Immobilization of heavy metals in contaminated soils by modified hydrochar: Efficiency, risk assessment and potential mechanisms

Biochar Blended with Alkaline Mineral Can Better Inhibit Lead and Cadmium Uptake and Promote the Growth of Vegetables

Three successive vegetable pot experiments were conducted to assess the effects on the long-term immobilization of heavy metals in soil and crop yield improvement after the addition of peanut shell biochar and an alkaline mineral to an acidic soil contaminated with lead and cadmium. Compared with the CK treatment, the change rates of biomass in the edible parts of the three types of vegetables treated with B0.3, B1, B3, B9, R0.2 and B1R0.2 were -15.43%~123.30%, 35.10%~269.09%, 40.77%~929.31%, -26.08%~711.99%, 44.14%~1067.12% and 53.09%~1139.06%, respectively. The cadmium contents in the edible parts of the three vegetables treated with these six additives reduced by 2.08%~13.21%, 9.56%~24.78%, 9.96%~35.61%, 41.96%~78.42%, -4.19%~57.07% and 12.43%~65.92%, respectively, while the lead contents in the edible parts reduced by -15.70%~59.47%, 6.55%~70.75%, 3.40%~80.10%, 55.26%~89.79%, 11.05%~70.15% and 50.35%~79.25%, respectively. Due to the increases in soil pH, soil cation-exchange capacity and soil organic carbon content, the accumulation of Cd and Pb in the vegetables was most notably reduced with a high dosage of 9% peanut shell biochar alone, followed by the addition of a low dosage of 1% peanut shell biochar blended with 0.2% alkaline mineral. Therefore, the addition of a low dosage of 1% peanut shell biochar blended with 0.2% alkaline mineral was the best additive in increasing the vegetable biomass, whereas the addition of 9% peanut shell biochar alone was the worst. Evidently, the addition of 0.2% alkaline mineral can significantly reduce the amount of peanut shell needed for passivating heavy metals in soil, while it also achieves the effect of increasing the vegetable yield.

Co-pyrolysis of alkali-fused fly ash and corn stover to synthesize biochar composites for remediating lead-contaminated soil

Fly ash (FA) is mainly composed of silica, alumina, and other metal oxide components, and has a positive stabilizing effect on soil heavy metals. Biochar composites produced from FA and corn stover (CS) can improve its remediation performance. Therefore, a batch of biochar composites (alkali-fused FA-CS biochars, ABs), synthesized via co-pyrolysis of CS and alkali-fused FA (AFFA) at different temperatures of 300, 500, and 700 °C (AB300-1, AB500-1, and AB700-1) and CS to AFFA mass ratios of 10:1, 10:2, and 10:5 (AB500-1, AB500-2, and AB500-5), was used to remediate lead (Pb)-contaminated soil. Compared with pristine biochars (BCs), ABs were enriched with oxygen-containing functional groups (Si-O-Si and Si-O) and aromatic structures. The ABs prepared at lower pyrolytic temperature (≤500 °C) and lower ratio of CS to AFFA (10:1) showed higher yield and stability. The contents of Toxicity Characteristic Leaching Procedure (TCLP)-extractable Pb and DTPA-CaCl2-triethanolamine (DTPA)-extractable Pb were generally lower in the soils amended with ABs than BCs. Compared with other ABs such as AB300-1, AB500-2, AB500-5, and AB700-1, the soil amended with AB500-1 had lower contents of TCLP and DTPA-extractable Pb (24% reduction), exhibiting superior performance in stabilizing Pb in the soil. The gradual decrease of DTPA-extractable Pb content in the soil with increasing dosage of AB500-1 amendments suggests that AB500-1 facilitated the conversion of bioavailable Pb to the stable and less toxic residual fractions. Specifically, the highest percentage of residual fraction of Pb in soil amended with AB500-1 was 14%. Correlation analyses showed that the soil DTPA-extractable Pb content decreased with the increase of soil pH and cation-exchange capacity (CEC) value. ABs stabilize Pb in the soils mainly via electrostatic attraction, precipitation, cation-π interaction, cation exchange, and complexation. These findings provide insights for producing functionalized biochar composites from industrial waste like FA and biomass waste for remediating the soils polluted by heavy metals.

Comparative role of charcoal, biochar, hydrochar and modified biochar on bioavailability of heavy metal(loid)s and machine learning regression analysis in alkaline polluted soil

Pot experiment was performed aimed to assess the comparative role of charcoal, biochar, hydrochar and thiourea-vegetable modified biochar at 1 and 2 % doses, and <1 mm particle size on the bioavailability of Cd, Pb, As, Ni, Cu and Zn, and enhance NPK, and mustard growth in a slightly alkaline polluted soil. Furthermore, machine learning method was used to examine the systematic evaluation of the impact of feature selection based on Pearson's correlation on the performance of the linear regression model. The results revealed that maximum fresh and dry biomass of mustard was observed by 26.38 and 38.18 % with hydrochar 1 %, whereas lemon biochar at 2 % reduced fresh and dry biomass up to 34.0 and 53.0 % than control. The immobilization of Cd and Pb was observed by 83.70 and 71.15 % with thiourea-vegetable modified biochar at 2 %, As 71.62 % with hydrochar 2 %, Ni 80.84 % with thiourea-vegetable modified biochar 2 %, Cu 66.32 % with and Zn 36.30 % with thiourea-vegetable modified biochar at 2 % than control. However, the maximum mobilization of Cu in soil was observed by 30.3 % with lemon biochar 2 %, similarly for Zn 37.36 % with hydrochar 2 % as compared with other treatments. The phyto-availability of Cd, Pb, As and Cu in the mustard shoot and root biomass was reduced except Ni and Zn in soil than control. It was observed that using the machine learning regression analysis approach, variability in treatments effectiveness is evident across different feature correlation thresholds. This study clearly shows that the beneficial role of studied amendments on mustard growth and reduced bioavailability of heavy metal(loid)s and enhance primary macronutrients in alkaline polluted soil. It is suggested that future studies may be conducted on combined application of studies amendments on plant growth, immobilization of heavy metal(loid)s in multi-metal polluted soil under different field conditions.

Recent advances in environmental and agricultural applications of hydrochars: A review

Hydrochar is a carbonaceous material that is generated through the process of hydrothermal carbonization (HTC) from biomass, which has garnered considerable attention in recent years owing to its potential applications in a diverse range of fields, such as environmental remediation and agriculture. Hydrochar is produced from a diverse range of biomass waste materials and retains exceptional properties, including high carbon content, stability, and surface area, making it an optimal candidate for various enviro-agricultural applications. Moreover, it delves into the production process of hydrochar, with explicit emphasis on the optimization of certain properties during the production of hydrochar from bio-waste. Furthermore, the potential of hydrochar as an adsorbent and catalyst support for heavy metals and dyes was extensively explored, along with a soil remediation potential that can improve the physical, chemical and biological properties of soil. This comprehensive review aims to provide a thorough overview of hydrochar with a particular focus on its production, properties, and prospective applications. The significance of hydrochar is accentuated and the growing need for alternative sources of energy and materials that are environmentally sustainable is highlighted in this paper. Besides, the consequence of hydrochar on soil properties such as water-holding capacity, nutrient retention, and total soil porosity, as well as its influence on soil chemical properties such as cation exchange capacity, electrical conductivity, and surface functionality is scrutinized.

