Thermal conversion of a promising phytoremediation plant (Symphytum officinale L.) into biochar: Dynamic of potentially toxic elements and environmental acceptability assessment of the biochar

Hydrothermal synthesized kaolin group lamellar/spongy aluminosilicates for enhanced lead vapor capture

Developing high-temperature-resistant adsorbents with superior porous properties is crucial for safely disposing of heavy metal-containing solid waste via pyrolysis. We synthesized aluminosilicates hydrothermally and observed that acidic conditions, especially HCl (pH=2.6), favored sponge-like mineral (NC2.6) formation with a specific surface area of 500.31 m²/g and pore volume of 0.986 cm³ /g, while alkaline conditions (pH=12.0) promoted spherical particle growth. NC2.6 exhibited higher adsorption capacity compared to kaolinite and halloysite in the PbCl2 vapor adsorption, reaching a maximum of 137.68 mg/g at 700 ℃ (75.91 % stable). We examined the effect of CO2 and H2O on adsorption efficiency and explored the mechanisms using DFT and GCMC simulations. From GCMC results, CO2 negatively impacted PbCl2 adsorption due to competitive adsorption, while H2O increased adsorption content (144.24 mg/g at 700 ℃) by converting PbCl2 into oxides. DFT revealed the presence of CO2 enhanced the adsorption stability of PbCl2 via the formation of covalent bonds between O in CO2 and Pb, and active O on the aluminosilicate surface. H2O increased PbCl2 adsorption energy, as O in H2O occupied an active Al that originally formed a covalent bond with Cl, while the H formed a weak hydrogen bond with this Cl.

GLDA exhibits advantages in the phytoextraction of Cd and Ni in land-applied municipal sludge

Landscape utilization is a green and environment-friendly way of disposing of compost sludge. Garden plants can extract heavy metals from the sludge of land use, but the effect is not enough to be widely used. Chelating agents have been found to facilitate the extraction of heavy metals from plants and are expected to be popularized if they are also environmentally friendly. In this study, the effects of methylglycinediacetic acid trisodium salt (MGDA), tetrasodium glutamate diacetate (GLDA), and ethylene diamine tetraacetate (EDTA) on the extraction of Ni and Cd from compost sludge by Symphytum officinale L. were studied through the pot experiment. Compared with the control group, the application of 5-10 mmol kg-1 MGDA and 1-9 mmol kg-1 GLDA promoted plant growth, while the application of 3-4 mmol kg-1 EDTA inhibited plant growth. The highest Ni content in shoots appeared in 4 mmol kg-1 GLDA treatment, which was 4.2 times that of the CK group. The highest shoot Cd concentration appeared in 4 mmol kg-1 EDTA treatment, 6.5 times that of CK. The promotion effects of the three reagents on the acid-extractable state of Cd were similar, while that of GLDA on the acid-extractable state of Ni was outstanding. The results of this study suggested that S. officinale could be a potential phytoextraction plant for Cd and Ni, and GLDA could friendly promote the Ni phytoextraction ability of the plant. The study provides a new and efficient method for phytoremediation of heavy metals in soil.

Environmental remediation potential of a pioneer plant (Miscanthus sp.) from abandoned mine into biochar: Heavy metal stabilization and environmental application

Pyrolysis stands out as an effective method for the disposal of phytoremediation residues in abandoned mines, yielding a valuable by-product, biochar. However, the environmental application of biochar derived from such residues is limited by the potential environmental risks of heavy metals. Herein, Miscanthus sp. residues from abandoned mines were pyrolyzed into biochars at varied pyrolysis temperatures (300-700 °C) to facilitate the safe reuse of phytoremediation residues. The results showed that pyrolysis significantly stabilizes heavy metals in biomass, with Cd exhibiting the most notable stabilization effect. Acid-soluble/exchangeable and reducible fractions of Cd decreased significantly from 69.91 % to 2.52 %, and oxidizable and residue fractions increased approximately 3.24 times at 700 °C. The environmental risk assessment indicated that biochar pyrolyzed over 500 °C pose lower environmental risk (RI < 30), making them optimal for the safe utilization of phytoremediation residues. Additionally, adsorption experiments suggested that biochars prepared at higher temperature (500-700 °C) exhibit superior adsorption capacity, attributed to alkalinity and precipitation effect. This study highlights that biochars produced by pyrolyzing Miscanthus sp. from abandoned mines above 500 °C hold promise for environmental remediation, offering novel insight into the reutilization of metal-rich biomass.

