Simultaneous removal of Cd(II) and As(III) by graphene-like biochar-supported zero-valent iron from irrigation waters under aerobic conditions: Synergistic effects and mechanisms

Enhancing irrigation water quality efficiently with potassium feldspar-derived adsorbent: Heavy metal detoxification and nutrient augmentation

The pollution of Pb2+ and Cd2+ in both irrigation water and soil, coupled with the scarcity of vital mineral nutrition, poses a significant hazard to the security and quality of agricultural products. An economical potassium feldspar-derived adsorbent (PFDA) was synthesized using potassium feldspar as the main raw material through ball milling-thermal activation technology to solve this problem. The synthesis process is cost-effective and the resulting adsorbent demonstrates high efficiency in removing Pb2+ and Cd2+ from water. The removal process is endothermic, spontaneous, and stochastic, and follows the quasi-second-order kinetics, intraparticle diffusion, and Langmuir model. The adsorption and elimination of Pb2+ and Cd2+ is largely dependent on monolayer chemical sorption. The maximum removal capacity of PFDA for Pb2+ and Cd2+ at room temperature is 417 and 56.3 mg·g-1, respectively, which is superior to most mineral-based adsorbents. The desorption of Pb2+/Cd2+ on PFDA is highly challenging at pH≥3, whereas PFDA and Pb2+/Cd2+ are recyclable at pH≤0.5. When Pb2+ and Cd2+ coexisted, Pb2+ was preferentially removed by PFDA. In the case of single adsorption, Pb2+ was mainly adsorbed onto PFDA as Pb2SiO4, PbSiO3·xH2O, Pb3SiO5, PbAl2O4, PbAl2SiO6, PbAl2Si2O8, Pb2SO5, and PbSO4, whereas Cd2+ was primarily adsorbed as CdSiO3, Cd2SiO4, and Cd3Al2Si3O12. After the complex adsorption, the main products were PbSiO3·xH2O, PbAl2Si2O8, Pb2SiO4, Pb4Al2Si2O11, Pb5SiO7, PbSO4, CdSiO3, and Cd3Al2Si3O12. The forms of mineral nutrients in single and complex adsorption were different. The main mechanisms by which PFDA removed Pb2+ and Cd2+ were chemical precipitation, complexation, electrostatic attraction, and ion exchange. In irrigation water, the elimination efficiencies of Pb2+ and Cd2+ by PFDA within 10 min were 96.0 % and 70.3 %, respectively, and the concentrations of K+, Si4+, Ca2+, and Mg2+ increased by 14.0 %, 12.4 %, 55.7 %, and 878 %, respectively, within 60 min. PFDA holds great potential to replace costly methods for treating heavy metal pollution and nutrient deficiency in irrigation water, offering a sustainable, cost-effective solution and paving a new way for the comprehensive utilization of potassium feldspar.

Nanoconfined Fe(II) releaser for long-term arsenic immobilization and its sustainability assessment

Ferrous (Fe(II))-based oxygen activation for pollutant abatements in soil and groundwater has attracted great attention, while the low utilization and insufficient longevity of electron donors are the primary challenges to hinder its practical applications. Herein, we propose a nanoconfined Fe(II) releasing strategy that enables stable long-term electron donation for oxygen activation and efficient arsenic (As) immobilization under oxic conditions, by encapsulating zero-valent iron in biomass-derived carbon shell (ZVI@porous carbon composites; ZVI@PC). This strategy effectively enhances the generation of reactive oxygen species, enabling efficient oxidation and subsequent immobilization of As(III) in soils. Importantly, this Fe(II) releaser exhibits strong anti-interference capability against complex soil matrices, and the accompanying generation of Fe(III) enables As immobilization in soils, effectively lowering soil As bioavailability. Soil fixed-bed column experiments demonstrate a 79.5 % reduction of the total As in effluent with a simulated rainfall input for 10 years, indicating the excellent long-term stability for As immobilization in soil. Life cycle assessment results show that this Fe(II) releaser can substantially mitigate the negative environmental impacts. This work offers new insights into developing green and sustainable technologies for environmental remediation.

Adsorptive removal of heavy metals from aqueous solutions: Progress of adsorbents development and their effectiveness

Increased levels of heavy metals (HMs) in aquatic environments poses serious health and ecological concerns. Hence, several approaches have been proposed to eliminate/reduce the levels of HMs before the discharge/reuse of HMs-contaminated waters. Adsorption is one of the most attractive processes for water decontamination; however, the efficiency of this process greatly depends on the choice of adsorbent. Therefore, the key aim of this article is to review the progress in the development and application of different classes of conventional and emerging adsorbents for the abatement of HMs from contaminated waters. Adsorbents that are based on activated carbon, natural materials, microbial, clay minerals, layered double hydroxides (LDHs), nano-zerovalent iron (nZVI), graphene, carbon nanotubes (CNTs), metal organic frameworks (MOFs), and zeolitic imidazolate frameworks (ZIFs) are critically reviewed, with more emphasis on the last four adsorbents and their nanocomposites since they have the potential to significantly boost the HMs removal efficiency from contaminated waters. Furthermore, the optimal process conditions to achieve efficient performance are discussed. Additionally, adsorption isotherm, kinetics, thermodynamics, mechanisms, and effects of varying adsorption process parameters have been introduced. Moreover, heavy metal removal driven by other processes such as oxidation, reduction, and precipitation that might concurrently occur in parallel with adsorption have been reviewed. The application of adsorption for the treatment of real wastewater has been also reviewed. Finally, challenges, limitations and potential areas for improvements in the adsorptive removal of HMs from contaminated waters are identified and discussed. Thus, this article serves as a comprehensive reference for the recent developments in the field of adsorptive removal of heavy metals from wastewater. The proposed future research work at the end of this review could help in addressing some of the key limitations facing this technology, and create a platform for boosting the efficiency of the adsorptive removal of heavy metals.

Biochar derivation at low temperature: A novel strategy for harmful resource usage of antibiotic mycelial dreg

Antibiotic mycelial dreg (AMD) has been categorized as hazardous waste due to the high residual hazardous contaminants. Inappropriate management and disposal of AMD can cause potential environmental and ecological risks. In this study, the potential of pleuromutilin mycelial dreg (PMD) as a novel feedstock for preparing tetracycline hydrochloride (TC) adsorbent was explored to achieve safe management of PMD. The results suggested that residual hazardous contaminants were completely eliminated after pyrolysis. With the increase of pyrolysis temperature, the yields, H/C, O/C, (O + N)/C, and pore size in PMD-derived biochars (PMD-BCs) decreased, while BET surface area and pore volume increased, resulting in the higher stability of the PMD-BCs prepared from higher temperatures. The TC adsorption of the PMD-BCs increased from 27.3 to 46.9 mg/g with the increase of the pyrolysis temperature. Surprisingly, pH value had a strong impact on the TC adsorption, the adsorption capacity of BC-450 increased from 6.5 to 71.1 mg/g when the solution pH value increased from 2 to 10. Lewis acid-base interaction, pore filling, π-π interaction, hydrophobic interaction, and charge-assisted hydrogen bond (CAHB) are considered to drive the adsorption. This work provides a novel pathway for the concurrent detoxification and reutilization of AMD.

A novel magnetic graphene-loaded biochar gel for the remediation of arsenic- and antimony-contaminated mining soil

Metalloid co-contamination such as arsenic (As) and antimony (Sb) in soils has posed a significant threat to ecological balance and human well-being. In this study, a novel magnetic graphene-loaded biochar gel (FeBG) was developed, and its remediation potential for the reclamation of AsSb spoiled soil was assessed through a six-month soil incubation experiment. Results showed that the incorporation of iron substances and graphene imparted FeBG with enhanced surface characteristics, such as the formation of a new FeO bond and an enlarged surface area compared to the pristine biochar (BC) (80.5 m2 g-1 vs 57.4 m2 g-1). Application of FeBG significantly decreased Na2HPO4-extractable concentration of As in soils by 9.9 %, whilst BC addition had a non-significant influence on As availability, compared to the control. Additionally, both BC (8.2 %) and FeBG (16.4 %) treatments decreased the Na2HPO4-extractable concentration of Sb in soils. The enhanced immobilization efficiency of FeBG for As/Sb could be attributed to FeBG-induced electrostatic attraction, complexation (Fe-O(H)-As/Sb), and π-π electron donor-acceptor coordination mechanisms. Additionally, the FeBG application boosted the activities of sucrase (9.6 %) and leucine aminopeptidase (7.7 %), compared to the control. PLS-PM analysis revealed a significant negative impact of soil physicochemical properties on the availability of As (β = -0.611, P < 0.01) and Sb (β = -0.848, P < 0.001) in soils, in which Sb availability subsequently led to a suppression in soil enzyme activities (β = -0.514, P < 0.01). Overall, the novel FeBG could be a potential amendment for the simultaneous stabilization of As/Sb and the improvement of soil quality in contaminated soils.