Efficient Immobilization of heavy metals using newly synthesized magnetic nanoparticles and some bacteria in a multi-metal contaminated soil

Simultaneous application of modified Fe3O4 with biological treatments in remediating multi-metal polluted soils, has rarely been investigated. Thus, a pioneering approach towards sustainable environmental remediation strategies is crucial. In this study, we aimed to improve the efficiency of Fe3O4 as adsorbents for heavy metals (HMs) by applying protective coatings. We synthesized core-shell magnetite nanoparticles coated with modified nanocellulose, nanohydrochar, and nanobiochar, and investigated their effectiveness in conjunction with bacteria (Pseudomonas putida and Bacillus megaterium) for remediating a multi-metal contamination soil. The results showed that the coatings significantly enhanced the immobilization of heavy metals in the soil, even at low doses (0.5%). The coating of nanocellulose had the highest efficiency in stabilizing metals due to the greater variety of surface functional groups and higher specific surface area (63.86 m2 g-1) than the other two coatings. Interestingly, uncoated Fe3O4 had lower performance (113.6 m2 g-1) due to their susceptibility to deformation and oxidation. The use of bacteria as a biological treatment led to an increase in the stabilization of metals in soil. In fact, Pseudomonas putida and Bacillus megaterium increased immobilization of HMs in soil successfully because of extracellular polymeric substances and intensive negative charges. Analysis of metal concentrations in plants revealed that Ni and Zn accumulated in the roots, while Pb and Cd were transferred from the roots to the shoots. Treatment Fe3O4 coated with modified nanocellulose at rates of 0.5 and 1% along with Pseudomonas putida showed the highest effect in stabilizing metals. Application of coated Fe3O4 for in-situ immobilization of HMs in contamination soils is recommendable due to their high metal stabilization efficiency and suitability to apply in large quantities.

Fe-CGS Effectively Inhibits the Dynamic Migration and Transformation of Cadmium and Arsenic in Soil

The iron-modified coal gasification slag (Fe-CGS) material has excellent performance in purifying heavy-metal-contaminated water due to its good surface properties and adsorption capacities. However, it is unclear whether it can provide long-term simultaneous stabilization of Cd and As in composite-contaminated soils in extreme environments. This study investigated the long-term stabilization of Cd and As in acidic (JLG) and alkaline (QD) soils by simulating prolonged heavy rainfall with the addition of Fe-CGS. Multiple extraction methods were used to analyze the immobilization mechanisms of Cd and As in soil and their effects on bioavailability. The results indicate that the stabilization efficiency was related to the dosage of Fe-CGS. The concentrations of Cd and As in the JLG soil leachate were reduced by 77.6% (2.0 wt%) and 87.8% (1.0 wt%), respectively. Additionally, the availability of Cd and As decreased by 46.7% (2.0 wt%) and 53.0% (1.0 wt%), respectively. In the QD soil leachate, the concentration of Cd did not significantly change, while the concentration of As decreased by 92.3% (2.0 wt%). Furthermore, the availability of Cd and As decreased by 22.1% (2.0 wt%) and 40.2% (1.0 wt%), respectively. Continuous extraction revealed that Fe-CGS facilitated the conversion of unstable, acid-soluble Cd into oxidizable Cd and acid-soluble Cd. Additionally, it promoted the transformation of both non-specifically and specifically adsorbed As into amorphous iron oxide-bound and residual As. Fe-CGS effectively improved the soil pH, reduced the bioavailability of Cd and As, and blocked the migration of Cd and As under extreme rainfall leaching conditions. It also promoted the transformation of Cd and As into more stable forms, exhibiting satisfactory long-term stabilization performance for Cd and As.

Towards engineering mitigation of leaching of Cd and Pb in co-contaminated soils using metal oxide-based aerogel composites and biochar

Applications of metal-based nanomaterials for the remediation of heavy metal (HM) contaminated environments are of great importance. The ability of metal oxide-based carbon aerogel composite to immobilize HMs in multi-metal contaminated soils has not yet been investigated, particularly under acidic conditions. Herein, we investigate the performance of metal oxides (Sr0.7 Mn0.3 Co0.5 Fe0.5O3-δ)-based carbon aerogel composite (MO-CAg) compared with coconut coil fiber biochar (CCFB) and carbon aerogel (CAg) for Cd and Pb immobilization in contaminated soil. The MO-CAg, applied at 2% (w/w), significantly decreased Pb leaching by 67-75% and Cd by 60-65%, CAg decreased Cd by 54% and Pb by 46%, while biochar decreased Cd by 40-44% and Pb by 43%. The addition of MO-CAg altered Cd and Pb geochemical fractions by increasing their residual fraction, i.e., stabilized both metals compared to the control. This presents a comprehensive elaboration on the probable reaction interactions between the MO-Cag and heavy metals, including a combination of (co)precipitation, and reduction-oxidation as the predominant mechanisms of metal stabilization with MO-CAg. Moreover, MO-CAg increased Pb and Cd stabilization in soils by strengthening the bonding between metal oxides and Cd/Pb. By imbedding MO into the CAg, in MO-CAg, the immobilization of Cd(II) and Pb(II) occurred through inner-sphere complexation, while with CCFB and CAg metals, immobilization occurred through outer-sphere complexation. MO-CAg is a promising and highly efficient material that could be recommended for the remediation of Cd- and Pb-contaminated soils in subsequent studies.

Simultaneous adsorption of Cd and As by a novel coal gasification slag based composite: Characterization and application in soil remediation

Cadmium (Cd) and arsenic (As) co-contamination has become increasingly serious in China agricultural soil due to rapid industrialization and urbanization. The opposite geochemical behaviors of Cd and As pose huge challenges for developing a material for their simultaneous immobilization in soils. Coal gasification slag (CGS) as a by-product of coal gasification process, is always dumped into a local landfill, which has a negative impact on environment. Few reports have been available on applying CGS as a material to immobilize simultaneously multiple soil heavy metals. A series of iron-modified coal gasification slag (IGS) composites IGS3/5/7/9/11 (with different pH values) were synthesized by alkali fusion and iron impregnation. After modification, carboxyl groups were activated, and Fe was successfully loaded onto the surface of IGS in the form of FeO and Fe₂O₃. The IGS7 exhibited the best adsorption capacity with the maximum Cd and As adsorption capacity of 42.72 mg/g and 35.29 mg/g, respectively. The Cd was mainly adsorbed through electrostatic attraction and precipitation, while the As through complexation with iron (hydr)oxides. IGS7 significantly reduced the bioavailability of Cd and As in soil with Cd bioavailability reduced from 1.17 mg/kg to 0.69 mg/kg and As bioavailability reduced from 10.59 mg/kg to 6.86 mg/kg at 1 % IGS7 addition. The Cd and As were all transformed to more stable fractions after IGS7 addition. The acid soluble and reducible Cd fractions were transformed into oxidizable and residual Cd fractions, and the non-specifically and specifically adsorbed As fractions were transformed to amorphous iron oxide-bound As fraction. This study provides valuable references for the application of CGS to the remediation of Cd and As co-contaminated soil.

Immobilization of Cd in the soil of mining areas by FeMn oxidizing bacteria

Microorganisms are widely used in large-scale pollution remediation due to their rapid reproduction and low cost. In this study, bioremediation batch experiments and characterization methods were adopted to investigate the mechanism of FeMn oxidizing bacteria on the immobilization of Cd in mining soil. The results showed that the FeMn oxidizing bacteria successfully reduced 36.84 % of the extractable Cd in the soil. The exchangeable forms, carbonate-bound forms, and organic-bound forms of Cd in the soil decreased by 11.4 %, 8 %, and 7.4 %, respectively, due to the addition of FeMn oxidizing bacteria, while FeMn oxides-bound and residual forms of Cd increased by 19.3 % and 7.5 %, as compared to the control treatments. The bacteria promotes the formation of amorphous FeMn precipitates such as lepidocrocite and goethite, which have high adsorption capacity on soil Cd. The oxidation rates of Fe and Mn in the soil treated with the oxidizing bacteria reached 70.32 % and 63.15 %, respectively. Meanwhile, the FeMn oxidizing bacteria increased soil pH and decreased soil organic matter content, further decreasing the extractable Cd in the soil. The FeMn oxidizing bacteria have the potential to be used in large mining areas to assist in the immobilization of heavy metals.

