Enhanced Arsenate Immobilization by Kaolinite via Heterogeneous Pathways during Ferrous Iron Oxidation.

Clay minerals are ubiquitous in subsurface environments and have long been recognized as having a limited or negligible impact on the fate of arsenic (As) due to their negatively charged surfaces. Here, we demonstrate the significant role of kaolinite (Kln), a pervasive clay mineral, in enhancing As(V) immobilization during ferrous iron (Fe(II)) oxidation at near-neutral pH. Our results showed that Fe(II) oxidation alone was not capable of immobilizing As(V) at relatively low Fe/As molar ratios (≤2) due to the generation of Fe(III)-As(V) nanocolloids that could still migrate easily as truly dissolved As did. In the presence of kaolinite, dissolved As(V) was significantly immobilized on the kaolinite surfaces via forming Kln-Fe(III)-As(V) ternary precipitates, which had large sizes (at micrometer levels) to reduce the As mobility. The kaolinite-induced heterogeneous pathways for As(V) immobilization involved Fe(II) adsorption, heterogeneous oxidation of adsorbed Fe(II), and finally heterogeneous nucleation/precipitation of Fe(III)-As(V) phases on the edge surfaces of kaolinite. The surface precipitates were mixtures of amorphous basic Fe(III)-arsenate and As-rich hydrous ferric oxide. Our findings provide new insights into the role of clay minerals in As transformation, which is significant for the fate of As in natural and engineered systems.

Can simultaneous immobilization of arsenic and cadmium in paddy soils be achieved by liming?

Liming acidic paddy soils to near-neutral pH is the most cost-effective strategy to minimize cadmium (Cd) accumulation by rice. However, the liming-induced effect on arsenic (As) (im)mobilization remains controversial and is called upon for further investigation, particularly for the safe utilization of paddy soils co-contaminated with As and Cd. Here, we explored As and Cd dissolution along pH gradients in flooded paddy soils and extracted key factors accounting for their release discrepancy with liming. The minimum As and Cd dissolution occurred concurrently at pH 6.5-7.0 in an acidic paddy soil (LY). In contrast, As release was minimized at pH < 6 in the other two acidic soils (CZ and XX), while the minimum Cd release still appeared at pH 6.5-7.0. Such a discrepancy was determined largely by the relative availability of Fe under overwhelming competition from dissolved organic carbon (DOC). A mole ratio of porewater Fe/DOC at pH 6.5-7.0 is suggested as a key indicator of whether co-immobilization of As and Cd can occur in flooded paddy soils with liming. In general, a high mole ratio of porewater Fe/DOC (≥ 0.23 in LY) at pH 6.5-7.0 can endow co-immobilization of As and Cd, regardless of Fe supplement, whereas such a case is not in the other two soils with lower Fe/DOC mole ratios (0.01-0.03 in CZ and XX). Taking the example of LY, the introduction of ferrihydrite promoted the transformation of metastable As and Cd fractions to more stable ones in the soil during 35 days of flooded incubation, thus meeting a class I soil for safe rice production. This study demonstrates that the porewater Fe/DOC mole ratio can indicate a liming-induced effect on co-(im)mobilization of As and Cd in typical acidic paddy soils, providing new insights into the applicability of liming practice for the paddy soils.

Magnetic confinement-enabled membrane reactor for enhanced removal of wide-spectrum contaminants in water: Proof of concept, synergistic decontamination mechanisms, and sustained treatment performance.

Membrane chemical reactors (MCRs) have demonstrated a great potential for simultaneous removal of wide-spectrum pollutants in advanced water treatment. However, current catalyst (re)loading and catalytic reactivity limitations obstruct their practical applications. Herein, as a proof-of-concept, we report a hollow fiber membrane chemical reactor (HF-MCR) with high and sustainable catalytic reactivity, enabled by novel magnetic confinement engineering of the catalysts. Namely, the zerovalent iron (ZVI) nanocatalysts were spatially dispersed and confined to nearly parallel magnetic induction lines, forming forest-like microwire arrays in the membrane lumen. Such arrays exhibited ultrahigh hydrodynamic stability. The HF-MCR integrated sequential membrane separation and Fenton-like catalysis, thus being capable of high and synergistic wide-spectrum decontamination. The membrane separation process completely removed large nanoplastics (NPs) via size exclusion, and thus the subsequent Fenton-like catalysis process enhanced removal efficiency of otherwise permeated bisphenol A (BPA) and phosphate (P) by in situ generated reactive oxygen species (primarily 1O2) and iron (oxyhydr)oxides, respectively. Furthermore, highly dispersed ZVI arrays and their continuous surface depassivation driven by magnetic gradient and hydrodynamic forces conferred abundant accessible catalytic sites (i.e., Fe0 and FeII) to stimulate Fenton-like catalysis. The consequent enhancement of BPA and P removal kinetics was 3-765 and 49-492 folds those in conventional (flow-through or batch) systems, respectively. Periodic ZVI reloading ensured sustained decontamination performance of the HF-MCR. This is the first demonstration of the magnetic confinement engineering that enables efficient and unlimited catalyst (re)loading and sustainable catalytic reactivity in the MCR for water treatment, which is beyond the reach of current approaches.