Pristine and Fe-functionalized biochar for the simultaneous immobilization of arsenic and antimony in a contaminated mining soil

This study examined the effectiveness of pristine biochar (BC) and Fe-functionalized biochar (FBC) in remediating As-Sb co-contaminated soil, and revealed the resulting impact on soil enzymatic activities and bacterial communities. Results from incubation experiments showed that the 1.5% FBC treatment reduced the bioavailable As and Sb concentration by 13.5% and 27.1%, respectively, in compared to the control, and reduced the proportion of specifically adsorbed and amorphous Fe-Mn oxide-bound metal(loid) fractions in the treated soil. Among the BC treatments, only the 1.5% BC treatment resulted in a reduction of bioavailable As by 11.7% and Sb by 21.4%. The 0.5% BC treatment showed no significant difference. The FBC achieved high As/Sb immobilization efficiency through Fe-induced electrostatic attraction, π-π electron donor-acceptor coordination, and complexation (Fe-O(H)-As/Sb) mechanisms. Additionally, the 1.5% FBC treatment led to a 108.2% and 367.4% increase in the activities of N-acetyl-β-glucosaminidase and urease in soils, respectively, compared to the control. Furthermore, it significantly increased the abundance of Proteobacteria (15.2%), Actinobacteriota (37.0%), Chloroflexi (21.4%), and Gemmatimonadota (43.6%) at the phylum level. Co-occurrence network analysis showed that FBC was better than BC in increasing the complexity of bacterial communities. Partial least squares path modeling further indicated that the addition of biochar treatments can affect soil enzyme activities by altering soil bacterial composition. This study suggests that FBC application offers advantages in simultaneous As and Sb immobilization and restructuring the bacterial community composition in metal(loid)-contaminated soil.

In situ carbothermal synthesis of carbonized bacterial cellulose embedded with nano zero-valent iron for removal of Cr(VI)

Carbonized bacterial cellulose embedded with highly dispersed nano zero-valent iron (nZVI), denoted as nZVI@CBC, was prepared through one-step in situ carbothermal treatment of bacterial cellulose adsorbing iron(III) nitrate. The structure characteristics of nZVI@CBC and its performance in removing hexavalent chromium Cr(VI) were investigated. Results showed the formation of nZVI@CBC with a surface area of 409.61 m2/g at 800 °C, with nZVI particles of mean size 28.2 nm well distributed within the fibrous network of CBC. The stability of nZVI was enhanced by its carbon coating, despite some inevitable oxidation of exposed nZVI. Batch experiments demonstrated that nZVI@CBC exhibited superior removal efficiency compared to bare nZVI and CBC. Under optimal conditions, nZVI@CBC exhibited a high Cr(VI) adsorption capacity of up to 372.42 mg/g. Therefore, nZVI@CBC shows promise as an effective adsorbent for remediating Cr(VI) pollution in water.

Simultaneous removal of Cd(II) and As(V) by ferrihydrite-biochar composite: Enhanced effects of As(V) on Cd(II) adsorption

The coexistence of cadmium (Cd(II)) and arsenate (As(V)) pollution has long been an environmental problem. Biochar, a porous carbonaceous material with tunable functionality, has been used for the remediation of contaminated soils. However, it is still challenging for the dynamic quantification and mechanistic understanding of the simultaneous sequestration of multi-metals in biochar-engineered environment, especially in the presence of anions. In this study, ferrihydrite was coprecipitated with biochar to investigate how ferrihydrite-biochar composite affects the fate of heavy metals, especially in the coexistence of Cd(II) and As(V). In the solution system containing both Cd(II) and As(V), the maximum adsorption capacities of ferrihydrite-biochar composite for Cd(II) and As(V) reached 82.03 µmol/g and 531.53 µmol/g, respectively, much higher than those of the pure biochar (26.90 µmol/g for Cd(II), and 40.24 µmol/g for As(V)) and ferrihydrite (42.26 µmol/g for Cd(II), and 248.25 µmol/g for As(V)). Cd(II) adsorption increased in the presence of As(V), possibly due to the changes in composite surface charge in the presence of As(V), and the increased dispersion of ferrihydrite by biochar. Further microscopic and mechanistic results showed that Cd(II) complexed with both biochar and ferrihydrite, while As(V) was mainly complexed by ferrihydrite in the Cd(II) and As(V) coexistence system. Ferrihydrite posed vital importance for the co-adsorption of Cd(II) and As(V). The different distribution patterns revealed by this study help to a deeper understanding of the behaviors of cations and anions in the natural environment.

Fast and efficient As(III) removal from water by bifunctional nZVI@NBC

As a notable toxic substance, metalloid arsenic (As) widely exists in water body and drinking As-contaminated water for an extended period of time can result in serious health concerns. Here, the performance of nanoscale zero-valent iron (nZVI) modified N-doped biochar (NBC) composites (nZVI@NBC) activated peroxydisulfate (PDS) for As(III) removal was investigated. The removal efficiencies of As(III) with initial concentration ranging from 50 to 1000 μg/L were above 99% (the residual total arsenic below 10 μg/L, satisfying the contaminant limit for arsenic in drinking water) within 10 min by nZVI@NBC (0.2 g/L)/PDS (100 μM). As(III) removal efficiency influenced by reaction time, PDS dosage, initial concentration, pH, co-existing ions, and natural organic matter in nZVI@NBC/PDS system were investigated. The nZVI@NBC composite is magnetic and could be conveniently collected from aqueous solutions. In practical applications, nZVI@NBC/PDS has more than 99% As(III) removal efficiency in various water bodies (such as deionized water, piped water, river water, and lake water) under optimized operation parameters. Radical quenching and EPR analysis revealed that SO4·- and ·OH play important roles in nZVI@NBC/PDS system, and the possible reaction mechanism was further proposed. These results suggest that nZVI@NBC activated peroxydisulfate may be an efficient and fast approach for the removal of water contaminated with As(III).

Synergistic removal performance and mechanism of Cd(II) and As(III) from irrigation water by iron sulfide-based porous biochar

Since Cd(II) and As(III) have extremely opposite chemical characteristics, it is a huge challenging to simultaneously remove these two ions from aqueous solutions. Therefore, a novel iron sulfide-based porous biochar (FSB) was synthesized and used to evaluate its Cd(II) and As(III) removal performance and mechanisms. The characterization and batch experiments results indicated that FeS was successfully loaded on the surface of biochar and increased its adsorption sites. The iron sulfide-based porous biochar was very favorable for the removal of Cd(II) and As(III) in the weakly acidic environment. The maximum adsorption of Cd(II) and As(III) by FSB was 108.8 mg g-1 and 76.3 mg g-1, respectively, according to the Langmuir and Freundlich isothermal adsorption model, and the adsorption equilibrium time was 12 h and 4 h, respectively, according to the pseudo-second-order kinetic model. In the coexisting ion system, Cd(II) adsorption was suppressed by Ca2+, Mg2+, and humic acid, but enhanced by PO43- and As(III). As(III) adsorption was inhibited by PO43- and humic acid. Precipitation and complexation are the predominant adsorption mechanisms of Cd(II) and As(III), which contribute to the formation of Cd-O, Fe-O-Cd, As-O, Fe-O-As, ternary complex Cd-Fe-As, and stable compounds FeAsO4·2H2O and CdS. Therefore, The iron sulfide-based porous biochar can be an efficient and environmentally friendly candidate for the treatment of Cd(II) and As(III) co-polluted irrigation water.

Application of biochar on soil bioelectrochemical remediation: behind roles, progress, and potential

Bioelectrochemical systems (BESs) that combine electrochemistry with biological methods have gained attention in the remediation of polluted environments, including wastewater, sludge, sediments, and soils. The most attractive advantage of BESs is that the solid electrode is used as an inexhaustible electron acceptor or donor, and biocurrent directly converted from organics can afford the reaction energy of contaminant breakdown, crossing the internal energy barrier of endothermic degradation, which achieves a continuous biodegradation process without the simultaneous use of exogenetic chemicals and bioelectricity recovery. However, soil BESs are hindered by expensive electrode materials, difficult pollutant and electron transfer, low microbial competitive activity, and biocompatibility in contamination remediation. Fortunately, introducing biochar into soil BESs could reveal a high potential in addressing these BES inadequacies. The characteristics of biochar, e.g., conductivity, transferability, high specific surface area, high porosity, large functional groups, and biocompatibility, can improve the performance of soil BESs. In fact, biochar not only carries electrons but also transfers nutrients, pollutants, and even bacteria by facilitating transmission in the bioelectric field of BESs. Consequently, the abilities of biochar make for better functionality of BESs. This review collates information on the roles, application, and progress of biochar in soil BESs, and future prospects are given. It is beneficial for environmental researchers and engineers to extend BES application in environmental remediation and to assist the progress of carbon sequestration and emission reduction based on the inertia of biochar and the blocking of electron flow to form methane.