A review on hydrothermal carbonization of potential biomass wastes, characterization and environmental applications of hydrochar, and biorefinery perspectives of the process

It is imperative to search for appropriate processes to convert wastes into energy, chemicals, and materials to establish a circular bio-economy toward sustainable development. Concerning waste biomass valorization, hydrothermal carbonization (HTC) is a promising route given its advantages over other thermochemical processes. From that perspective, this article reviewed the HTC of potential biomass wastes, the characterization and environmental utilization of hydrochar, and the biorefinery potential of this process. Crop and forestry residues and sewage sludge are two categories of biomass wastes (lignocellulosic and non-lignocellulosic, respectively) readily available for HTC or even co-hydrothermal carbonization (Co-HTC). The temperature, reaction time, and solid-to-liquid ratio utilized in HTC/Co-HTC of those biomass wastes were reported to range from 140 to 370 °C, 0.05 to 48 h, and 1/47 to 1/1, respectively, providing hydrochar yields of up to 94 % according to the process conditions. Hydrochar characterization by different techniques to determine its physicochemical properties is crucial to defining the best applications for this material. In the environmental field, hydrochar might be suitable for removing pollutants from aqueous systems, ameliorating soils, adsorbing atmospheric pollutants, working as an energy carrier, and performing carbon sequestration. But this material could also be employed in other areas (e.g., catalysis). Regarding the effluent from HTC/Co-HTC, this byproduct has the potential for serving as feedstock in other processes, such as anaerobic digestion and microalgae cultivation. These opportunities have aroused the industry interest in HTC since 2010, and the number of industrial-scale HTC plants and patent document applications has increased. The hydrochar patents are concentrated in China (77.6 %), the United States (10.6 %), the Republic of Korea (3.5 %), and Germany (3.5 %). Therefore, considering the possibilities of converting their product (hydrochar) and byproduct (effluent) into energy, chemicals, and materials, HTC or Co-HTC could work as the first step of a biorefinery. And this approach would completely agree with circular bioeconomy principles.

Rethinking quantified methods for arsenic speciation and risk in a biowaste hydrothermal liquefaction system

Controversy exists to quantify the fate and speciation of Arsenic (As). We investigated its characteristics by As-containing algae in various pH hydrothermal liquefaction (HTL) system, specifically via two classical methods, i.e. the European Community Bureau of Reference (BCR) and Wenzel's method. Solid residue immobilized 11.23-16.55% of As, and 88.07-82.44% was in aqueous by the pH regulators (e.g., CH₃COOH, HCl, and KOH). ICP-MS and XRD analysis revealed that As (V) was converted into As (III) and As (0) in the solid residue, while the As (V) was mainly converted into As (III) in the aqueous phase during HTL. When the classified forms of As in solid residue are compared, Wenzel's method was more appropriate for dividing the bio-availability forms of As, whereas BCR was better for estimating the toxic-potential forms of As. Subsequently, pH regulators raised the risk of As in solid residue associated with the increasing of unstable forms. The amide was hydrolyzed to carboxylic acid with acidic additives, which weakened the reducing environment in the HTL process. In contrast, the amide was hydrolyzed to ammonia with the alkaline additives, which enhanced the reducing environment and increased the risk of As in products. This work provided a new insight in systematically evaluating the risk and speciation of As in HTL.

Development of chitosan-magnetic sawdust hydrochar for Pb and Zn immobilization process on various soil conditions

A series of 60-day soil immobilized incubations were performed to explore the impacts of various factors (incubation time, chitosan modified magnetic sawdust hydrochar (CMSH) dosages, initial pH values, moisture contents, and humic acid (HA)) on CMSH immobilization of Pb and Zn. DTPA and BCR extraction techniques were undertaken to study the distribution of form transformations of Pb and Zn. CMSH showed significant immobilization ability for both DTPA-Pb and DTPA-Zn, and the highest removal rates were shown to be 57.40% and 90.00% for Pb and Zn respectively. After 60 days of incubation, the residual Pb was enhanced by 34-61% and residual Zn increased by 25-41%, which indicated that CMSH was effective in immobilizing Pb and Zn. Meanwhile, the immobilization efficiency improved with increasing incubation time, CMSH dosage, HA dosage, and initial solution pH. In particular, 5% HA application increased the soil TOC and accelerated the metal stabilization processes, with the residual forms of Pb and Zn eventually reaching a maximum of 73% and 71%, respectively. In addition, the alkaline initial solution promoted the ion exchange, surface complexation reaction, and cationic-π interaction, resulting in a better immobilization of Pb and Zn by CMSH. Finally, according to the orthogonal analysis of BCR results, HA dosage was the major factor affecting Pb and Zn immobilization by CMSH compared to soil pH and moisture content in this study.

Multi-walled carbon nanotube-modified hydrothermal carbon: A potent carbon material for efficient remediation of cadmium-contaminated soil in coal gangue piling site

Coal gangue contains numerous metal ions. After rainwater leaching, the metal ions in coal gangue migrate into the surrounding soil, posing significant threats to human health and ecosystems. To remove Cd from contaminated soil surrounding a coal gangue pile area, loofah, oak chips, and corn stalks were used as raw materials to prepare loofah hydrothermal carbon (LH), oak chip hydrothermal carbon (OH), corn stalk hydrothermal carbon (CH). Next, LH, OH, and CH were modified using multiwalled carbon nanotubes (MWCNTs) to obtain loofah-, oak-chip-, and corn-stalk-MWCNTs hydrothermal carbon composites (LMH, OMH, and CMH). The loofah hydrothermal carbon with excellent pore structure, specific surface area and pore capacity was selected, and the loading of MWCNTs was varied to give it better adsorption performance. The N₂ adsorption-desorption experiments showed that the specific surface area and pore volume of LMH, OMH and CMH were significantly higher than those of LH, OH and CH, respectively. The specific surface area and pore volume of LMH are 101.948 m² g^-1 and 6.226 cm³ g^-1, respectively. By EDS analysis and infrared spectroscopy, it can be seen that LMH has more o-containing functional groups than OMH and CMH. Remediation experiments were carried out on the actual contaminated soil obtained from Chaili Coal Mine. It was found that the mass fractions of Cd in the acid-soluble state in the soil samples that were treated with LH and LMH decreased by 18.54% and 26.3%, respectively, after 20 d. Therefore, LMH significantly affected Cd fixation and promoted Cd pollution remediation in the coal gangue pile area.

Remediation of cadmium and lead contaminated soils using Fe-OM based materials

Iron oxide-lignin composites (GLS) were prepared based on the significant role of Fe-OM in the environmental behaviour of heavy metals and lignin binding with iron oxide preferentially in soil. GLS was applied in Cd/Pb immobilization and the stability under acid rain was investigated. The results show that the iron oxide appeared weakly crystalline or amorphous similar to 2-line ferrihydrite after the addition of lignin. Agglomerates of nanoparticles with higher adsorption capacity were observed for GLS. The mobility factor (MF) of Cd/Pb in the soil decreased rapidly after adding GLS. At the 3% dosage, the MF of Cd and Pb in the soil was decreased by 58.94% and 78.15% respectively, which was approximately 5 times that of goethite (GE). The mobile and exchangeable Cd/Pb were converted to organic, amorphous Fe oxide-bound and residue fractions. Under acid rain conditions, MF continues to decline for the GLS group, increasing the organic and amorphous Fe oxide-bound fractions, while for control group (CK) and GE, the trend was the opposite. Lignin could inhibit iron oxide dissolution and stabilize the combination of Cd/Pb and iron oxides in soil. The better stability performance of GLS for Cd/Pb may be related to the higher adsorption capacity and microstructural difference after iron oxide combined with lignin.

Efficient removal of mercury and chromium from wastewater via biochar fabricated with steel slag: Performance and mechanisms

Biochar derived from biomass is regarded as a promising adsorbent for wastewater treatment, but the high cost of modification is still a challenge for its large-scale practical applications. In this study, we employed steel slag as a low-cost fabricant and synthesized hydrothermally carbonized steel slag (HCSS), as a stable environmentally functional material for heavy metal removal. Typically, positively and negatively charged heavy metal contaminants of Hg²⁺ and Cr₂O₇²⁻ were employed to testify the performance of HCSS as an adsorbent, and good capacities [(283.24 mg/g for Hg (II) and 323.16 mg/g for Cr (VI)] were found. The feasibility of HCSS on real wastewater purification was also evaluated, as the removal efficiency was 94.11% and 88.65% for Hg (II) and Cr (VI), respectively. Mechanism studies revealed that the modification of steel slag on bio-adsorbents offered copious active sites for pollutants. As expected, oxygen-containing functional groups in HCSS acted as the main contributor to adsorption capacity. Moreover, some reactive iron species (i.e., Fe²⁺) played an essential role in chemical reduction of Cr (VI). The adsorptive reactions were pH-dependent, owing to other more mechanisms, such as coprecipitation, ion-exchange, and electrostatic attraction. This promising recycling approach of biomass waste and the design of agro-industrial byproducts can be highly suggestive of the issues of resource recovery in the application of solid waste-derived environmentally functional materials for heavy metal remediation.