Effect of ammonium sulfate combined with aqueous bio-chelator on Cd uptake by Cd-hyperaccumulator Solanum nigrum L

The efficacy of using plants to phytoremediate heavy metal (HM) contaminated soils can be improved using soil amendments. These amendments may both increase plant biomasses and HMs uptake. We aimed to determine the composite effect of ammonium sulfate ((NH4)2SO4) combined with the application of an aqueous stem-extracted bio-chelator (Bidens tripartita L) on the plant biomasses and cadmium (Cd) phytoextraction by Solanum nigrum L. The constant (NH4)2SO4 application mode plus bio-chelator additives collectively enhanced the shoot Cd extraction ability owing to the increased plant biomass and shoot Cd concentration by S. nigrum. The shoot Cd extraction and the soil Cd decreased concentration confirmed the optimal Cd phytoextraction pattern in K8 and K9 treatments (co-application of (NH4)2SO4 and twofold/threefold bio-chelators). Accordingly, Cd contamination risk in the soil (2 mg kg-1) could be completely eradicated (<0.2 mg kg-1) after three rounds of phytoremediation by S.nigrum based on K8 and K9 treatments through calculating soil Cd depletion. The microorganism counts and enzyme activities in rhizosphere soils at treatments with the combined soil additives apparently advanced. In general, co-application mode of (NH4)2SO4 and aqueous bio-chelator was likely to be a perfect substitute for conventional scavenger agents on account of its environmental friendliness and cost saving for field Cd contamination phytoremediation by S. nigrum.

Biochar from phytoremediation plant residues: a review of its characteristics and potential applications

Phytoremediation is a cost-effective and eco-friendly plant-based approach promising technique to repair heavy metal-contaminated soils. However, a significant quantity of plant residues needs to be properly treated and utilized. Pyrolysis is an effective technology for converting residues to biochar, which can solve the problem and avoid secondary contamination. This paper reviews the generation, and physicochemical properties of biochar from phytoremediation residues, and its application in soil improvement, environmental remediation, and carbon sequestration. In spite of this, it is important to be aware of the potential toxicity of heavy metals in biochar and the environmental risks of biochar before applying it to practical applications. Future challenges in the production and application of residue-derived biochar include the rational selection of pyrolysis parameters and proper handling of potentially hazardous components in the biochar.

Recent progress in TiO2-biochar-based photocatalysts for water contaminants treatment: strategies to improve photocatalytic performance

Toxic organic pollutants in wastewater have seriously damaged human health and ecosystems. Photocatalytic degradation is a potential and efficient tactic for wastewater treatment. Among the entire carbon family, biochar has been developed for the adsorption of pollutants due to its large specific surface area, porous skeleton structure, and abundant surface functional groups. Hence, combining adsorption and photocatalytic decomposition, TiO2-biochar photocatalysts have received considerable attention and have been extensively studied. Owing to biochar's adsorption, more active sites and strong interactions between contaminants and photocatalysts can be achieved. The synergistic effect of biochar and TiO2 nanomaterials substantially improves the photocatalytic capacity for pollutant degradation. TiO2-biochar composites have numerous attractive properties and advantages, culminating in infinite applications. This review discusses the characteristics and preparation techniques of biochar, presents in situ and ex situ synthesis approaches of TiO2-biochar nanocomposites, explains the benefits of TiO2-biochar-based compounds for photocatalytic degradation, and emphasizes the strategies for enhancing the photocatalytic efficiency of TiO2-biochar-based photocatalysts. Finally, the main difficulties and future advancements of TiO2-biochar-based photocatalysis are highlighted. The review gives an exhaustive overview of recent progress in TiO2-biochar-based photocatalysts for organic contaminants removal and is expected to encourage the development of robust TiO2-biochar-based photocatalysts for sewage remediation and other environmentally friendly uses.