Milk vetch returning reduces rice grain Cd concentration in paddy fields: Roles of iron plaque and soil reducing-bacteria.

Milk vetch (MV, Astragalus sinicus L.) is used in agricultural production as a green manure; however, its impact on accumulation levels of heavy metals (e.g., Cd) in rice remains poorly understood. This study investigated the effects of MV on Cd accumulation in rice, iron plaque formation, soil properties, and the soil microbial community structure through field experiments. The results showed that MV reduced Cd concentration in the roots, stem, leaves, and grains by 33%, 60%, 71%, and 49%, respectively. Chemical fertilizer and MV treatment promoted iron plaque formation, and MV considerably increased the Fe/Mn ratio in the iron plaque. More importantly, MV inhibited Cd transportation from the root iron plaque to the root by 74%. The concentrations of CaCl2-extractable Cd, available phosphorus, and available potassium, as well as the cation exchange capacity and urease activity, were significantly reduced in the MV treatment. Furthermore, 16 S rDNA high-throughput sequencing results of the soil microbial community structure showed that compared with the control, MV increased the soil microbial richness, increased the relative abundance of anaerobic microorganisms, and significantly increased the relative abundance of Thermodesulfovibrio and Geobacter at the genus level. The increase in anaerobic microbial abundance was closely related to the decrease in CaCl2-extractable Cd concentration. The application of MV promoted the formation of iron plaque, inhibited the transport of Cd, increased the abundance of anaerobic microorganisms, decreased the CaCl2-extractable Cd concentration, and reduced the Cd concentration in rice grain.

Reduction of cadmium bioavailability in paddy soil and its accumulation in brown rice by FeCl3 washing combined with biochar: A field study.

Cadmium (Cd) removal from paddy soil to reduce Cd accumulation in brown rice is essential for agroecology, food safety, and human health. In this study, we demonstrate that ferric chloride (FeCl3) washing combined with biochar treatment efficiently remediates Cd-contaminated paddy soil in field trials. Our results showed that 30.9 % of total Cd and 41.6 % of bioavailable Cd were removed by the addition of 0.03 M FeCl3 at a liquid/soil ratio of 1.5:1. The subsequent addition of 1 % biochar further reduced bioavailable Cd by 36.5 and 41.5 %, compared with FeCl3 washing or biochar treatment alone. The principal component regression analysis showed that the Cd content in brown rice was primarily affected by the bioavailable Cd in soil. The combined remediation contributed to the decreased Cd contents in brown rice by 45.5-62.5 %, as well as a 2.7-11.8 % increase in rice yield. The Cd contents in brown rice decreased to 0.12 and 0.04 mg kg-1 in two cultivars of rice (Zhuliangyou189 and Zhuliangyou929), lower than the national food safety standard limit value of China (0.2 mg kg-1). Meanwhile, the combined remediation promoted the restoration of soil pH and organic matter as well as the improvement of available nutrients. This finding suggests that the combination of FeCl3 washing and biochar is an effective remediation strategy to minimize Cd bioavailability in paddy soil, and improves soil quality, thus contributing to food safety.

In Situ-Formed Phenoxyl Radical on the CuO Surface Triggers Efficient Persulfate Activation for Phenol Degradation.