Clay-hydrochar composites return to cadmium contaminated paddy soil: Reduced Cd accumulation in rice seed and affected soil microbiome

Excess cadmium (Cd) in rice precipitated by Cd contamination in paddy soils is a global human health threat and rational response is urgently needed. In this study, attapulgite-modified hydrochar (CA) and the montmorillonite-modified hydrochar (CM) were utilized in Cd-contaminated paddy soils at 0.5% (w/w) and 1% (w/w) application rates to investigate the effects of these clay-hydrochar composites on rice growth and soil Cd availability. The results show that the utility of CA and CM resulted in a significant increase in rice yield, especially at 1% application rate, which extended rice yield by 46.7-50.0% compared to 0.5% application rate. This is related to the Cd fixation and nutrient sequestration of the acidic functional groups on the surface of CA and CM. Additionally, CA-1% and CM-1% reduced the Cd concentration in rice seeds by 26.9-28.1% relative to the control. Notably, CA-1% showed the capacity to passivate Cd at the early stage of rice transplanting, lowering the proportion of Cd in the ion exchange state by 41.6% compared to the control, and this passivation effect persisted until the late stage of transplanted rice. The soil microbial community consequences showed that CA and CM did not significantly change the horizontal composition of the soil bacterial phylum and species diversity, indicating that CA and CM had excessive soil microbial adaptability. Moreover, results of correlation and Canonical Correspondence Analysis confirm that microbial genera responded significantly to the soil Cd morphologies, revealing the importance of CA and CM in the remediation of Cd-contaminated soils by influencing microorganisms. Our findings provide clay-hydrochar composites as a low-cost approach to effectively mitigate soil Cd contamination and improve the security and quality of rice.

Recent progress on the phytotoxic effects of hydrochars and toxicity reduction approaches

Hydrothermal carbonization of wet biomasses has been known to produce added-value materials for a wide range of applications. From catalyst substrates, to biofuels and soil amendments, hydrochars have distinct advantages to offer compared to conventional materials. With respect to the agricultural application of hydrochars, both positive and negative results have been reported. The presence of N, P and K in certain hydrochars is appealing and may contribute to the reduction of chemical fertilizer application. However, regardless of biomass, hydrothermal carbonization results in the production of phytotoxic organic compounds. Additionally, hydrochars from sewage sludge often contain heavy metal concentrations which exceed the regulatory limits set for agricultural use. This review critically discusses the phytotoxic aspects of hydrochar and provides an account of the substances commonly responsible for these. Furthermore, phytotoxicity reduction approaches are proposed and compared with each other, in view of field-scale applications.

Speciation of heavy metals in soils and their immobilization at micro-scale interfaces among diverse soil components

Heavy metal (HM) pollution of soils is a globally important ecological and environmental problem. Previous studies have focused on i) tracking pollution sources in HM-contaminated soils, ii) exploring the adsorption capacity and distribution of HMs, and iii) assessing phyto-uptake of HMs and their ecotoxicity. However, few reviews have systematically summarized HM pollution in soil-plant systems over the past decade. Understanding the mechanisms of interaction between HMs and solid soil components is consequently key to effectively controlling and remediating HM pollution. However, the compositions of solid soil phases are diverse, their structures are complex, and their spatial arrangements are heterogeneous, all leading to the formation of soil micro-domains that exhibit different particle sizes and surface properties. The various soil components and their interactions ultimately control the speciation, transformation, and bioavailability of HMs in soils. Over the past few decades, the extensive application of advanced instrumental techniques and methods has greatly expanded our understanding of the behavior of HMs in organic mineral assemblages. In this review, studies investigating the immobilization of HMs by minerals, organic compounds, microorganisms, and their associated complexes are summarized, with a particular emphasis on the interfacial adsorption and immobilization of HMs. In addition, methods for analyzing the speciation and distribution of HMs in aggregates of natural soils with different particle sizes are also discussed. Moreover, we also review the methods for speciating HMs at mineral-organic micro-scale interfaces. Lastly, developmental prospects for HM research at inorganic-organic interfaces are outlined. In future research, the most advanced methods should be used to characterize the interfaces and in situ characteristics of metals and metal complexes. In particular, the roles and contributions of microorganisms in the immobilization of HMs at complex mineral-organic interfaces require significant further investigation.

Stabilization of heavy metals in biochar derived from plants in antimony mining area and its environmental implications

Heavy metals pollution in mining soils seriously threatens the ecological environment and human health worldwide. Phytoremediation is considered to be an ideal method to reduce the toxicity, mobility, and bioavailability of heavy metals in the soils. However, the disposal of plant-enriched heavy metals has become a thorny problem. To estimate the effect of pyrolysis on the stabilization of heavy metals in post-phytoremediation plant residues, different biochars were prepared from Conyza canadensis (CC), Gahnia tristis (GT), and Betula luminifera (BL) at different pyrolysis temperatures (300, 450, and 600 °C). Results indicated that pyrolysis was effective in the stabilization of heavy metals (Cr, Ni, As, Sb, Hg, and Pb) in plants and significantly (P < 0.05) decreased the bioavailability of most heavy metals. Among them, GT₆₀₀ prepared by pyrolysis of GT at 600 °C has the best stabilization effect on Sb, which increases the residual fraction by 7.32 times, up to 82.05%. The results of environmental risk assessment show that pyrolysis of biomass at high temperature (600 °C) can effectively mitigate the environmental impact of As, Sb, and Hg. Additionally, the reutilization potential of biochar produced by post-phytoremediation plant residues as adsorbents was investigated. The results of adsorption experiments revealed that all biochars have an excellent performance to adsorb Pb(II), and the maximum adsorption capacity is 139.16 mg g^-1 for CC₄₅₀. The adsorption mechanism could be attributed to complexation, electrostatic attraction, and cation exchange. This study demonstrates that pyrolysis is an effective and environment-friendly alternative method to stabilize heavy metals in plants, and their pyrolysis products can be reused for heavy metal adsorption.

A critical review of biochar-based materials for the remediation of heavy metal contaminated environment: Applications and practical evaluations

The contamination of heavy metals (HMs) in the environment has aroused a global concern. The valid remediation of HM contaminated environment is a highly significant issue. As alternative to carbon materials, biochar has been vastly documented for the remediation of HM contaminated environment. However, there are some possible imperfections to meet the actual remediation tasks as the finite properties of raw biochar, and the remediation process is complex and unexpectedly. This review focuses on the progress made on environmental HM remediation by biochar-based materials within the past six years. The property analysis and key modifications of biochar are summarized inspired by their applicability or necessity for HM decontamination, and the environmental remediation as well as the implicated mechanisms are thoroughly elaborated from multiple pivotal sides. The evaluations of practical application associated with biochar amendment are also presented. Finally, some pertinent improvements and research directions are proposed. To our knowledge, this article is the first time to make a systematic summary on the reliability and practicability of biochar-based materials for environmental HM remediation, and critically pointed out the existing issues to facilitate the judicious design of biochar-based materials and understanding the research trends. It is also aims to provide reference for subsequent research and propel the practical applications.