Multitask Deep Learning Enabling a Synergy for Cadmium and Methane Mitigation with Biochar Amendments in Paddy Soils

Biochar has demonstrated significant promise in addressing heavy metal contamination and methane (CH4) emissions in paddy soils; however, achieving a synergy between these two goals is challenging due to various variables, including the characteristics of biochar and soil properties that influence biochar's performance. Here, we successfully developed an interpretable multitask deep learning (MTDL) model by employing a tensor tracking paradigm to facilitate parameter sharing between two separate data sets, enabling a synergy between Cd and CH4 mitigation with biochar amendments. The characteristics of biochar contribute similar weightings of 67.9% and 62.5% to Cd and CH4 mitigation, respectively, but their relative importance in determining biochar's performance varies significantly. Notably, this MTDL model excels in custom-tailoring biochar to synergistically mitigate Cd and CH4 in paddy soils across a wide geographic range, surpassing traditional machine learning models. Our findings deepen our understanding of the interactive effects of Cd and CH4 mitigation with biochar amendments in paddy soils, and they also potentially extend the application of artificial intelligence in sustainable environmental remediation, especially when dealing with multiple objectives.

Effective reducing the mobility and health risk of mercury in soil under thiol-modified biochar amendment

Soil mercury contamination poses health risks. However, effective immobilization techniques are lacking with challenges including low efficiency, limited long-term stability, susceptibility to multi-medium interference, and difficulty in controlling health risks. This study confirmed the feasibility of thiol-modified biochar, and elucidated the underlying mechanisms. Within 32 days of treatment, the leachable mercury decreased from 184.7 μg/L to below the hazardous waste threshold (100 μg/L, HJ/T299-2007, China). After 198 days of treatment, the soil achieved a safe ecological state with a mercury immobilization rate of 79.8-98.2% and a 50% reduction in available methylmercury. Thiol-modified biochar facilitated the conversion of active mercury species (exchangeable, carbonate, and oxide) into stable forms (organic and residual) through complexation and precipitation (e.g., HgS). Soil quality improvements were observed, including enhanced cation exchange capacity, available nitrogen, and total organic carbon. Thiol-modified biochar exhibits long-term effectiveness. After one and two years of treatment, the leachable mercury remained within acceptable health risk limit (hazard quotient < 1) for adults and children, respectively. After three years, the leachable mercury met the Level III groundwater quality standard (< 1 μg/L, GB 14848-2017, China). This study demonstrates an effective strategy for long-term diminishing mercury mobility and health risks in soil.

Critical layer in liquid-solid system influencing the remediation of chromium using zeolite-supported sulfide nano zero-valent iron

Sulfidated nano zero-valent iron particles were immobilized on ZSM-5 zeolite (Z/S-nZVI) and used for hexavalent chromium (Cr(VI)) remediation. The performance of Z/S-nZVI improved with the increase in Cr(VI) concentration (< 60 mg/L), while the performance significantly decreased for a Cr(VI) concentration of more than 60 mg/L. The adsorption behavior for Cr(VI) was different from that reported in previous studies. The improved performance can be tailored for increasing efficiency of nano zero-valent iron (nZVI) corrosion, while the degree of corrosion of nZVI was affected by the concentration of the pollutant as discussed by kinetics, X-ray diffraction (XRD) and X-ray photoelectron spectrometer (XPS) analyses. The experiments for the dissolution of ferrous ions and the dosage of adsorbent demonstrated that the critical layer in the liquid-solid system changed with the increase in the concentration of Cr(VI) (Cr(VI): Z/S-nZVI > 0.6). Moreover, the removal mechanisms of Cr(VI) were elucidated through XRD, transmission electron microscopy (TEM) and XPS techniques. This results demonstrate that the species of chromium in the critical layer changed from Cr(III) to Cr(VI) as the concentration of chromium increased from low to high. Furthermore, the critical layer was composed of Cr(VI), Fe(II), O and H elements. Additionally, the experiments of coexisting ions and aging time confirmed that Z/S-nZVI possessed high selectivity and stability to ensure efficiency and cost-effectiveness in practical applications.

Preparation of chitosan mesoporous membrane/halloysite composite for efficiently selective adsorption of Al(III) from rare earth ions solution through constructing pore structure on substrate

The removal of impurity Al(III) from rare earth ion solution by selective adsorption method was one of the challenging tasks. Herein, calcination and acid dissolution treatment were used to construct the pore structure for the halloysite substrate (Hal-650-H) and provide conditions for the formation of the chitosan mesoporous membrane to prepare composite (Hal-H-2CS). The selective adsorption properties and mechanism of the Hal-H-2CS for Al(III) in the rare earth ion solution were studied. The results showed that the formation of mesoporous structures for chitosan provided abundant sites for the adsorption of Al(III). Hal-H-2CS showed remarkable selective adsorption properties for Al(III) in a wide pH range and the binary mixtures with high content of Al(III) or La(III). The maximum adsorption capacity of Al(III) was 106 mg/g, while the adsorption capacity of La(III) was only 1.41 mg/g at pH 4.0. In addition, the Hal-H-2CS exhibited excellent regeneration and structural stability. The remarkable selective properties of Hal-H-2CS was achieved by the synergistic effect between chitosan mesoporous membrane and Hal-650-H, the main adsorption sites were the OH, NH2, CONH2 of chitosan and the oxygen sites of the Hal-650-H. This work provides a new strategy for the design and preparation of outstanding selective adsorbent for Al(III).

Visualizing the bibliometrics of biochar research for remediation of arsenic pollution

Arsenic-contamination of soil and water has always been a topic of considerable concern, and the potential of biochar for remediation of arsenic contamination has been widely recognized due to its advantages, including abundant sources, simple preparation, large surface area, significant pore size, and rich functional groups. To gain insights into the development trends in this field and provide suggestions for future research directions, scientometric analysis was conducted on articles sourced from the Web of Science core collection database by using the CiteSpace and VOSviewer software. In total, 637 bibliographic records, retrieved using the keywords "biochar" and "arsenic" were analyzed based on publication distribution over the years, contributing countries, keywords, authors, cited authors, publishing journals, and highly cited articles. Further, progress maps were generated from these data sets to assess the current research landscape. Results revealed a steady increase in annual publications since 2009, and China has the most publications. Notably, Daniel C. W. Tsang stood out as a representative author. The journal "Science of the Total Environment" published the most articles related to biochar and arsenic. "Adsorption" is the most frequently occurring keyword. The investigations of the impact and mechanism of biochar and modified biochar on inorganic arsenic removal from water and immobilization in soil have been identified as current research focal points. In order to realize the efficient and safe use of biochar, the future necessitates the implementation of advanced technology to conduct further comprehensive research. This study highlights the ongoing advancements in the research field on biochar and arsenic. Valuable insights are provided for future researchers and policymakers to guide their significant efforts toward addressing the issue of soil and water contamination caused by arsenic and exploring the potential of biochar for effective remediation strategies.

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.

A novel magnetic sludge biochar was prepared by making full use of internal iron in sludge combining KMnO4-NaOH modification to enhance the adsorption of Pb (Ⅱ), Cu (Ⅱ) and Cd (Ⅱ)

This study synthesized novel magnetic biochar (PCMN600) by KMnO4-NaOH combined modification using iron-containing pharmaceutical sludge to remove toxic metals from wastewater effectively. Various characterization experiments of engineered biochar showed that the modification process introduced ultrafine MnOx particles on the carbon surface and resulted in higher BET surface area and porosity along with more oxygen-containing surface functional groups. Batch adsorption studies indicated that the maximum adsorption capacities of PCMN600 for Pb2+, Cu2+ and Cd2+ were 181.82 mg/g, 30.03 mg/g and 27.47 mg/g, respectively, at a temperature of 25 °C and pH of 5.0, which were much higher than that of pristine biochar (26.46 mg/g, 6.56 mg/g and 6.40 mg/g). The adsorption datums of three toxic metal ions fitted well to the pseudo-second-order model and Langmuir isotherm, and the sorption mechanisms were identified as electrostatic attraction, ion exchange, surface complexation, cation-π interaction and precipitation. The strong magnetic properties of the engineered biochar endowed the adsorbent with remarkable reusability, and after five cycles of recycling, PCMN600 still retained nearly 80% of its initial adsorption capacities.