Toxic elements pollution risk as affected by various input sources in soils of greenhouses, kiwifruit orchards, cereal fields, and forest/grassland

Increasing food demand has led to more intensive farming, which threatens our ecosystem and human health due to toxic elements accumulation. This study aimed to estimate the vulnerability of different agricultural systems with unequal high fertilizer input practices regarding toxic element pollution in the greenhouse, kiwifruit orchard, cereal field, and forest/grassland. Soil samples were collected from 181 sites across Shaanxi Province, China, and analyzed for selected characteristics and toxic elements (As, Cd, Cr, Cu, Hg, Pb, and Zn). The contamination factor (CFx) represents the ratio of the measured value of the toxic element in the soil over the soil background values. The CFx values of all the toxic elements were above background values, while Cd and Hg contamination levels were more severe than those of Zn, Cu, As, Cr, and Pb. Kiwifruit orchards and greenhouse soils were contaminated with Cd, Hg, Cu, and Zn, but cereal fields and forest/grassland soils were contaminated with As, Cd, Hg, and Hg. Overall, the cumulative pollution load (PLI) of toxic elements indicated moderate contamination. The cumulative ecological risk (RI) results indicated that greenhouse (178.81) and forest/grassland (156.25) soils were at moderate ecological risks, whereas kiwifruit orchards (120.97) and cereal field (139.72) soils were at low ecological risks. According to a Pearson correlation analysis, Cd, Hg, Cu, and Zn were substantially linked with soil organic matter (SOM), total nitrogen (TN), total phosphorous (TP), and total potassium (TK). The primary sources of toxic elements were phosphate and potash fertilizers, manure, composts, and pesticides in a greenhouse, kiwifruit orchards, and cereal fields, whereas, in forest/grassland soils parent material and atmospheric deposition were the sources identified by positive matrix factorization (PMF). Furthermore, the partial least square structural equation model (PLS-SEM) demonstrated that agriculture inputs largely influenced toxic elements accumulation. We conclude that high fertilizer inputs in greenhouse soils should be considered carefully so that toxic element pollution may be minimized.

Roles and significance of chelating agents for potentially toxic elements (PTEs) phytoremediation in soil: A review

Phytoremediation is a biological remediation technique known for low-cost technology and environmentally friendly approach, which employs plants to extract, stabilise, and transform various compounds, such as potentially toxic elements (PTEs), in the soil or water. Recent developments in utilising chelating agents soil remediation have led to a renewed interest in chelate-induced phytoremediation. This review article summarises the roles of various chelating agents and the mechanisms of chelate-induced phytoremediation. This paper also discusses the recent findings on the impacts of chelating agents on PTEs uptake and plant growth and development in phytoremediation. It was found that the chelating agents have increased the rate of metal absorption and translocation up to 45% from roots to the aboveground plant parts during PTEs phytoremediation. Besides, it was also explored that the plants may experience some phytotoxicity after adding chelating agents to the soil. However, due to the leaching potential of synthetic chelating agents, the use of organic chelants have been explored to be used in PTEs phytoremediation. Finally, this paper also presents comprehensive insights on the significance of using chelating agents through SWOT analysis to discuss the advantages and limitations of chelate-induced phytoremediation.

Use of local waste for biochar production: Influence of feedstock and pyrolysis temperature on chromium removal from aqueous solutions

Sediment enrichment with biochar, a high-carbon material produced by the pyrolysis of biomass, is a promising remediation strategy for metal pollution. The metal immobilization capacity of biochar can be explained by its porous structure, surface functional groups, pH greater than 7, and cation exchange capacity. However, the effectiveness in reducing metal bioavailability depends on the physicochemical characteristics of the biochar, which are strongly associated with the process conditions and feedstock. The aims of this study were to analyze the effect of pyrolysis temperature on the properties of biochars derived from different locally available biomass materials, biochar potential to adsorb Cr, and biochar phytotoxicity in seed germination. Poultry litter (PL), maize straw, the macrophyte Juncus imbricatus, and phytoremediation wastes from the macrophyte previously exposed to Cr were pyrolyzed into biochar at 300 °C and 600 °C. The properties and capacity of biochar to remove Cr from the aqueous phase were determined. Finally, a germination assay was performed to evaluate biochar phytotoxicity. Biochar yield decreased with increasing pyrolysis temperature, whereas ash content and pH increased. Biochar C content and total surface area increased with temperature. Biochar Cr removal capacity improved under the highest temperature, reaching a maximum sorption value of 13.7 mg g^-1 Cr at 300 °C in PL biochar and of 42.6 mg g^-1 Cr at 600 °C in J. imbricatus biochar. Despite the comparatively high metal content in the biochar, the germination indices of all biochars produced at 600 °C were higher than 80%, suggesting no phytotoxicity. Considering the metal sorption capacity and the phytotoxicity, biochars produced from J. imbricatus, PL, and phytoremediation residues at 600 °C were suitable for use in the removal of Cr from water. Integr Environ Assess Manag 2023;19:717-725. © 2022 SETAC.