Changes and release risk of typical pharmaceuticals and personal care products in sewage sludge during hydrothermal carbonization process

Hydrochars were obtained by hydrothermal carbonization treatment of municipal sewage sludge. Effects of reaction temperature (180-300 °C) and reaction time (2-15 h) on structural characteristics of the hydrochars, and changes and release risk of typical pharmaceuticals and personal care products (PPCPs) in the hydrochars were investigated. Reaction temperature played a more important role than reaction time on hydrochar properties and decarboxylation reaction was the primary process during the converting of sludge to hydrochars. The sludge hydrochars had higher yields, carbon recovery rates, polarity and less aromaticity than biochars. Hydrothermal process reduced PPCPs' load in sludge hydrochars effectively except caffeine and acetaminophen. The hydrochars prepared at intermediate and high temperatures (240 and 300 °C) had higher caffeine concentrations than the original sludge, which can be ascribed to the transformation of N-containing precursors. The highest CaCl₂ extracted caffeine concentration occurred at intermediate temperature of 240 °C (48.1 μg/kg) due to the stronger affinity of caffeine in the high-temperature hydrochars. Caffeine was not detected in hydroxypropyl-β-cyclodextrin (HPCD) extract. Hydrochars prepared at low temperature (180 °C) had a higher acetaminophen concentration than the original sludge, which was attributed to the high thermal stability temperature of acetaminophen. Low- and intermediate-temperature hydrochars had higher CaCl₂ extracted acetaminophen concentrations. The HPCD extracted acetaminophen was low with a range of nd to 6.72 μg/kg. In conclusion, PPCPs are less likely to constitute a limiting factor on the farm application of sludge hydrochar. This study provides theoretical support for the safe application of sludge hydrochar in the farmland.

Co-pyrolysis of corn stover with industrial coal ash for in situ efficient remediation of heavy metals in multi-polluted soil

Coal ash incorporated biochar (CA/BC) composite was prepared by co-pyrolysis of agricultural residue and industrial coal ash and applied for remediation of soils polluted by lead (Pb) and cadmium (Cd). The results showed that immobilization efficiency of CA/BC for heavy metals (HMs) was significantly enhanced by 77.1 % (Pb) and 42.7 % (Cd) compared to pristine biochar (BC), and this was mainly due to the increased pH value, surface functionality and surface negative charge. By the introduction of 5 % CA/BC, the polluted soils showed the highest reduction of leaching toxicity by 67.9 % (Pb) and 49.7 % (Cd), respectively. The chemical speciation of Pb and Cd in soils was changed remarkably and the reduced bioavailable Pb and Cd were mainly transformed from acid-soluble fraction into the most stable form of residual fraction. The mechanism study showed that surface precipitation, complexation, cation exchange and cation-π interaction of CA/BC mainly contributed to heavy metals (HMs) immobilization. The pot experiments further confirmed that incorporation of 5 % CA/BC effectively reduced plant Pb and Cd accumulation by 81 % and 62.5 % respectively, and significantly promoted the plant growth of paddy rice by 3.1, 2.2 and 2.0 times in terms of root, stem length and dry mass parameters. The present study offered a cost-effective and green method to prepare soil amendment with great potential for remediation of soils polluted by HMs and realized the value-added utilization of waste agricultural residue and industrial coal ash.

Effect of Manure and Compost on the Phytostabilization Potential of Heavy Metals by the Halophytic Plant Wavy-Leaved Saltbush

This study aimed to use organic fertilizers, e.g., compost and manures, and a halophytic plant [wavy-leaved saltbush (Atriplex undulata)] to remediate an agricultural soil polluted with toxic elements. Compost or manure (1% w/w) was added to a polluted soil in a pot trial. The application of the organic fertilizer, whether compost or manure, led to a significant improvement in the growth of the tested plant. From the physiological point of view, the application of organic fertilizers to polluted soil significantly increased the content of chlorophyll, carotenoid, and proline and, furthermore, led to a clear decrease in malondialdehyde (MDA) in the plant leaves. The highest significant values of organic carbon in the polluted soil (SOC) and cation exchange capacity (CEC) were found for the soil amended by compost and planted with wavy-leaved saltbush. Manure significantly reduced the soil pH to 7.52. Compost significantly decreased Zn, Cu, Cd, and Pb availability by 19, 8, 12, and 13%, respectively, compared to the control. On the other hand, manure increased Zn, Cu, Cd, and Pb availability by 8, 15, 18, and 14%, respectively. Compost and manure reduced the bioconcentration factor (BCF) and translocation factor (TF) of Cd and Pb. Compost was more effective in increasing the phytostabilization of toxic metals by wavy-leaved saltbush plants compared to manure. The results of the current study confirm that the application of non-decomposed organic fertilizers to polluted soils increases the risk of pollution of the ecosystem with toxic elements. The cultivation of contaminated soils with halophytic plants with the addition of aged organic materials, e. g., compost, is an effective strategy to reduce the spreading of toxic metals in the ecosystem, thus mitigating their introduction into the food chain.

Hierarchically porous magnetic biochar as an efficient amendment for cadmium in water and soil: Performance and mechanism

Three types of hierarchically porous magnetic biochars (HMBs) were prepared by pyrolyzing low-cost wheat straw and potassium ferrate (K₂FeO₄) under a nitrogen atmosphere at 600, 700 and 800 °C, respectively, which could be used as amendments for cadmium (Cd) in water and soil. HMB fabricated at 700 °C (HMB700) had the best remediation performance for Cd in water and soil, which was mainly due to its largest specific surface area and micropore volume. Batch sorption experiments showed that Cd(II) sorption onto HMBs were well-described by a pseudo-second-order model and Sips (Freundlich-Langmuir) model, indicating that HMBs removed Cd(II) mainly through chemical adsorption. MINTEQ modeling evidenced that HMBs adsorbed Cd(II) mainly through precipitation rather than surface complexation. The adsorption behavior of HMB700 to Cd(II) could be explained by surface complexation (-OCd, -COOCd), precipitation (Cd(OH)₂ and CdCO₃), physical adsorption (rich pore structure) and ion exchange (K⁺, Ca²⁺, Mg²⁺). Furthermore, adding HMBs (1 wt%) (incubation 60 days) could also significantly increase soil pH and electrical conductivity (EC), and significantly reduce the available Cd content in soil (47.97%-61.38%). Adding HMBs could promote the conversion of bioavailable to less bioavailable Cd forms. These results provided a new idea for fabricating agricultural waste-based HMBs to remediate Cd in water and soil.

P-enriched hydrochar for soil remediation: Synthesis, characterization, and lead stabilization

One-step synthesis of multifunctional materials using biomass waste for environmental remediation is a current research hotspot. In this study, a novel P-enriched hydrochar was obtained by co-hydrothermal treatment of biomass (bamboo or hickory) with concentrated H₃PO₄ (biomass: H₃PO₄ = 1:4) at 200 °C for 7 h. The characteristics of the P-enriched hydrochar were determined and its effect on the stabilization of Pb in soils was investigated. Compared to pristine hydrochar, the weight yield of the P-enriched hydrochar was greater (by over 2 times). This was due to the enrichment of P (over 20% by weight), as the C, N, and H weight content was reduced. Moreover, the aromaticity, thermal stability, and surface functionality of P-enriched hydrochar were all higher than that of pristine hydrochar. Addition of the pristine hydrochar to a simulated 1300 mg·kg^-1 Pb-contaminated soil at 3% (w/w) resulted in a 20%-40% reduction in leached Pb only after 4 weeks, compared to the control without hydrochar amendment. However, addition of the P-enriched hydrochar to the spiked Pb-contaminated soil reduced Pb leaching by about 60% after only 1 week and about 90% after 3 weeks. Besides, using a real Pb-contaminated soil (149,000 mg·kg^-1 Pb), P-enriched hydrochar addition at 5% (w/w) resulted in a 100% decrease in Pb leaching in the first week and maintained leached Pb levels at <2 mg L^-1, meeting U.S.-E.P.A. standards. Thus, P-enriched hydrochar stabilized Pb in both simulated and real Pb-contaminated soil quickly and efficiently. Hence, the potential of one-step co-hydrothermal carbonization of biomass with H₃PO₄ to produce a novel and sustainable P-enriched hydrochar with properties suitable for environmental remediation of cationic metals.