A novel biochar-based composite hydrogel for removing heavy metals in water and alleviating cadmium stress in tobacco seedlings

A novel composite hydrogel (AM/CMC/B) synthesized from peanut shell biochar effectively adsorbs heavy metal Cd in water and reduces its toxicity to tobacco seedlings. The hydrogel, prepared via hydrothermal polymerization using acrylamide (AM), carboxymethyl cellulose (CMC), and peanut shell biochar (B), exhibited a maximum adsorption capacity of 164.83 mg g-1 for Cd2+ and followed a pseudo-second-order kinetic model. In pot experiments, the application of exogenous AM/CMC/B mitigated the inhibitory effects of Cd-contaminated soil on tobacco seedling growth. Addition of 10 mg kg-1 Cd resulted in improved phenotype, root system development, enhanced photosynthetic capacity, stomatal conductance (Gs), stomatal number, and increased antioxidant activity while reducing MDA content and leaf cell death. These findings highlight the potential of AM/CMC/B as an environmentally friendly adsorbent for Cd removal from water and for reducing Cd stress toxicity in tobacco and other plants.

All-in-one strategy to prepare molded biochar with magnetism from sewage sludge for high-efficiency removal of Cd(Ⅱ)

Biochar in powder could lead to the separation difficulties after using and easy dispersion by wind with non-necessary consumption during the practical application. The current method for preparing molded biochar is multi-step, tedious, and required exogenous reagents. Moreover, the dehydration of sewage sludge with high water content (>85%) causes expensive production cost, limiting its secondary utilization. Therefore, an "all-in-one" strategy was developed to prepare molded biochar with magnetism by using sewage sludge as endogenetic binder, water source, carbon source, as well as magnetic source, and biomass wastes as water moderator and pore-forming agent. The molded biochar showed high removal capacity towards Cd(Ⅱ) of 456.2 mg/g, which was 6 times higher than the commercial activated carbon in powder (69.1 mg/g). The excellent removal performance of the molded biochar was in linear correlation the O/C ratio (R² =0.855), resulting in the complexation with Cd(Ⅱ). DFT calculations indicated the amounts and species of oxygen changed the electron distribution and electron-donation properties of biochar for Cd(Ⅱ). Moreover, the Na⁺ exchanges with Cd(Ⅱ) were also an important removal mechanism. This study provided a novel synthesis strategy for the molded biochar with both high particle density and high adsorption capability.

Safety of African grown rice: Comparative review of As, Cd, and Pb contamination in African rice and paddy fields

This review aimed to investigate the reported concentrations of arsenic (As), cadmium (Cd), and lead (Pb) in rice cultivated in Africa and African rice paddies compared to other regions. It also aimed to explore the factors influencing these concentrations and evaluate the associated health risks of elevated As, Cd, and Pb exposure. Relevant data were obtained from electronic databases such as PubMed, Scopus, and Google Scholar using specific keywords related to arsenic, cadmium, lead, rice, Africa, paddy, and grain. While the number of studies reporting the concentrations of As, Cd, and Pb in rice and rice paddies in Africa is relatively low compared to other regions, this review revealed that most of the African rice and paddy soils have low concentrations of these metals. However, some studies have reported elevated concentrations of As, Cd, and Pb in paddy fields, which is concerning due to the increased use of agrochemicals containing heavy metals in rice production. Nonetheless, agronomical interventions such as implementing alternate wetting and drying water management, cultivating cultivars with low accumulation of As, Cd, and Pb, amending rice fields with sorbents, and screening irrigation water can limit the bioaccumulation of these carcinogens in paddy fields using phytoremediation techniques. Therefore, we strongly urge African governments and organizations operating in Africa to enhance the capacity of rice farmers and extension officers in adopting approaches and practices that reduce the accumulation of these carcinogenic metals in rice. This is essential to achieve the sustainable development goal of providing safe food for all.

Qualitative and quantitative analysis for Cd2+ removal mechanisms by biochar composites from co-pyrolysis of corn straw and fly ash

Removal of heavy metals (e.g., Cd) from contaminated water using waste-converted adsorbents is promising, but the efficiency still needs to be improved. Here, we prepared a functional biochar composite as novel Cd adsorbents by co-pyrolysis of two typical solid wastes, i.e., agricultural corn straw and industrial fly ash. The adsorption behavior and mechanism were investigated using batch and column adsorption experiments and modern characterization techniques. Results showed that alkali-modified fly ash (AMFA) was loaded onto the surface of the corn straw biochar as some fine particle forms, with quartz (SiO₂) and silicate being the main mineral phases on the surface. The maximum sorption capacity fitted by Langmuir model for functionalized biochar composite (FBC700) was up to 137.1 mg g^-1, which was 7.7 times higher than that of the original corn straw biochar (BC700). Spectroscopic analysis revealed that adsorption mechanisms of Cd onto the FBC700 included mainly precipitation and ion exchange, with complexation and Cd-π interaction also contributing. The AMFA could effectively improve the mineral precipitation with Cd. The adsorption columns filled with FBC700 exhibited a longer breakthrough time than that filled with BC700. The adsorption capacity calculated by Thomas model for FBC700 was also approximately 6.0 times higher than that for BC700, showing that FBC700 was more suited to practical applications. This study provided a novel perspective for recycling solid wastes and treating Cd-contaminated water.

Biochar modification to enhance arsenic removal from water: a review

Arsenic (As) contamination is a major threat to drinking water quality throughout the world, and the development of appropriate remediation methods is critical. Adsorption is considered the most effective method for remediation of As-contaminated water. Biochar is a promising adsorbent and widely discussed for As removal due to its potential low cost and environmental friendliness. However, pristine biochar generally exhibited relatively low adsorption capacity for As mainly due to the electrostatic repulsion between the negatively charged biochar and As. Biochar modification, especially metal modification, was developed to boost the adsorption capacity for As. A systematic analysis of As removal as affected by biochar properties and modification will be of great help for As removal. This paper presents a comprehensive review on As removal by biochars from different feedstock, preparation procedures, and modification methods, with a major focus on the possible mechanisms of interaction between As and biochar. Biochar derived from sewage sludge exhibited relatively high adsorption capacity for As. Considering energy conservation, biochars prepared at 401-500 °C were more favorable in adsorbing As. Fe-modified biochar was the most popular modified biochar for As remediation due to its low cost and high efficiency. In addition, the limitations of the current studies and future perspectives are presented. The aim of this review is to provide guidance for the preparation of low-cost, environmentally friendly, and high efficiency biochar for the remediation of As-contaminated water.

Synergistic effect and mechanism of Cd(II) and As(III) adsorption by biochar supported sulfide nanoscale zero-valent iron

Biochar derived from bamboo was used to support sulfide nanoscale zero-valent iron (S-nZVI@BC) for simultaneous removal of Cd(II) and As (III) from aqueous media. Scanning electron microscopy (SEM) and X-ray diffraction spectroscopy (XRD) characterization confirmed the successful synthesis of the S-nZVI@BC. Adsorption kinetics and isotherms indicated that co-adsorption of Cd(II) and As(III) onto S-nZVI@BC was well represented by pseudo-second-order model (R²_Cd(II) = 0.990, R²_As(III) = 0.995) and Langmuir model (R²_Cd(II) = 0.954, R²_As(III) = 0.936). The maximum adsorption was 162.365 and 276.133 mg/g for Cd(II) and As(III), respectively, in a co-adsorption system, which was significantly higher than that in a single adsorption system (103.195 and 223.736 mg/g, respectively). Batch experiments showed that the Cd(II)-to-As(III) concentration ratio significantly affected the co-adsorption with the optimal ratio of 1:2. Ca²⁺ and Mg²⁺ significantly inhibited Cd(II) removal. In contrast, phosphate and humic acid significantly inhibited As(III) removal. Electrochemical analysis indicated S-nZVI@BC had a lower corrosion potential and resistance than nZVI@BC, making it more conducive to electron transfer and chemical reaction. Electrostatic adsorption, complexation, co-precipitation, and redox were the primary mechanisms for Cd(II) and As(III) removal. Overall, the present study provides new insights into the synergistic removal of Cd(II) and As(III) by S-nZVI@BC, which is a very promising adsorbent for the effective removal of Cd(II) and As(III) from contaminated wastewater.