Carbon content determines the aggregation of biochar colloids from various feedstocks

The aggregation kinetics of biochar colloids (BCs) play a crucial role in the fate and transport of contaminants, as well as the carbon (C) cycle in the environment. However, the colloidal stability of BCs from various feedstocks is very limited. In this study, the critical coagulation concentration (CCC) of twelve standard biochars pyrolyzed from various feedstocks (municipal source, agricultural waste, herbaceous residue, and woody feedstock) at 550 °C and 700 °C were investigated, and the relationship between the physicochemical characteristics of biochar and the colloidal stability of BCs was further analyzed. The CCC of BCs in the NaCl solution followed the trend of municipal source < agricultural waste < herbaceous residue < woody feedstock, which was similar to the order of C content in biochar. The CCC of BCs showed a strong positive correlation with the C content of various biochars, especially pyrolyzed at a higher temperature of 700 °C. The BCs derived from lignin-rich feedstock (e.g., woody feedstock) had the highest colloidal stability, followed by cellulose-rich feedstock (e.g., agricultural waste and herbaceous residue). The BCs derived from organic matter-rich feedstock (municipal source) were easy to aggregate in the aqueous environment. This study quantitatively provides new insights into the relationship between BCs stability and biochar characteristics from various feedstocks, which is critical to assess biochar environmental behavior in aqueous environments.

Co-pyrolysis of sewage sludge as additive with phytoremediation residue on the fate of heavy metals and the carbon sequestration potential of derived biochar

Considering the characteristics of municipal sewage sludge (MS) and Sedum alfreddi L. (SA, a hyperaccumulator plant), we attempted to use MS to enhance the enrichment and stability of heavy metals (HMs) in pyrolysis residue during SA pyrolysis. The effects of pyrolysis temperature (400-800 °C) and co-pyrolysis on migration behavior, chemical speciation, long-term leaching toxicity of HMs, and the environmental risk and carbon sequestration potential of biochar were systematically investigated. Besides, thermodynamic equilibrium simulations were performed to study the transformation of HM compounds during pyrolysis. When the pyrolysis temperature increased from 400 °C to 800 °C, the unstable fractions (F1+F2) of Cd, Pb, Cu, and Cr in MS1SA3 800 had decreased to less than 6% and Zn to 20.4%, and long-term leachability of HMs decreased continuously. Meanwhile, biochar's ecological risk was reduced to a low level, while its carbon sequestration potential improved with little released HMs. Compared with SA pyrolysis alone, adding MS increased the relative residue content of Cd and Zn in biochar, whereas no apparent effect on Pb, Cu, and Cr, and the proportion of stable fractions (F3+F4) increased. Co-pyrolysis enhanced the carbon sequestration potential of biochar, attributed to the inherent minerals of MS. Equilibrium calculations showed that the influence of MS on the fate of HMs during SA pyrolysis is mainly attributed to its high sulfur content, while Si and Al preferentially combine with alkali metal (K)/alkaline earth metal (Ca) and then interact with Zn. The findings in this paper suggest that co-pyrolysis of MS as an additive with hyperaccumulator plants is a feasible proposal, and the co-pyrolysis biochar obtained at suitable temperatures has the potential for safe application.

Review of biochar production via crop residue pyrolysis: Development and perspectives

Worldwide surge in crop residue generation has necessitated developing strategies for their sustainable disposal. Pyrolysis has been widely adopted to convert crop residue into biochar with bio-oil and gas being two co-products. The review adopts a whole system philosophy and systematically summarises up-to-date knowledge of crop residue pyrolysis processes, influential factors, and biochar applications. Essential process design tools for biochar production e.g., cost-benefit analysis, life cycle assessment, and machine learning methods are also reviewed, which has often been overlooked in prior reviews. Important aspects include (a) correlating techno-economics of biochar production with crop residue compositions, (b) process operating conditions and management strategies, (c) biochar applications including soil amendment, fuel displacement, catalytic usage, etc., (d) data-driven modelling techniques, (e) properties of biochar, and (f) climate change mitigation. Overall, the review will support the development of application-oriented process pipelines for crop residue-based biochar.