A Strategy to Revalue a Wood Waste for Simultaneous Cadmium Removal and Wastewater Disinfection

In this investigation, the possibility of wood waste (hardwoods such as oaks’ and alternatives’ staves from Balkan cooperage) revalorization for simultaneous cadmium removal and wastewater disinfection was examined. All samples were characterized in terms of their crystallinity index and crystallite size, amount of functional groups, and surface chemistry (determined by ATR-FTIR) as well as antibacterial activity. Mulberry is characterized by the lowest crystallinity index which can be ascribed to the highest crystallite size disabling crystallite denser packaging, while myrobalan plum has about 23% lower crystallite size that enables crystallite better packaging, thus resulting in a 42.4% higher crystallinity index compared to the mulberry. All oaks have a significantly higher amount of carboxyl groups compared to the alternatives (0.23-0.28 vs. 0.12-0.19 mmol/g). The adsorption experiments revealed that with increasing the initial cadmium concentration from 15 up to 55 mg/g, samples’ adsorption capacity increases by 89-220%. The equilibrium data fit well with the Langmuir isotherm model implying monolayer coverage of cadmium ions over a homogeneous wood surface. The relationship between the samples’ maximum adsorption capacities (ranged from 5.726 to 12.618 mg/g), their crystallinity index, and crystallite size was established. According to ATR-FTIR spectra, aldehyde, carboxyl, hydroxyl, and phenyl groups present on the wood waste surface are involved in Cd2+ adsorption which proceeds via the interplay of the complexation, cation-π interactions, and ion-exchange mechanisms. Mulberry and myrobalan plum showed about 89% and 80% of the total uptake capacity of cadmium within 60 min, while the equilibrium was attained after 240 min of contact time. Good compliance with pseudo-second kinetic order indicated that cadmium adsorption was mediated by chemical forces. Thermodynamic parameters revealed the spontaneous and exothermic character of cadmium ion adsorption onto mulberry and myrobalan plum. All studied samples provide maximum bacterial reduction (>99%) for E. coli and S. aureus. Wood waste from Balkan cooperage can be successfully used for simultaneous cadmium removal and wastewater disinfection.

Biowaste hydrothermal carbonization aqueous product application in rice paddy: Focus on rice growth and ammonia volatilization

Hydrothermal carbonization (HTC) is known as a green biomass conversion technology. However, it often suffers from the issue of disposing hydrothermal carbonization aqueous products (HCAP). Based on the characterization and composition of acidic HCAP, a rice paddy soil column experiment was conducted to observe the effects of HCAP on ammonia (NH₃) volatilization form paddy soil and rice yield. The experiment was designed with five treatments. HCAPs were produced at 220 °C and (SHC220-L) and 260 °C (SHC260-L) derived from poplar sawdust, HCAP produced at 220 °C (WHC220-L) and 260 °C (WHC260-L) derived from wheat straw, and a control group without HCAP application (termed CKU hereafter). The results showed that HCAP treatments increased the rice yield by 4.30%-26.0% compared to CKU. HACPs prepared at lower temperatures (SHC220-L and WHC220-L) mitigated the cumulative NH₃ volatilization by 11.2% and 7.6%, respectively, and mitigated yield-scale NH₃ volatilization (cumulative NH₃ volatilization/total yield) by 14.2% ∼ 22.4%. HCAP significantly improved the N use efficiency of rice. We found that the NH₃ volatilization was related to NH₄⁺-N concentration and pH of surface water, soil TOC and NH₄⁺-N oxidation functional genes. This study implied that HCAP could be potentially used as a liquid fertilizer, which will be a potential substitute for chemical N fertilizers. There is still a long way before HCAP can be applied in full-scale for N fertilizer reduction and waste recycle.

Interactive assessment of lignite and bamboo-biochar for geochemical speciation, modulation and uptake of Cu and other heavy metals in the copper mine tailing

This study was designed to examine the combined effect of bamboo-biochar (BC) and water-washed lignite (LGT) at copper mine tailings (CuMT) sites on the concentration of Cu and other metals in pore water (PW), their bioavailability, and change in geochemical speciation. Rapeseed (first cropping-season) and wheat (second cropping-season) were grown for 40-days each and the influence of applied-amendments on both cropping seasons was observed and compared. A significant increase in pH, water holding capacity (WHC), and soil organic carbon (SOC) was observed after the applied amendments in second cropping-seasons. The BC-LGT significantly reduced the concentration of Cu in PW after second cropping seasons; however, the concentration of Pb and Zn were increased with the individual application of biochar and LGT, respectively. BC-LGT and BC-2% significantly reduced the bioavailability of Cu and other HMs in both cropping seasons. The treated-CuMT was subjected to spectroscopic investigation through X-ray photoelectron spectroscopy (XPS), Fourier transform Infrared spectroscopy (FTIR), and X-ray powder diffraction (XRD). The results showed that Cu sorption mainly involved the coordination with hydroxyl and carboxyl functional groups, as well as the co-precipitation or complexation on mineral surfaces, which vary with the applied amendment and bulk amount of Mg, Mn, and Fe released during sorption-process. The co-application of BC-LGT exerted significant effectiveness in immobilizing Cu and other HMs in CuMT. The outcomes of the study indicated that co-application of BC-LGT is an efficacious combination of organic and inorganic materials for Cu adsorption which may provide some new information for the sustainable remediation of copper mine tailing.

Enhancement of exchangeable Cd and Pb immobilization in contaminated soil using Mg/Al LDH-zeolite as an effective adsorbent

In the present study, experiments using zeolite and Mg/Al LDH-zeolite for immobilization of Cd and Pb ions in artificial soil were conducted. The conditions which affect Cd and Pb ion immobilization in soil were evaluated, namely soil pH (5-7), the mass ratio of adsorbents (1%, 3% and 5%), incubation time (15 days, 30 days and 45 days) and soil moisture (30%, 50% and 70%). The results indicated that the optimal soil pH, mass ratio of adsorbents, incubation time and soil moisture for immobilization of Cd and Pb ions by the adsorbent were, respectively, 7.0, 3%, 30 days and 70%. The exchangeable Cd ion content in the contaminated soil dropped from 22.17 mg kg-1 (87.65%) to 11.03 mg kg-1 (43.48%) and 6.47 mg kg-1 (26.36%) on incubation with zeolite and Mg/Al LDH-zeolite, respectively, while the exchangeable Pb content fell from 23.28 mg kg-1 (90.02%) to 14.12 mg kg-1 (54.04%) and 9.47 mg kg-1 (35.24%) using zeolite and Mg/Al LDH-zeolite as absorbents in contaminated soil, respectively. Fe-Mn oxide occluded (F2), carbonate bound (F3) and organically complexed (F4) were the main forms for immobilization of the exchangeable Cd and Pb when the zeolite and Mg/Al LDH-zeolite absorbents were separately cultivated into soil. Precipitation, co-precipitation and electrostatic attraction were the main mechanisms of exchangeable Cd and Pb immobilization onto the Mg/Al LDH-zeolite to form carbonate metals (CdCO3 and PbCO3). This was due to the surface functional groups of the adsorbent and the presence of Fe and Al oxyhydroxides, Mn oxides, and Si and O elements in the Mg/Al LDH-zeolite's constituents. The efficiency of Cd and Pb immobilization by the Mg/Al LDH-zeolite was higher than that by zeolite from 1.5 to 1.6 times. The Mg/Al LDH-zeolite showed an enhanced ability of exchangeable Cd and Pb immobilization in contaminated soil.

A meta-analysis of heavy metal bioavailability response to biochar aging: Importance of soil and biochar properties

Biochar has been widely applied to remediate the heavy metal-polluted soils, whereas biochar aging can induce the changes of the biochar physic-chemical properties. Afterwards, the bioavailability of heavy metals (BHM) will vary in soils which likely increase the unstable fractions of heavy metals and the following environmental risks. To explore the biochar aging effects on the BHM changes in responses to the variation of experimental conditions and biochar properties, a meta-analysis for the literatures published before May 2020 was conducted. A sum of 257 independent observations from 22 published papers was obtained. The results from the analysis of boosted regression tree showed that the soil pH was the most important factor influencing the BHM changes in biochar amended soil, followed by soil texture, aging time and biochar pyrolysis temperature. The results of this review showed that the BHM was decreased by 16.9%, 28.7% and 6.4% in weakly acid soil (pH 6.00-6.99), coarse- and medium-textured soils, respectively, but increased by 149% and 121% in the alkaline (pH > 8.00) and fine-textured soils. The BHM declined in the soils amended with biochar pyrolyzed at relative high temperature (> 500 °C), and increased during aging in soils amended with biochar pyrolyzed at relatively low temperature (401-500 °C). In terms of diverse immobilized heavy metals, only bioavailable Zn in soil decreased after aging. However, there was no significant changes in Cd, Cu and Pb's bioavalability. Besides, the BHM was decreased by 18.6% within the short-term (less than one year) biochar aging, while showed inverse trend during the longer aging processes. Besides, the application of lignin-enriched biochar may counteract the positive effects of the biochar aging on BHM. Our works may promote the interpretation of the interference factors on the BHM changes and filled the research gaps on biochar aging process in soils.