Synergistic effect in simultaneous removal of cationic and anionic heavy metals by nitrogen heteroatom doped hydrochar from aqueous solutions

Industrial wastewater typically contains both cationic and anionic heavy metals; therefore, their simultaneous removal must be considered to ensure environmental sustainability. Herein, nitrogen heteroatom (N) doped hydrochar derived from corncob was prepared via facile NH₄Cl-aided hydrothermal carbonization and used for the simultaneous adsorption of divalent copper (Cu(II)) and hexavalent chromium (Cr(VI)) in aqueous solutions. During hydrothermal carbonization, NH₄Cl played a vital role as the porogen and N dopant, which contributed to the efficient adsorption affinity toward coexisting Cu(II) and Cr(VI). The theoretical maximum adsorption capacities of the N-doped hydrochar were determined to be 1.223 mmol/g for Cu(II) and 1.995 mmol/g for Cr(VI), which were much better than those of the pristine hydrochar. Furthermore, in the binary-component system, the synergistic effect between Cu(II) and Cr(VI) significantly promoted the adsorption affinity of N-doped hydrochar, resulting in adsorption capacities for Cu(II) and Cr(VI) 9.48 and 1.92 times higher than those of the single-component system, respectively. A series of adsorption experiments and spectroscopic analyses demonstrated that multiple mechanisms, including electrostatic shielding, cation bridging, and redox reactions, mutually contributed to the synergistic effect in the adsorption of coexisting Cu(II) and Cr(VI). Overall, the N-doped hydrochar proved to be effective in simultaneously removing both cationic and anionic heavy metal pollutants.

Removal of heavy metals from binary and multicomponent adsorption systems using various adsorbents - a systematic review

The ecosystem and human health are both significantly affected by the occurrence of potentially harmful heavy metals in the aquatic environment. In general, wastewater comprises an array of heavy metals, and the existence of other competing heavy metal ions might affect the adsorptive elimination of one heavy metal ion. Therefore, to fully comprehend the adsorbent's efficiency and practical applications, the abatement of heavy metals in multicomponent systems is important. In the current study, the multicomponent adsorption of heavy metals from different complex mixtures, such as binary, ternary, quaternary, and quinary solutions, utilizing various adsorbents are reviewed in detail. According to the systematic review, the adsorbents made from locally and naturally occurring materials, such as biomass, feedstocks, and industrial and agricultural waste, are effective and promising in removing heavy metals from complex water systems. The systematic study further discovered that numerous studies evaluate the adsorption characteristics of an adsorbent in a multicomponent system using various important independent adsorption parameters. These independent adsorption parameters include reaction time, solution pH, agitation speed, adsorbent dosage, initial metal ion concentration, ionic strength as well as reaction temperature, which were found to significantly affect the multicomponent sorption of heavy metals. Furthermore, through the application of the multicomponent adsorption isotherms, the competitive heavy metals sorption mechanisms were identified and characterized by three primary kinds of interactive effects including synergism, antagonism, and non-interaction. Despite the enormous amount of research and extensive data on the capability of different adsorbents, several significant drawbacks hinder adsorbents from being used practically and economically to remove heavy metal ions from multicomponent systems. As a result, the current systematic review provides insights and perspectives for further studies through the thorough and reliable analysis of the relevant literature on heavy metals removal from multicomponent systems.

Using organo-mineral complex material to prevent the migration of soil Cd and As into crops: An agricultural practice and chemical mechanism study

The migration and transformations of Cd and As in soil are different, so it is difficult to simultaneously control them. In this study, an organo-mineral complex (OMC) material was prepared using modified palygorskite and chicken manure, the Cd and As adsorption capacities and mechanism of the OMC were explored, and the response of the crop to the OMC was clarified. The results show that the maximum Cd and As adsorption capacities of the OMC under pH values of 6-8 are 12.19 mg·g^-1 and 5.07 mg·g^-1, respectively. In the OMC system, the modified palygorskite contributed more to the adsorption of the heavy metals than the organic matter. Cd²⁺ may form CdCO₃ and CdFe₂O₄, and AsO₂^- may form FeAsO₄, As₂O₃, and As₂O₅ on the surfaces of the modified palygorskite. Organic functional groups such as hydroxyl, imino, and benzaldehyde groups can participate in the adsorption of Cd and As. The Fe species and carbon vacancy in the OMC system promote the conversion of As³⁺ into As⁵⁺. A laboratory experiment was conducted to compare five commercial remediation agents with OMC. Planting Brassica campestris in the OMC remediated soil with excessive contamination increased the crop biomass and decreased the Cd and As accumulation sufficiently to meet the current national food safety standards. This study emphasizes the effectiveness of OMC in preventing the migration of Cd and As into crops while promoting crop growth, which can provide a feasible soil management strategy for CdAs co-contaminated farmland soil.

Removal of As(III) using a microorganism sustained secrete laccase-straw oxidation system

While laccase oxidation is a novel and promising method for treating arsenite-containing wastewater, the high cost and unsustainability of commercially available enzymes indicate a need to investigate more cost-effective viable alternatives. Here, a microorganism sustained secrete laccase-straw oxidation system (MLOS) was established and subsequently evaluated for the removal of As(III). MLOS showed efficient biological As(III) oxidation, with an As(III) removal efficiency reaching 99.9% at an initial As(III) concentration of 1.0 mg·L^-1. IC-AFS and XPS analysis showed that As(III) was partially oxidized to As(V), and partially As(III) adsorbed on the surface of rice straw. FTIR analysis revealed that hydroxyl, amine and amide groups were all involved in the As(III) removal process. SEM-EDS demonstrated that the surface structure of rice straw was destroyed following Comamonas testosteroni FJ17 (C. testosteroni FJ17) treatment, and the metal ions binding sites of rice straw were increased resulting in elemental arsenic being detected on the material surface. Molecular docking revealed the interaction between key residues of laccase and As(III). Laccase activity was negatively correlated with Cu(II) concentration in the As(III) oxidation. EEM showed that humic-like acids were also involved in the interaction with As(III). Overall, a MLOS derived from biomass waste has a significant potential to be developed as a green and sustainable technology for the treatment of wastewater containing As(III).

Treatment of Cd(Ⅱ) and As(Ⅴ) co-contamination in aqueous environment by steel slag-biochar composites and its mechanism

As toxic elements of concern, Cd(II) and As(V) pose a threat to human health. In this study, a new type of magnetic adsorbent (SBNa800) was prepared using a mixture of industrial waste steel slag and ginkgo leaves to treat wastewater contaminated with Cd(II) and As(V). The maximum adsorption capacities of SBNa800 for Cd(II) and As(V) were 109.17 (pH 5, 1.82 times that of the original biochar) and 59.79 (pH 3) mg/g, respectively. Cd(II) and As(V) adsorption capacities was above 90 % at pH = 4. Cd(II) and As(V) were synergistic and competitive adsorption. The results of μ-XRF, XANES and XPS showed that Cd(II) was adsorbed by SBNa800 in the forms of Cd(OH)₂, CdCO₃, Cd₅H₂(AsO₄)₄·4 H₂O, CdCl₂·2.5 H₂O and Cd(NO₃)₂. About 52.79-64.61 % As(V) was reduced to As(III) by Fe(0) on SBNa800 and then adsorbed. The adsorption mechanisms of Cd(II), As(V) and As(III) were hydrogen bonding/electrostatic attraction, inner-sphere complexation and precipitation. The saturation magnetisation of SBNa800, which was easy to separate from wastewater, was 6.54 emu/g. Therefore, SBNa800 can be used as a potential adsorbent to treat wastewater contaminated with Cd(II) and As(V).

Humic acid and fulvic acid facilitate the formation of vivianite and the transformation of cadmium via microbially-mediated iron reduction

The effects of humic acids (HA) and fulvic acids (FA) on the fate of Cd in anaerobic environment upon microbial reduction of Cd-bearing ferrihydrite (Fh) with Geobacter metallireducens were investigated. The results showed that HA and FA could promote the reductive dissolution of Fh and the formation of vivianite. After incubation of 38 d, vivianite accounted for 47.19%, 59.22%, and 48.53% of total Fe in biological control batch (BCK), HA and FA batches (C/Fe molar ratio of 1.0), respectively, by Mössbauer spectroscopy analysis. In terms of Cd, HA and FA could promote the release of adsorbed Cd during the initial bioreduction process, but reassuringly, after 38 d the dissolved Cd with HA and FA addition batches were 0.58-0.91 and 0.99-1.08 times of the BCK, respectively. The proportions of residual Cd in HA batches were higher than FA and BCK batches, indicating that HA was better than FA in immobilizing Cd. This might be because the quinone groups in HA could act as electron shuttle. This study showed that HA facilitated the transformation of vivianite better than FA, and Cd can be stabilized by resorption or co-precipitation with vivianite, providing a theoretical support for the translocation of Cd in sediment-water interface.