Transition-metal oxide (MxOy)-based persulfate (PDS) activation processes have demonstrated enormous potential for pollutant degradation in water purification. However, the mechanistic insight of PDS activation by a MxOy catalyst concerning the mediate role of the organic substrate remains obscure. Here, we demonstrated that the in situ-formed phenoxyl radical on the CuO surface can trigger efficient persulfate activation for phenol degradation. The formation of the phenoxyl radical was an inner-sphere process, which involved the successive steps of chemisorption through surface hydroxyl group substitution and the subsequent spontaneous electron transfer reaction from adsorbed phenol to CuO. The organic substrate phenol can be oxidized by the PDS molecule and surface-bound SO4•- through the nonradical and free-radical pathways, respectively. Such a unique "half-radical" mechanism resulted in an extraordinarily high PDS utilization efficiency of 188.9%. More importantly, a general rule for phenoxyl radical formation was concluded; it can be formed in the cases of organic substrates with a Hammett constant σ+ lower than -0.02 and metal ion of a 3d subshell between half-filled and fully filled. This study clarifies the mediate role of the organic substrate for interfacial PDS activation on MxOy and also gives new insights into the rational design of a highly efficient MxOy catalyst for selective phenolic/aniline pollutant degradation in wastewater.

Rice husk-derived biochar can aggravate arsenic mobility in ferrous-rich groundwater during oxygenation.

Elevated As(III) and Fe(II) in shallow reducing groundwater can be frequently re-oxidized by introducing O2 due to natural/anthropogenic processes, thus leading to oxidative precipitation of As as well as Fe. Nevertheless, the geochemical process may be impacted by co-existing engineered black carbon due to its considerable applications, which remains poorly understood. Taking rice husk-derived biochar prepared at 500 °C as an example, we explored its impact on the process particularly for the As(III) oxidation and (im)mobilization during the oxygenation. The presence of the biochar had a negligible effect on the As(III) oxidation and immobilization extents within 1 d, while accelerating their rates. However, the immobilized As(III) was significantly liberated from the formed Fe(III) minerals afterward within 21 d, which was 2.2-fold higher than that in the absence of the biochar. The enhanced As(III) liberation was attributed to the presence of the surface silicon-carbon structure, consisting of the outer silicon and inner carbon layers, of the rice husk-derived biochar. The outer silicon components, particularly for the dissolved silicate primarily promoted the As(III) release via ligand exchange, while significantly impeding the transformation of ferrihydrite to lepidocrocite and goethite still resulted secondarily in the As(III) release. Our findings reveal the possible impact of biochar on the environmental behavior and fate of As(III) in the Fe(II)-rich groundwater during the oxygenation. This work highlights that biochar, particularly for its structural features should be a concern in re-mobilizing As in such scenarios when the oxygenation time reaches several days or weeks.

Contrasting abiotic As(III) immobilization by undissolved and dissolved fractions of biochar in Ca2+-rich groundwater under anoxic conditions.

Engineered black carbon (biochar) can be introduced into groundwater through its extensive engineered applications (e.g., in-situ remediation of groundwater/soils), which can participate in geochemical processes that may alter the fate of trace contaminants such as arsenic (As(III)). Here we examined the impacts of the undissolved and dissolved fractions of reduced biochar (hereafter denoted as rUBC and rDBC, respectively) on the As(III) immobilization in the absence/presence of Ca2+ (50 mM) at pH 11.5 under anoxic conditions. While neither rUBC nor rDBC alone was capable of immobilizing As(III), the apparent As(III) immobilization by rUBC and rDBC synergistically occurred in the presence of Ca2+, with an efficiency of 73.1% and 89.6% within 24 h, respectively. In the rUBC/Ca2+/As(III) system, rUBC enabled full oxidation of As(III) to As(V) by its residual redox-active moieties such as quinoid CO and persistent free radicals, thereby facilitating precipitation of the newly generated As(V) with Ca2+ adsorbed onto the rUBC's surface. In contrast, rDBC induced in-situ local enrichment of Ca2+ in the nascent rDBC-derived flocs with predominant non-oxidative and slight oxidative precipitation of As(III) via ternary rDBC-Ca-As complexation. This ternary complex was created by Ca2+-bridging interactions between As species and oxygen-containing functional groups of rDBC, as evidenced by the FTIR results and the Ca2+-impeded As(III) oxidation. The generation of the flocs physically trapped a small amount of As species particularly As(III). Both the increases in Ca2+ concentration (0-100 mM) and solution pH (10.0-12.5) enhanced the apparent As(III) immobilization. This study provides new insights into the environmental impacts of two reduced biochar fractions released into typical Ca2+-rich aquifers on the fate and transport of As species.

pH Dependence of Arsenic Oxidation by Rice-Husk-Derived Biochar: Roles of Redox-Active Moieties.