Improving biochar properties by co-pyrolysis of pig manure with bio-invasive weed for use as the soil amendment

Over recent years, pyrolysis has grown into a mature technology with added value for producing soil improvers. Further innovations of this technology lie in developing tailor-made products from specific feedstocks (or mixtures thereof) in combination with adjusted mixing ratio-temperature regimes. In this context, co-pyrolysis of pig manure (PM) and the invasive plant Japanese knotweed (JK) at different mixture ratios (w/w) of 3:1 (P3J1), 1:1 (P1J1), and 1:3 (P1J3) and varying temperatures (400-700 °C) was studied to address the low carbon properties and heavy metals (HMs) risks of manure-derive biochars and beneficially ameliorate the bio-invasion situation by creating value from the plant biomass. Co-pyrolysis of PM with JK increased by nearly 1.5 folds the fixed carbon contents in the combined feedstock biochars obtained at 600 °C compared with PM-derived biochar alone, and all combined feedstock biochars met the requirements for soil improvement and carbon sequestration. The total HMs in PM biochars were significantly reduced by adding JK. The combined feedstock biochar P1J1 generated at 600 °C was the most effective in transforming Cu and Zn into more stable forms, accordingly reducing the associated environmental risk of heavy metal leaching from the biochar. In addition, the accumulation of macronutrients can be an added benefit of the co-pyrolysis process, and P1J1-600 was also the biochar that retained the most nutrients (P, Ca, Mg, and K).

Phytoremediation plants (ramie) and steel smelting wastes for calcium silicate coated-nZVI/biochar production: Environmental risk assessment and efficient As(V) removal mechanisms

Arsenic is one of the most common and harmful pollutants in environment throughout the world, especially in aqueous solutions. In this study, two kinds of industrial solid wastes (Oxide scale (OS) and Blast furnace slag (BFS)) and one kind of phytoremediation plant waste (Ramie stalk) were used to prepare an environmentally friendly, low-cost, and efficient calcium silicate coated nano zero-valent iron (nZVI)/biochar composite (BOS) for As(V) adsorption. The potential environmental risks of BOS and their effects on removal of arsenic ions from aqueous media were investigated. The adsorption mechanism was explored and discussed based on XRD, SEM-EDS, XPS, etc. The results suggested that the environmental risk and heavy metals toxicity in BOS by co-pyrolysis were significantly reduced compared to the original materials, and no additional contaminant was observed in the subsequent experiments. Simultaneously, the BOS showed excellent As(V) removal capacity (>99%) and regenerative properties. The As(V) removal mechanisms are mainly ascribed to the complexation and co-precipitation between Fe and As, and the hydrogen bond between CO functional group of BOS and As. The mechanism of enhanced nZVI activity for As(V) removal was revealed. A protective layer of Ca₂SiO₄ was formed on the surface of nZVI during the co-pyrolysis process to prevent the passivation of nZVI. During the reaction process, the Ca₂SiO₄ covering the nZVI surface would be continuously detached to expose the fresh surface of nZVI, thus providing more redox activity and adsorption sites. This study provides a new way to treat and recycle industrial steel solid wastes and phytoremediation plant wastes, and the produced calcium silicate coated-nZVI/biochar composite is proposed to be a very promising material for practical remediation of As(V)-contaminated water bodies.

A Mini Review on Persulfate Activation by Sustainable Biochar for the Removal of Antibiotics

Antibiotic contamination in water bodies poses ecological risks to aquatic organisms and humans and is a global environmental issue. Persulfate-based advanced oxidation processes (PS-AOPs) are efficient for the removal of antibiotics. Sustainable biochar materials have emerged as potential candidates as persulfates (Peroxymonosulfate (PMS) and Peroxydisulfate (PDS)) activation catalysts to degrade antibiotics. In this review, the feasibility of pristine biochar and modified biochar (non-metal heteroatom-doped biochar and metal-loaded biochar) for the removal of antibiotics in PS-AOPs is evaluated through a critical analysis of recent research. The removal performances of biochar materials, the underlying mechanisms, and active sites involved in the reactions are studied. Lastly, sustainability considerations for future biochar research, including Sustainable Development Goals, technical feasibility, toxicity assessment, economic and life cycle assessment, are discussed to promote the large-scale application of biochar/PS technology. This is in line with the global trends in ensuring sustainable production.