Enhanced cadmium removal by biochar and iron oxides composite: Material interactions and pore structure

The combination of biochar (BC) and iron minerals improves their pollutant adsorption capacity. However, little is known about the reactivity of BC-iron mineral composites regarding their interaction and change in the pore structure. In this study, the mechanism of cadmium (Cd) adsorption by BC-iron oxide composites, such as BC combined with ferrihydrite (FH) or goethite (GT), was explored. The synergistic effect of the BC-FH composite significantly improved its Cd adsorption capacity. The adsorption efficiencies of BC-FH and BC-GT increased by 15.0% and 10.8%, respectively, compared with that of uncombined BC, FH, and GT. The strong Cd adsorption by BC-FH was attributed to stable interactions and stereoscopic pore filling between BC and FH. The scanning electron microscopy results showed that FH particles entered the BC pores, whereas GT particles were loaded onto the BC surface. FTIR spectroscopy showed that GT covered a larger area of the BC surface than FH. After loading FH and GT, BC porosities decreased by 9.3% and 4.1%, respectively. Quantum chemical calculations and independent gradient mode analysis showed that van der Waals interactions, H-bonds, and covalent-like interactions maintained stability between iron minerals and BC. Additionally, humic acid increased the agglomeration of iron oxides and formed larger particles, causing additional aggregates to load onto the BC surface instead of entering the BC pores. Our results provide theoretical support to reveal the interfacial behavior of BC-iron mineral composites in soil and provide a reference for field applications of these materials for pollution control and environmental remediation.

A chrysotile-based Fe/Ti nanoreactor enables efficient arsenic capture for sustainable environmental remediation

Iron-based materials for arsenic (As) immobilization in practical groundwater and soil remediation suffer from a low removal capacity and an insufficient long-term stability. Herein, a unique chrysotile-based nanoreactor has been developed by incorporating iron/titanium oxides into the cylindrical cavity of chrysotile (TiFe-Chy), providing sufficient internal reaction sites for As immobilization. Results reveal that the adsorption capacities of TiFe-Chy for As(III) and As(V) are considerably higher than the commonly used amendments, i.e., layered double hydroxide (LDH) and Phoslock®, respectively. More importantly, TiFe-Chy exhibits a strong anti-interference capability of As immobilization in soils compared to those commercial products due to this unique incorporation approach. Fixed-bed leaching experiments indciate that this TiFe-Chy nanoreactor can efficiently decrase the As(III) and As(V) concentrations by 81.8-87.3% within a period of ten years, significantly improving the long-term stability of As immobilization in soils. Life cycle assessment analysis reveals that TiFe-Chy can reduce negative environmental impacts (such as carbon emissions), resulting in a low cost for soils and groundwater remediation. The findings of this work open a new avenue for sustainable heavy metal(loid)s remediation in groundwater and soils.

Exploration on the role of different iron species in the remediation of As and Cd co-contamination by sewage sludge biochar

Numerous studies have explored the adsorption of cadmium (Cd) and arsenic (As) by iron (Fe)-modified biochar, but few studies have examined in-depth the similarities and differences in the adsorption behavior of different iron types on Cd and As. In this study, sewage sludge biochar (BC) was co-pyrolyzed with self-made Fe minerals (magnetite, hematite, ferrihydrite, goethite, and schwertmannite) to treat Cd and As co-contaminated water. The adsorption of Cd and As on the Fe-modified biochar was further analyzed by adsorption kinetics, adsorption isotherms, and adsorption thermodynamics combined with a series of characterization experiments. Both SEM-EDX and XRD results confirmed the successful loading of iron minerals onto BC. Both adsorption kinetics and adsorption isotherms experiments showed that the adsorption of Cd and As by BC and the other five Fe-modified biochar was mainly controlled by chemical interactions. The results also indicated that goethite biochar (GtBC) was the most effective for the adsorption of Cd among the five Fe-modified biochar. Ferrihydrite biochar (FhBC) formed more diverse complexes, coupled with the relatively stronger electrons accepting ability, thus making it more effective for As adsorption than the others. Additionally, GtBC and hematite biochar (HmBC) were found effective for the adsorption of both Cd and As, whereas MBC was not found effective for either metal. Furthermore, combined with XPS results, the adsorption of Cd by the materials was mainly governed by Cd²⁺-π interactions, complexation precipitation, and co-precipitation, while oxidation reactions also existed for As.

Facile preparation of activated carbon supported nano zero-valent iron for Cd(Ⅱ) removal in aqueous environment

Activated carbon-supported nano-zero-valent iron (nZVI@AC) is considered to be one of the most promising materials for in-situ remediation of pollutants in aqueous environment, while liquid phase reduction (LPR) is one of the most commonly used preparation methods for nZVI@AC. However, the complex operation and the requirement of various agents limit the practical application of the traditional liquid-phase reduction (TLPR). In this study, an improved liquid phase reduction method (ILPR) was proposed, which was characterized by solid-state dosing of reducing agents. Compared with TLPR, ILPR simplified the preparation process, while there was no requirement of polyethylene glycol and ethanol. When the Cd(II) removal efficiency was used as the evaluation index, the preferred parameters of ILPR were as follows: AC/FeSO₄·7H₂O mass ratio was 15:1; NaBH₄ dosage was 8 g; ultrasonic time was 1 h; stirring time was 20 min. Moreover, the Cd(II) removal efficiency of nZVI@AC prepared by ILPR (nZVI@AC-I) was greater than 92.00%, which was superior to that of nZVI@AC prepared by TLPR (nZVI@AC-T). The characterization results showed that the pore parameters, surface functional groups and iron contents of nZVI@AC-I and nZVI@AC-T were basically the same. However, the distribution of iron-containing particles on the surface of nZVI@AC-I was more uniform. Furthermore, the Fe⁰ in nZVI@AC-I had a smaller particle size and a higher content. Overall, this study provided a promising approach for nZVI@AC preparation.

Kill two birds with one stone: The management of hazardous waste and the preparation of efficient adsorbents for Pb(II) were realized by the pyrolysis of penicillin mycelial dreg

The penicillin industry produces a large amount of penicillin mycelial dreg (PMD), potentially causing severe environmental problems without proper treatment and disposal. To achieve the goals of PMD management, the present work explored the potential of PMD as a novel feedstock to produce biochar with very high adsorption performance. PMD was pyrolyzed at 400-800 °C to prepare biochars (PMD-BCs), and the physical and chemical properties were characterized using various methods. The adsorption capacities of Pb²⁺ on PMD-BC400, PMD-BC600, and PMD-BC800 were 37.04, 62.89, and 107.53 mg/g, respectively, at a temperature of 25 °C and pH of 5.0. The adsorption process of Pb²⁺ on PMD-BCs can be well described by the Langmuir model and pseudo-second-order model. Mineral precipitation, ion exchange, functional group complexation and Pb²⁺-π interaction were involved in the adsorption of Pb²⁺ on PMD-BCs. Moreover, mineral precipitation and ion exchange dominated Pb²⁺ sorption on PMD-BCs (84.71-92.73%). This study indicates the transition of PMD to biochar for Pb²⁺ adsorption is a promising method for PMD utilization.

Removal of cadmium in aqueous solutions using a ball milling-assisted one-pot pyrolyzed iron-biochar composite derived from cotton husk

A novel iron-biochar composite adsorbent was produced via ball milling-assisted one-pot pyrolyzed BM-nZVI-BC 800. Characterization proved that nano zero valent iron was successfully embedded in the newly produced biochar, and the nZVI payload was higher than that of traditional one-pot pyrolyzed methods. BM-nZVI-BC 800 provided a high adsorption performance of cadmium reaching 96.40 mg·g^-1 during batch testing. Alkaline conditions were beneficial for cadmium removal of BM-nZVI-BC 800. The pseudo-second-order kinetic model and Langmuir isotherm fitted better, demonstrating that the Cd adsorption on the BM-nZVI-BC 800 was a chemical and surface process. The intraparticle diffusion controlled the adsorption of BM-nZVI-BC 800. The physisorption dominated by high specific surface area and mesoporous structure was the primary mechanism in the removal of cadmium, though electrostatic attraction and complexation also played a secondary role in cadmium adsorption. Compared to adsorbents prepared by more traditional methods, the efficiencies of the ball milling-assisted one-pot pyrolyzed method appears superior.

Remediation of cadmium or arsenic contaminated water and soil by modified biochar: A review

Biochar has a high specific surface area with abundant pore structure and functional groups, which has been widely used in remediation of cadmium or arsenic contaminated water and soil. However, the bottleneck problem of low-efficiency of pristine biochar in remediation of contaminated environments always occurs. Nowadays, the modification of biochar is a feasible way to enhance the performance of biochar. Based on the Web of science™, the research progress of modified biochar and its application in remediation of cadmium or arsenic contaminated water and soil have been systematically summarized in this paper. The main modification strategies of biochar were summarized, and the variation of physicochemical properties of biochar before and after modification were illustrated. The efficiency and key mechanisms of modified biochar for remediation of cadmium or arsenic contaminated water and soil were expounded in detail. Finally, some constructive suggestions were given for the future direction and challenges of modified biochar research.