Hydrochar and pyrochar for sorption of pollutants in wastewater and exhaust gas: A critical review

Pollutants in wastewater and exhaust gas bring out serious concerns to public health and the environment. Biochar can be developed as a sustainable adsorbent originating from abundant bio-wastes, such as agricultural waste, forestry residue, food waste and human waste. Here we highlight the state-of-the-art research progress on pyrochar and hydrochar for the sorption of pollutants (heavy metal, organics, gas, etc) in wastewater and exhaust gases. The adsorption performance of pyrochar and hydrochar are compared and discussed in-depth, including preparation procedures (carbonization and activation), sorption possible mechanisms, and physiochemical properties. Challenges and perspective for designing efficient and environmental benign biochar-based adsorbents are finally addressed.

Integrated remediation for organic-contaminated site by forcing running-water to modify alkali-heat/persulfate via oxidation process transfer

As organic pollution of soil and groundwater increases, the effective and economical remediation of contaminated sites has drawn growing attention. In this study, running-water (RW) was designed to modify alkali-heat/persulfate (MAH/PS) for integrated remediation of an actual organic-contaminated site. The degradation efficiency mainly reached 60%-99% for Benz[a]anthracene, Benzo[a]pyrene and total petroleum hydrocarbons (TPHs). MAH/PS was more effective in degrading Benzene and 1,2-Dichloroethane with simple molecular configurations. The pollutant degradation efficiencies decreased with increasing site depth and increased with increasing pollutant concentrations. Migration with RW enhanced site remediation. By monitoring the groundwater after remediation, it was found that residual TPHs presented anomalous diffusion; SO₄^2- ranged from 8.00 to 237.00 mg L^-1 to 8.00-290.00 mg L^-1 and pH presented alkalescence (7.00-8.20). Mathematical models were established to describe the reaction process including the solubility equilibrium of calcium hydroxide, temperature equilibrium, and reaction kinetics. Moreover, MAH/PS provided a cost-saving approach for site remediation.

Performance of microbial induced carbonate precipitation for immobilizing Cd in water and soil

Microbial induced carbonate precipitation (MICP) is known as a significant process for remediating heavy metals contaminated environment. In this study, a novel Cd-resistant ureolytic bacteria was isolated and identified as Enterobacter sp. Its performances for immobilizing Cd in solution and soil were systematically discussed at different treatment conditions. Results showed that initial pH and Cd concentration were important parameters to influence Cd removal rate. The maximal Cd removal rate in solution reached 99.50 % within 7 days by MICP. The precipitation produced in Cd removal process were characterized by X-ray diffraction, scanning electron microscopy and energy dispersive spectrometer to understand the removal mechanism. Analyses showed that Cd removal mechanism of CJW-1 was predominately via biominerals including calcites and vaterites to absorb Cd²⁺. Cd immobilization tests demonstrated that the highest Cd-immobilization rate in soil could reach 56.10 %. Although all treatments contribute to soil pH, fertility, and enzyme activities improvement, oyster shell wastes (OS) had a better effect on soil cation exchange capacity. All treatments had negative effects on soil respiration and bacterial community, but OS can alleviate such adverse influence. Our results emphasized that Cd-resistant ureolytic bacteria strain CJW-1 combined with OS had excellent ability and reuse value to remediate Cd-contaminated environment.

Effect of spent liquor recycle during hydrothermal carbonization on the properties of hydrochar

Hydrothermal carbonization is a feasible way to convert biomass into valuable hydrochar, the recycle of spent liquor during HTC is beneficial to reduce the output of spent liquor. The effects of spent liquor recycle on the properties of hydrochar and the biogas potential of spent liquor are investigated in this study. Part of the spent liquor (50% volume fraction) was recycled four times at 220 and 240 °C HTC, respectively. The results showed that the surface area of hydrochar was in the range of 7.2-8.6 m²/g after spent liquor recycle, and the peak of -OH, C = O and C-O became more intense. The Cd²⁺ adsorption capacity of hydrochar was around 1.3 mg/g and it decreased by 0.02-0.15 mg/g after spent liquor recycle. The biogas yield of spent liquor kept declining from 263 to 29 mL/g-COD with each cycle of spent liquor. Our results suggest that spent liquor recycle is a promising method to improve hydrothermal carbonization process.

Iron-modified rice husk hydrochar and its immobilization effect for Pb and Sb in contaminated soil

Cationic and anionic heavy metal contamination sometimes co-exists in soil systems, such as mining areas and shooting ranges, seriously threatens human health and ecological stability. In this study, iron-modified rice husk hydrochar showed commendable ability to immobilize both heavy metal cation (Pb) and anion (Sb) simultaneously in soils. Iron-modified rice husk hydrochar (HC12.5-180) (5%) amendment reduced the bioavailability (EX- and CB-fraction) of Pb and Sb by 25 and 40%, respectively, which were 8 and 5 times higher than that of pristine rice husk hydrochar (HC0-180) (5%) amendment. The cation (Pb) immobilization mainly depends on cation exchange with mineral components (K⁺, Ca²⁺, Na⁺, Mg²⁺), precipitation with nonmetallic anions (Cl- and SO₄^2-), and complexation. Meanwhile, the iron oxides (FeO, Fe₂O₃, Fe₃O₄), formed during hydrothermal process, can be easily combined with anion (Sb) to form geochemically stable minerals. In conclusion, this work offered a practical and cost-effective technology based on the iron modification rice husk hydrochar for the immobilization of both anionic and cationic heavy metal contaminants in soils.

Bamboo-biochar and hydrothermally treated-coal mediated geochemical speciation, transformation and uptake of Cd, Cr, and Pb in a polymetal(iod)s-contaminated mine soil

In this study, polymetal(iod)s-contaminated mining soil from the Huainan coalfield, Anhui, China, was used to investigate the synergistic effects of biochar (BC), raw coal (RC), and hydrothermally treated coal (HTC) on the immobilization, speciation, transformation, and accumulation of Cd, Cr, and Pb in a soil-plant system via geochemical speciation and advanced spectroscopic approaches. The results revealed that the BC-2% and BC-HTC amendments were more effective than the individual RC, and/or HTC amendments to reduce ethylene-diamine-tetraacetic acid (EDTA)-extractable Cd, Cr, and Pb concentrations by elevating soil pH and soil organic carbon content. Soil pH increased by 1.5 and 2.5 units after BC-2% and BC-HTC amendments, respectively, which reduced EDTA-extractable Cd, Cr, and Pb to more stabilized forms. Metal speciation and X-ray photoelectron spectroscopy analyses suggested that the BC-HTC amendment stimulated the transformation of reactive Cd, Cr, and Pb (exchangeable and carbonate-bound) states to less reachable (oxide and residual) states to decrease the toxicity of these heavy metals. Fourier transform infrared spectroscopy and X-ray diffraction analyses suggested that reduction and adsorption by soil colloids may be involved in the mechanism of Cd(II), Cr(VI), and Pb(II) immobilization via hydroxyl, carbonyl, carboxyl, and amide groups in the BC and HTC. Additionally, the BC-2% and BC-HTC amendments reduced Cd and Pb accumulation in maize shoots, which could mainly be ascribed to the reduction of EDTA-extractable heavy metals in the soil and more functional groups in the roots, thus inhibiting metal ion translocation by providing the electrons necessary for immobilization, compared to those in roots grown in the unamended soil. Therefore, the combined application of BC and HTC was more effective than the individual application of these amendments to minimize the leaching, availability, and exchangeable states of Cd, Cr, and Pb in polymetal(iod)s-contaminated mining soil and accumulation in maize.