Biochars have demonstrated great potential for water decontamination and soil remediation; however, their redox reactivity toward trace contaminants and the corresponding redox-active moieties (RAMs, i.e., phenolic -OH, semiquinone-type persistent free radicals (PFRs), and quinoid C═O) remain poorly understood. Here we investigated the roles of the RAMs on biochar in oxidation of As(III) under varying pH and O2 conditions. The results showed that the promoted oxidation of As(III) by the RAMs is strongly pH dependent. Under acidic and neutral conditions, only the oxidation of As(III) by •OH and H2O2 produced from activation of O2 by phenolic -OH and semiquinone-type PFRs occurred. In contrast, the oxidation by semiquinone-type PFRs, quinoid C═O, and H2O2 (if O2 was introduced) appeared under alkaline conditions. This pH-dependent oxidation behavior was attributed to the varying redox activities of RAMs, as confirmed by multiple characterization and validation experiments using biochar with tuned RAMs compositions, as well as thermodynamics evaluation. Our findings provide new insights into the roles of the RAMs on biochar in the promoted oxidation of trace As(III) over a broader pH range under both anoxic and oxic conditions. This study also paves a promising way to oxidize As(III) with biochar.

Mechanistic insights into adsorption and reduction of hexavalent chromium from water using magnetic biochar composite: Key roles of Fe3O4 and persistent free radicals.

Magnetic biochar (MBC) has been used to remove hexavalent chromium (Cr(VI)) from water, but the roles of Fe3O4 and persistent free radicals (PFRs) in MBC in Cr(VI) removal are still less investigated. In this work, the MBC synthesized by microwave co-pyrolysis of solid-state FeSO4 and rice husk was employed to remove Cr(VI) from water. In comparison to the rice husk biochar (BC), the MBC exhibits the 3.2- and 11.7-fold higher adsorption and reduction efficiency of Cr(VI), resulting in the higher Cr(VI) removal efficiency (84.3%) and equilibrium adsorption capacity of MBC (8.35 mg g-1) than that (26.5% and 2.63 mg g-1) of BC. Multiple characterization results revealed that the high Cr(VI) removal performance of MBC was mainly attributed to the presence of active Fe3O4 and carbon-centered PFRs in the porous and graphitic MBC. The Fe3O4 not only provided active chemisorption/reduction sites for Cr(VI) via its Fe(II)oct and Fe(III)oct coordination, but also facilitated the generation of more active electron donating carbon-centered PFRs than carbon-centered PFRs with an oxygen atom in the graphitic structure to reduce Cr(VI). The presence of Fe3O4 also elevated 36.7 m2 g-1 of BET-surface area and 0.043 cm2 g-1 of pore volume of MBC, promoting the Cr(VI) removal. The Fe3O4 and carbon-centered PFRs contributed to ∼81.8% and ∼18.2% of total Cr(III) generation, respectively. In addition, the initial solution pH was responsible for determining the relative significance of Cr(VI) adsorption and reduction. This study provides new insights into the mechanisms of Cr(VI) removal from water by the MBC.

Anaerobic co-digestion of food waste and landfill leachate in single-phase batch reactors.

In order to investigate the effect of raw leachate on anaerobic digestion of food waste, co-digestions of food waste with raw leachate were carried out. A series of single-phase batch mesophilic (35±1°C) anaerobic digestions were performed at a food waste concentration of 41.8 g VS/L. The results showed that inhibition of biogas production by volatile fatty acids (VFA) occurred without raw leachate addition. A certain amount of raw leachate in the reactors effectively relieved acidic inhibition caused by VFA accumulation, and the system maintained stable with methane yield of 369-466 mL/g VS. Total ammonia nitrogen introduced into the digestion systems with initial 2000-3000 mgNH4-N/L not only replenished nitrogen for bacterial growth, but also formed a buffer system with VFA to maintain a delicate biochemical balance between the acidogenic and methanogenic microorganisms. UV spectroscopy and fluorescence excitation-emission matrix spectroscopy data showed that food waste was completely degraded. We concluded that using raw leachate for supplement water addition and pH modifier on anaerobic digestion of food waste was effective. An appropriate fraction of leachate could stimulate methanogenic activity and enhance biogas production.