Aquatic photochemistry of Cu(II) in the presence of As(III): Mechanistic insights from Cu(III) production and As(III) oxidation under neutral pH conditions

Surface complexation between arsenite (As(III)) and colloidal metal hydroxides plays an important role not only in the immobilization and oxidation of As(III) but also in the cycle of the metal and the fate of their ligands. However, the photochemical processes between Cu(II) and As(III) are not sufficiently understood. In this work, the photooxidation of As(III) in the presence of Cu(II) under neutral pH conditions was investigated in water containing 200 μM Cu(II) and 5 μM As(III) under simulated solar irradiation consisting of UVB light. The results confirmed the complexation between As(III) and Cu(II) hydroxides, and the photooxidation of As(III) is attributed to the ligand-to-metal charge transfer (LMCT) process and Cu(III) oxidation. The light-induced LMCT process results in simultaneous As(III) oxidation and Cu(II) reduction, then produced Cu(I) undergoes autooxidation with O₂ to produce O₂^•⁻ and H₂O₂, and further the Cu(I)-Fenton reaction produces Cu(III) that can oxidize As(III) efficiently (k_{Cu(III)+As(III)} = 1.02 × 10⁹ M^-1 s^-1). The contributions from each pathway (ρ_{rCu(II)-As(III)+hv} = 0.62, ρ_{rCu(III)+As(III)} = 0.38) were obtained using kinetic analysis and simulation. Sunlight experiments showed that the pH range of As(III) oxidation could be extended to weak acidic conditions in downstream water from acid mine drainage (AMD). This work helps to understand the environmental chemistry of Cu(II) and As(III) regarding their interaction and photo-induced redox reactions.

Co-incorporation of Chinese milk vetch (Astragalus sinicus L.), rice straw, and biochar strengthens the mitigation of Cd uptake by rice (Oryza sativa L.)

Soil cadmium (Cd) contamination is becoming a widespread concern because of its threat to global ecosystem health and food security. Co-incorporation of Chinese milk vetch (MV) and rice straw (RS) is a common agricultural practice in Southern China; however, the effects of combining these two materials with biochar on Cd bioavailability remain unclear. This study investigated the effects of MV, RS, rape straw biochar (RB), iron-modified biochar (FB), and their combinations on Cd uptake by rice through incubation and field experiments. The results showed that compared with the control without material input (CK), MV + RS (MR), MV + RS + RB (MRRB), and MV + RS + FB (MRFB) considerably reduced the Cd concentration in brown rice by 61.20 %, 65.38 %, and 62.65 %, respectively. Furthermore, the treatments increased the formation of iron‑manganese plaque (IMP) at different growth stages; MRRB and MRFB exhibited the highest increase rates among the treatments. Quantitatively, the Fe plaque and Mn plaque were increased by 20.61 %-47.23 % and 80.18 %-172.74 %, respectively. Compared with CK, the MRRB and MRFB treatments reduced the soil available Cd by 35.09 %-54.45 % and 38.20 %-50.20 %, respectively, at all stages. This decrease was substantially lower than that observed in the MV, RS, and MR treatments. Similar trends were observed in the incubation experiment. Additionally, the Community Bureau of Reference Sequential Extraction Analysis indicated that the MRRB and MRFB treatments converted the bioavailable Cd fractions into a stable form. Partial least squares path model and redundancy analysis revealed that pH was the major factor influencing Cd bioavailability. This study emphasized that the dual impact factors from the enhancement of Cd passivation capability and IMP formation jointly result in the reduction of Cd uptake by rice. Consequently, the co-incorporation of MV, RS, and biochar is promising for remediating Cd-contaminated paddy soils in Southern China.

Insights into enhanced removal of Cd2+ from aqueous solutions by attapulgite supported sulfide-modified nanoscale zero-valent iron

The sulfidation of nanoscale zerovalent iron (nZVI) has received increasing attention for reducing the oxidizability of nZVI and improving its reactivity toward heavy metal ions. Here, a sulfide (S)-modified attapulgite (ATP)-supported nanoscale nZVI composite (S-nZVI@ATP) was rapidly synthesized under acidic conditions and used to alleviate Cd²⁺ toxicity from an aqueous solution. The degree of oxidation of S-nZVI@ATP was less than that of nZVI@ATP, indicating that the sulfide modification significantly reduced the oxidation of nZVI. The optimal loading ratio was at an S-to-Fe molar ratio of 0.75, and the adsorption performance of S-nZVI@ATP for Cd²⁺ was significantly improved compared with that of nZVI@ATP. The removal of Cd²⁺ by S-nZVI@ATP was 100% when the adsorbent addition was 1 g/L, the solution was 30 mL, and the adsorption was performed at 25 °C for 24 h with an initial Cd²⁺ concentration of 100 mg/L. Kinetics studies showed that the adsorption process of Cd followed the pseudo-second-order model, indicating that chemisorption was the dominant adsorption mechanism. The adsorption of Cd²⁺ by S-nZVI @ATP is dominated by the complexation between the iron oxide or iron hydroxide shell of S-nZVI and Cd²⁺ and the formation of Cd(OH)₂ and CdS precipitates.

Carbothermal Synthesis of Sludge Biochar Supported Nanoscale Zero-Valent Iron for the Removal of Cd2+ and Cu2+: Preparation, Performance, and Safety Risks

The practical application of nanoscale zero-valent iron (NZVI) is restricted by its easy oxidation and aggregation. Here, sludge biochar (SB) was used as a carrier to stabilize NZVI for Cd²⁺ and Cu²⁺ removal. SB supported NZVI (SB-NZVI) was synthesized using the carbothermic method. The superior preparation conditions, structural characteristics, and performance and mechanisms of the SB-NZVI composites for the removal of Cd²⁺ and Cu²⁺ were investigated via batch experiments and characterization analysis. The optimal removal capacities of 55.94 mg/g for Cd²⁺ and 97.68 mg/g for Cu²⁺ were achieved at a Fe/sludge mass ratio of 1:4 and pyrolysis temperature of 900 °C. Batch experiments showed that the SB-NZVI (1:4-900) composite had an excellent elimination capacity over a broad pH range, and that weakly acidic to neutral solutions were optimal for removal. The XPS results indicated that the Cd²⁺ removal was mainly dependent on the adsorption and precipitation/coprecipitation, while reduction and adsorption were the mechanisms that play a decisive role in Cu²⁺ removal. The presence of Cd²⁺ had an opposite effect on the Cu²⁺ removal. Moreover, the SB-NZVI composites made of municipal sludge greatly reduces the leaching toxicity and bio-availability of heavy metals in the municipal sludge, which can be identified as an environmentally-friendly material.

An iron-biochar composite from co-pyrolysis of incinerated sewage sludge ash and peanut shell for arsenic removal: Role of silica

Modification of biochar by low-cost iron sources has gained increasing attention to improve pollutants removal performance and reduce production costs compared to conventional chemical modifications. While such iron sources generally have complex compositions, their effects on properties of the iron-biochar composite are not well investigated. This study produced an iron-biochar (RBC) composite from co-pyrolysis of incinerated sewage sludge ash (ISSA) and peanut shell, and examined the role of silica with widespread existence in ISSA and other low-cost iron sources on properties of the iron-biochar composite relevant to As(III)/As(V) removal. Silica was found to react with iron during the pyrolysis process at 850 °C and formed iron silicon at the expense of producing zero valent iron and Fe₃O₄ which resulted in a poorer removal efficacy for As(III) and As(V) compared to the iron-biochar (FBC) made from pure Fe₂O₃ and peanut shell. Moreover, a high leaching of reactive silica from RBC was observed which affected the formation of corrosion products of ZVI and competed with arsenic for active adsorption sites. Despite this, RBC still exhibited a maximum adsorption capacity of 17.44 and 57.56 mg/g towards As(III) and As(V) respectively at pH 3.0. Overall, this study provides an interesting insight into upcycling ISSA into useful media for sorptive removal of arsenic from aqueous solutions.

