, which have been systematically investigated by experimental studies and theoretical calculations. Many chloride/bromide-based coordination polymers show air-stable, broadband self-trapped emission with large Stokes shift and high color rendition, exhibiting the absolute quantum yields of 35-72%. Among them, the porous frameworks with low-dimensional (0D, 1D) inorganic blocks are recognized as a rare class of porous metal-organic frameworks (MOFs) constructed by lead halides as secondary building units (SBUs). They not only occupy substantially higher light-harvesting and carrier-transport properties than conventional metal oxide-based MOFs, but also allow for isoreticular modification to regulate the PL characteristics by guest molecules. More importantly, combining the high stability with excellent carrier characteristics, a layered organolead iodide coordination polymer shows the overall photocatalytic water splitting without the use of any sacrificial agent under simulated sunlight illumination. Given the wide choice of structurally diverse organocarboxylate linkers, we hope this Account provides deep insights on the importance of coordination chemistry in the discovery of a wide family of intrinsically stable organolead halides to expand their photophysical applications.

Oxyanion-modified zero valent iron with excellent pollutant removal performance

Oxyanion-modified zero valent iron (OM-ZVI), including oxyanion-modified microscale ZVI (OM-mZVI) and nanoscale zero valent iron (OM-nZVI), has attracted growing interest in recent years for their excellent pollutant removal performance. This feature article summarizes the recent progress of OM-ZVI materials, including their synthesis, characterization, enhanced pollutant removal performance, and structure-property relationships. Generally, OM-ZVI could be synthesized with wet chemical and mechanochemical (ball-milling) methods and then characterized with state-of-the-art characterization techniques (e.g., X-ray-based spectroscopy, electron microscopy) to reveal their structure and physicochemical properties. We found that phosphate modification could form iron-phosphate on the nZVI surface, facilitating Cr(VI) removal, while the phosphorylation process could induce tensile hoop stress to produce numerous radial nanocracks in the structurally-dense spherical nZVI for enhanced Ni(II) removal via a boosted Kirkendall effect. Oxalate modification could trigger electron delocalization to increase electron cloud density and surface-bound Fe(II) of mZVI for enhanced Cr(VI) removal, while oxalated mZVI exhibited more efficient Cr(VI) removal performance via an in situ formed FeC₂O₄·2H₂O shell of high proton conductivity, favoring Cr(VI) reduction. Differently, the mechanochemical treatment of mZVI with B₂O₃ could exert tensile strain on it through interstitial boron doping, thereby promoting the release and transfer of electrons from its Fe(0) core to its iron oxide shell for dramatic Cr(VI) reduction. This article aims to demonstrate the potential of OM-ZVI for pollution control and environmental remediation.

Simultaneous stabilization of Pb, Cd, and As in soil by rhamnolipid coated sulfidated nano zero-valent iron: Effects and mechanisms

Sulfidation effectively improves the electron transfer efficiency of nanoscale zero-valent iron (nZVI), but decreases the specific surface area of nZVI. In this study, sulfidated nZVI (S-nZVI) coated with rhamnolipid (RL-S-nZVI) was synthesized and used to stabilize Pb, Cd, and As in combined polluted soil. The stabilization efficiency of 0.3% (wt) RL-S-nZVI to water soluble Pb, Cd, and As in soil reached 88.76%, 72%, and 63%, respectively. Rhamnolipid coating inhibited the reduction of specific surface area and successfully encapsulated nZVI, thus reducing the oxidation of Fe⁰. The types of iron oxides in RL-S-nZVI were reduced compared to S-nZVI, but the content and strength of Fe⁰ iron were obviously enhanced. Furthermore, rhamnolipid functional groups (-COOH and -COO^-) were also involved in the stabilization process. In addition, the stabilization efficiency of RL-S-nZVI to the bioavailable Pb, Cd, and As in soil increased by 41%, 41%, and 50%, respectively, compared with nZVI. The presence of organic acids, especially citric acid, improved the stabilization efficiency of RL-S-nZVI to the three metals. The result of BCR sequential extraction indicated that RL-S-nZVI increased the residual state of Pb, Cd, and As and reduced the acid-soluble and reducible state after 28 days of soil incubation. XRD and XPS analyses showed that the stabilization mechanisms of RL-S-nZVI on heavy metals involved in ion exchange, surface complexation, adsorption, co-precipitation, chemisorption, and redox.

Modified biochar/humic substance/fertiliser compound soil conditioner for highly efficient improvement of soil fertility and heavy metals remediation in acidic soils

Fertile and uncontaminated soil with appropriate pH is crucial in terms of the agricultural sustainable development. Herein, a compound soil conditioner containing chitosan modified straw biochar (CBC), kitchen waste compost product-derived humic substance (HS), NPK compound fertiliser (NPK-CF) was prepared to simultaneously adjust acidic soil pH, improve fertility, and immobilize heavy metal. The results exhibited that the best Pb and NH₄⁺ adsorption performance was obtained in CBC with chitosan:biochar of 1:5. Then, the acid soil pH was improved from 5.03 to 6.66 in the presence of CBC/HS (5:5) with 3% addition weight (the mass ratio of conditioner to soil). Meanwhile, compared with the control, the contents of organic matter, available nitrogen, and available phosphorus significantly increased by 52.4%, 92.6%, and 136.3%, respectively. Moreover, Pb was highly efficient immobilised by CBC, and the concentration of Pb in the soil was decreased by 55.2%. The optimal growth trend of ryegrass was obtained in the presence of 3% addition weight (the mass ratio of conditioner to soil) CBC/HS (CBC:HS = 5:5) combined with 60% of the recommended NPK-CF application weight, which was mainly contributed by the improvement of the soil microbial abundance and community structure diversity. The addition of CBC/HS could effectively reduce the addition of NPK-CF and contribute to simultaneous controlling nitrogen loss, releasing phosphorus, immobilising Pb, adjusting pH, improving soil quality and controlling nonpoint pollution.