One-pot synthesis and characterization of engineered hydrochar by hydrothermal carbonization of biomass with ZnCl2

Hydrochar, the product of hydrothermal carbonization of biomass, is a sustainable alternative to other carbonaceous environmental sorbents. However, its use has been limited due to its low surface area. A one-pot biomass/metal salt co-hydrothermal synthesis method might improve its sorptive properties while retaining its efficient production characteristic. Thus, bamboo sawdust and zinc chloride (ZnCl₂) were combined in a hydrothermal reactor (200 °C, 7 h) for preparing modified hydrochar. Compared to the non-modified hydrochar, the hydrochar produced with the addition of ZnCl₂ during hydrothermal treatment was more fully carbonized (C content increased from 54% to 64%), of higher surface area after acid washing (30 versus 1.7 m² g^-1), and enriched in O-containing functional groups and of greater aromaticity (according to FTIR and XRD analysis). Because of these improved properties, Methylene blue adsorption capacity of the modified hydrochar increased by nearly 90% and by 257% after it was rinsed with acid. This study highlights the potential of this one-pot co-hydrothermal treatment of biomass in presence of metal salt to provide a simple and effective hydrochar with properties suitable for environmental remediation and water treatment.

Experimental and Computational Evaluation of Heavy Metal Cation Adsorption for Molecular Design of Hydrothermal Char

A model hydrochar was synthesized from glucose at 180 °C and its Cu(II) sorption capacity was studied experimentally and computationally as an example of molecular-level adsorbent design. The sorption capacity of the glucose hydrochar was less than detection limits (3 mg g−1) and increased significantly with simple alkali treatments with hydroxide and carbonate salts of K and Na. Sorption capacity depended on the salt used for alkali treatment, with hydroxides leading to greater improvement than carbonates and K+ more than Na+. Subsequent zeta potential and infrared spectroscopy analysis implicated the importance of electrostatic interactions in Cu(II) sorption to the hydrochar surface. Computational modeling using Density Functional Theory (DFT) rationalized the binding as electrostatic interactions with carboxylate groups; similarly, DFT calculations were consistent with the finding that K+ was more effective than Na+ at activating the hydrochar. Based on this finding, custom-synthesized hydrochars were synthesized from glucose-acrylic acid and glucose-vinyl sulfonic acid precursors, with subsequent improvements in Cu(II) adsorption capacity. The performance of these hydrochars was compared with ion exchange resins, with the finding that Cu(II)-binding site stoichiometry is superior in the hydrochars compared with the resins, offering potential for future improvements in hydrochar design.

Hydrothermal Carbonization as a Valuable Tool for Energy and Environmental Applications: A Review

Hydrothermal carbonization (HTC) represents an efficient and valuable pre-treatment technology to convert waste biomass into highly dense carbonaceous materials that could be used in a wide range of applications between energy, environment, soil improvement and nutrients recovery fields. HTC converts residual organic materials into a solid high energy dense material (hydrochar) and a liquid residue where the most volatile and oxygenated compounds (mainly furans and organic acids) concentrate during reaction. Pristine hydrochar is mainly used for direct combustion, to generate heat or electricity, but highly porous carbonaceous media for energy storage or for adsorption of pollutants applications can be also obtained through a further activation stage. HTC process can be used to enhance recovery of nutrients as nitrogen and phosphorous in particular and can be used as soil conditioner, to favor plant growth and mitigate desertification of soils. The present review proposes an outlook of the several possible applications of hydrochar produced from any sort of waste biomass sources. For each of the applications proposed, the main operative parameters that mostly affect the hydrochar properties and characteristics are highlighted, in order to match the needs for the specific application.

Efficient immobilization of toxic heavy metals in multi-contaminated agricultural soils by amino-functionalized hydrochar: Performance, plant responses and immobilization mechanisms

A novel amino-functionalized hydrochar material (referred to NH₂-HCs) was prepared and used as the soil amendment to immobilize multi-contaminated soils for the first time. The results showed that the application of NH₂-HCs significantly improved (P < 0.05) soil properties (i.e., pH value, cation exchange capacity and organic content). By introduction of NH₂-HCs, the contaminated soil showed the highest value of 96.2%, 52.2% and 15.5% reductions in Cu, Pb and Cd bioavailable concentrations and the leaching toxicity of Cu, Pb and Cd were remarkably reduced by 98.1%, 31.3% and 30.4%, respectively. Most of exchangeable Cu, Pb and Cd reduced were transformed into its less available forms of oxidizable and residual fractions. Potential ecological risk assessment indicated that the element Cd accounted for the most of total risks in NH₂-HCs amended soils. The mechanism study indicated that surface complexation, chemical chelating and cation-pi interaction of NH₂-HCs played a vital role in the immobilization of heavy metals. Pot experiments further verified that the application of NH₂-HCs significantly improved plant growth and reduced metal accumulations. The present study offered a novel approach to prepare amino-functionalized hydrochars with great potential as the green and alternative amendments for efficiently immobilizing heavy metals in multi-contaminated soil.

Green remediation of Cd and Hg contaminated soil using humic acid modified montmorillonite: Immobilization performance under accelerated ageing conditions

Solidification/Stabilization (S/S) is an effective way to immobilize toxic metals in contaminated soil. However, utilization of ordinary Portland cement (PC) in this process has raised environmental concerns owing to the high carbon footprint from PC manufacturing and the risk of toxic element leaching in the long term. Hence there is an urgent need to seek for "green" immobilization approaches with long-term stability. In this study, a clay-based material, humic acid modified montmorillonite (HA-Mont) was applied to a Cd and Hg contaminated soil. Field emission scanning electron microscopy combined with energy-dispersive X-ray spectroscopy (FESEM/EDS), N₂ adsorption-desorption, Fourier transform infrared (FTIR) spectroscopy and X-ray diffraction (XRD) analyses were performed to investigate the characteristics of this material. Compared to the soil without any treatment, dosage of 5 % HA-Mont could effectively reduce Cd and Hg concentrations by 94.1 % and 93.0 %, respectively and to below the regulatory limits in the TCLP (Toxicity Characteristic Leaching Procedure) leachates. Compared to the soil treated with virgin montmorillonite, HA modification resulted in the reduction of leachate concentrations of Cd and Hg by 69.5 % and 65.9 %, respectively. Long-term immobilization performance of the HA-Mont treatment was examined using a quantitative accelerated ageing method. In order to examine the ageing features, a novel method based on conditional probability was developed, and the reliability of HA-Mont immobilization was found to fit the Weibull model well, as the ageing rate of immobilization effect increased with time. After 120 years of ageing, reliability of both metals could still remain above 0.95. Cd concentration in TCLP leachates at 120^th year could still remain below the regulatory limit (294 μg/L vs 1000 μg/L), while Hg concentration reached the regulatory limit of 200 μg/L in 96^th year. This is the first attempt developing a green S/S method of Cd and Hg contaminated soil using HA-Mont and examining the long-term ageing characteristics of the stabilized soil using a probability-based approach.

Effects of Long-Term Repeated Freeze-Thaw Cycles on the Engineering Properties of Compound Solidified/Stabilized Pb-Contaminated Soil: Deterioration Characteristics and Mechanisms

The effects of long-term repeated freeze-thaw cycles and pollution levels on the engineering properties (q_u, E₅₀, φ, c, and k) of Pb-contaminated soils were investigated in various laboratory tests. These soils were solidified/stabilized (S/S) with three types of cement-based combined binders (C2.5S5F5, C5S2.5F2.5, and C5S5, cement, lime, and fly ash, mixed in different proportions; these materials are widely used in S/S technology). The strength and permeability coefficient of compound solidified/stabilized Pb-contaminated soils (Pb-CSCSs) were determined based on measurements of unconfined compressive strength (UCS), direct shear, and permeability. CT scanning, scanning electron microscopy (SEM), and Fourier transform infrared spectroscopy (FTIR) tests were employed to analyse the deterioration mechanisms under various repetitions of freeze-thaw cycles. The results showed that, under repeated freeze-thaw cycles, the engineering properties of Pb-CSCSs all degraded to varying degrees, though degradation tended to stabilise after 30 days of freeze-thaw cycles. The study also found that the pollutants obstruct hydration and other favourable reactions within the soil structure (such as ion exchanges and agglomerations and pozzolanic reactions). The activation of hydration reactions and the rearrangement of soil particles by freeze-thaw cycles thus caused the engineering properties to fluctuate, and soils exhibited different deterioration characteristics with changes in Pb²⁺ content.