Promotion of higher synthesis temperature for higher-efficient removal of antimonite and antimonate in aqueous solution by iron-loaded porous biochar

Iron-loaded porous biochar (FPBC) was synthesized by co-pyrolysis method using sawdust and potassium ferrate at 500 (FPBC500) and 800°C (FPBC800), then characterized and applied to eliminate antimonite (Sb(III)) and antimonate (Sb(V)) in aqueous. Due to alkali erosion on feedstock and K/Fe-oxides attacking carbon, FPBC800 obtained a larger specific surface area (SSA) (515.49 m²·g^-1) that was 5.48-fold that of PFBC500, meaning the exposure of more active sites. Fe₃O₄ was formed on FPBC500, but Fe⁰ and Fe₃C were generated on FPBC800. FPBC800 showed the optimal sorption performance for Sb(III) (144.48 mg·g^-1) and Sb(V) (45.29 mg·g^-1), which were much higher than that of FPBC500. Noteworthily, Sb(III) anchored on FPBC was oxidized to Sb(V) with less ecotoxicity; moreover, FPBC800 with Fe⁰ showed stronger oxidization. Although pH-dependent sorption of Sb(III)/Sb(V) on FPBC occurred, the resistance to environmental factors showed a potential for eliminating actual pollution, demonstrating an easy-to-operate construction strategy for modified biochar.

Synergistic removal of As(III) and Cd(II) by sepiolite-modified nanoscale zero-valent iron and a related mechanistic study

Arsenic (As) and cadmium (Cd) are two highly toxic elements. In recent years, many newly synthesized chemical materials have been used widely for treatments of As- and Cd-contaminated effluents. However, most materials do not exhibit high efficiencies for simultaneous removal of As and Cd from water systems. Our study established a simple scheme for synthesizing a sepiolite (SEP)-modified nanoscale zero-valent iron (S-nZVI) for simultaneous removal of coexisting As and Cd from water and illuminated a possible underlying mechanism. Batch experiments showed that the maximum capacities for adsorption of As(III) and Cd(II) by S-nZVI were 230.29 mg/g and 11.37 mg/g, respectively, which represented better effects than those of other materials, as reported previously. Removal of Cd(II) depended on pH, but As(III) removal showed little dependence on pH. Coexisting ions such as phosphate (PO₄^3-) and the conjugate base of humic acid (HA) significantly inhibited simultaneous removal of As(III) and Cd(II). In the mixed As(III)-Cd(II) system, the presence of As(III)-pretreated S-nZVI significantly enhanced Cd(II) adsorption by a factor of four over that seen for aqueous solution without As(III). XRD and XPS results showed that CdFe₂O₄ (Fe-O-Cd), Fe₂As₂O₁₄ or FeAsO₄ (Fe-O-As) were formed after As(III) and Cd(II) were captured by S-nZVI. However, a further zeta (ζ) potential analysis showed that the mechanism for As(III) and Cd(II) adsorption by S-nZVI is not just simple formation of the above chemicals, since the adsorbed As(III) increased the negative charge of S-nZVI; this suggested an electrostatic attraction between S-nZVI and Cd(II) and indicated that adsorbed As(III) created new sorption sites for Cd(II), which enhanced Cd(II) sorption via formation of ternary complexes (Fe-As-Cd). These results suggested that S-nZVI is a promising material for in situ remediation of heavy metal-contaminated groundwaters or paddy soils.

A high-efficient and recyclable aged nanoscale zero-valent iron compound for V5+ removal from wastewater: Characterization, performance and mechanism

An effective complex of nanoscale zero-valent iron (NZVI) supported on zirconium 1,4-dicarboxybenzene metals-organic frameworks (UIO-66) with strong oxidation resistance was synthesized (NZVI@UIO-66) for V⁵⁺ removal from wastewater. The results demonstrated that NZVI was successfully loaded on UIO-66 with a uniform dispersion, and then the composite was aged in the air which was named A-NZVI@UIO-66. V⁵⁺ could be removed quickly and completely using A-NZVI@UIO-66 in a wider pH range except for the pH = 1 condition. The reaction between A-NZVI@UIO-66 and V⁵⁺ was an endothermic process. Freundlich model with a better-fitted value showed the adsorption of V⁵⁺ on A-NZVI@UIO-66 was multi-layer heterogeneous adsorption and the adsorbed amount of V⁵⁺ was 397.23 mg V/g NZVI. Nitrate had a competitive inhibition on V⁵⁺ removal by A-NZVI@UIO-66. Mechanisms of vanadium elimination from the aqueous phase by A-NZVI@UIO-66 included physical adsorption, reduction, and complex co-precipitation, particularly the reduction dominated. The subsistent Zr-O bond in A-NZVI@UIO-66 provided a possible double reaction path by playing an electron donor, storage, or conductor role. After acid leaching, A-NZVI@UIO-66 represented good reusability in the removal of V⁵⁺ from the practical mine sewage.

Spatial and temporal variations of metal fractions in paddy soil flooding with acid mine drainage

Environmental release of acid mine drainage (AMD) poses a potential threat to the environment and human health due to its high content of heavy metals. The impact of AMD flooding on unpolluted soil leads to serious pollution over time via a complex process, related to the geochemical behavior of toxic metals that so far has only been partially investigated. Here, a soil column study was conducted to investigate the migration of Cu and Cd fractions in unpolluted paddy soil following treatment with AMD collected from the Dabaoshan Mining area. Tessier's sequential extraction was performed to fractionate the metals at various depths over time. After 160 days of experimental flooding, the soil pH stabilized at 2.52 at a column depth of 5 cm. The fractions of Cu and Cd that were highly mobile increased significantly during AMD flooding. For Cd, the latter already occurred on day 67. At a depth of 20 cm, the total content of Cu maximally increased from initially 26.89 mg kg^-1 to 696.96 mg kg^-1 on day 160, while the content of Cd maximally increased from 0.22 mg kg^-1 to 391.30 mg kg^-1 on day 67. Reduced partition index analysis conformed that the mobility of both Cu and Cd significantly increased in contaminated soil during continuous AMD flooding. Scanning electron microscopy coupled with energy dispersive spectroscopy (SEM-EDS) identified a changed distribution of the elements in the soil, with Fe appearing to have aggregated. The correlation analysis between Cu and Cd in pore water and in different fractions in the soil's solid phase identified a dynamic distribution of these metals in certain geochemical components during their migration. The results of this study contribute to a scientific foundation to describe the geochemical behavior of heavy metals in soil subject to AMD flooding.

Biochar-based microbial agent reduces U and Cd accumulation in vegetables and improves rhizosphere microecology

Microbial remediation of heavy metals in soil has been widely studied. However, bioremediation efficiency is limited in practical applications because of nutritional deficiency, low efficiency, and competition with indigenous microorganisms. Herein, we prepared a biochar-based microbial agent (BMA) by immobilizing the microbial agent (MA, containing Bacillus subtilis, Bacillus cereus, and Citrobacter sp.) on biochar for the remediation of U and Cd in soil. The results showed that BMA increased soil organic matter, cation exchange capacity, and fluorescein diacetate hydrolysis activity and dehydrogenase activity by 58.7%, 38.2%, 42.9%, and 51.1%. The availability of U and Cd were significantly decreased by 67.4% and 54.2% in BMA amended soil, thereby reducing their accumulation in vegetables. BMA greatly promoted vegetable growth. Additionally, BMA significantly altered the structure and function of rhizosphere soil microbial communities. Coincidently, more abundant ecologically beneficial bacteria like Nitrospira, Nitrosomonas, Lysobacter, and Bacillus were observed, whereas plant pathogenic fungi like Fusarium and Alternaria reduced in BMA amended soil. The network analysis revealed that BMA amendment increased the tightness and complexity of microbial communities. Importantly, the compatibility of niches and microbial species within co-occurrence network was enhanced after BMA addition. These findings provide a promising strategy for suppressing heavy metal accumulation in vegetables and promoting their growth.

Investigations of S-nZVI/AC composites for hexavalent chromium (Cr(VI)) elimination: synthesis and application

Sulfidated nano zero-valent iron supported by activated carbon (S-nZVI/AC) composites were synthesized via liquid phase reduction method, and then they were used for Cr(VI) elimination. Characterization results showed that Fe⁰ was the main component, besides, iron oxides and iron sulfides were also detected. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) results showed that S-nZVI nanoparticles were homogeneously distributed on the surfaces of AC. The influences of S/Fe ratio, C/Fe ratio, pH value, reaction temperature and co-existed ions (Cl^-, SO₄^2-, PO₄^3- and NO₃^-) on Cr(VI) removal performances were investigated. Furthermore, the corresponding mechanisms were also discussed. The S-nZVI/AC composites exhibited good aging-resistance performances that Cr(VI) removal efficiency still maintained at 83.1% after being sealed in water for seven days, and they also had satisfying cycling stabilities that Cr(VI) removal efficiency only decreased less than 10% after four cycles. The good performances of S-nZVI/AC composites for Cr(VI) removal are attributed to the protection effect of iron sulfides and immobilization effect of AC, making S-nZVI/AC as a promising candidate for Cr(VI) elimination in effluents.