Closed-loop evaluation on potential of three oil crops in remediation of Cd-contaminated soil

Cd-contaminated farmlands threaten food security and safety by inhibiting crop growth and Cd accumulating in edible parts. Phytoremediation is a promising option to remove Cd from farmland soil. An ideal option is to remediate Cd and produce crops simultaneously on the contaminated soil. Therefore, we chose widely planted oil crops (soybean, sunflower and rape) as experimental materials, cultured in pots filled with soils contaminated with different concentrations (10, 20, 50, and 100 mg kg^-1) Cd till harvest, and then took a closed-loop method to evaluate the remediation potential of the three oil crops, including the remediating ability, yield, and quality of seeds and environmental risk of pyrolytic biochar. The results show that the order of Cd accumulation capacity in the three oil crops was sunflower > rape > soybean. The yield and quality of the three oil crops were decreased by being treated with different concentrations of Cd. In addition, the order for a decreased degree in yield of the three oil crops was sunflower < rape < soybean, and the order for a decreased degree in protein and fat content was sunflower < soybean < rape. The potential risk of seeds of the three oil crops as food/feed was sunflower/soybean < soybean/sunflower < rape. After pyrolysis of harvested three oil crops, the order for leaching toxicity/leaching potential was sunflower-biochar < soybean-biochar/rape-biochar < rape-biochar/soybean-biochar. All three oil crops could remediate Cd-contaminated soils, and their seeds could generate economic value. Closed-loop evaluation of sunflower proved it might be a good option for removing Cd from farmland soil.

Release characteristics of phosphate from ball-milled biochar and its potential effects on plant growth

Ball-milled biochar could potentially supply phosphorus, an essential element for plant growth. To realize resource reuse and phosphorus recovery, three feedstocks (rice straw, distillers grains, and Eupatorium adenophorum) were used to prepare ball-milled biochar to evaluate its release characteristics of phosphorus and potential effects on germination and growth. The results showed that the phosphate release performance of ball-milled distillers grains biochar (DM) at 300 and 600 °C was better than that of other biochars ball-milled for 12 h. The DM prepared at 600 °C and incubated for 12 (DM-12) or 24 h (DM-24) had the best phosphate release capacity. The solution with pH 3.0 was beneficial to the release of phosphate from DM-12. The pseudo-second-order model could better fit the phosphate release of DM-12. A germination and seedling growth experiment suggested that adding 2.5 wt% DM-12 was beneficial to the height of mung beans. This study shows that DM-12 can be used as a slow-release fertilizer for the growth of mung beans, which provides a new way for resource utilization of distillers grains and phosphorus-rich biochar.

Co-pyrolysis of sewage sludge and Ca(H2PO4)2: heavy metal stabilization, mechanism, and toxic leaching

The presence of unstable heavy metals in sewage sludge (SS) restricts its resource utilization. In this study, Ca(H₂PO₄)₂ and SS were co-pyrolyzed to produce biochar, which contained relatively stable heavy metals. X-ray diffraction spectroscopy, Fourier transform infrared spectroscopy, and inductively coupled plasma atomic emission techniques were used to analyze the physical and chemical properties and heavy metal content of the biochar. The results indicated that co-pyrolysis of SS with Ca(H₂PO₄)₂ resulted in the production of more stable heavy metals in the SS. The optimal co-pyrolysis conditions were a blended ratio of 15% Ca(H₂PO₄)₂, 650 °C final temperature, 15 °C min^-1, and 60 min retention time. The potential stabilization mechanisms of heavy metals were as follows: (1) organic decomposition and moisture (sourced from Ca(H₂PO₄)₂ decomposition) evaporation resulted in greater biochar surface porosity; (2) phosphorous substances were complexed with heavy metals to form metal phosphates; and (3) the mixture reactions among inorganic substances, pyrolysis products of organics, and heavy metals resulted in the formation of highly aromatic metallic compounds. Additionally, the potential environmental risks posed by the heavy metals decreased from 65.73 (in SS) to 4.39 (in biochar derived from co-pyrolysis of SS and 15% of Ca(H₂PO₄)₂). This study reports on a good approach for the disposal of SS and the reduction of its environmental risk.

Metal-rich hyperaccumulator-derived biochar as an efficient persulfate activator: Role of intrinsic metals (Fe, Mn and Zn) in regulating characteristics, performance and reaction mechanisms

Biochar has been widely used in advanced oxidation processes (AOPs) for the decomposition of organic contaminants. However, the role of intrinsic metals in hyperaccumulator biomass in the physico-chemical properties and performance of peroxodisulfate (PDS) activation by biochar is still unclear. This work employed hyperaccumulator biomass containing Fe, Mn and Zn, respectively. Result showed that as the pyrolysis temperature of the biochar increased, Fe was gradually reduced to iron oxide and Fe⁰, and Zn was reduced and volatilized; however, Mn remained in biochar in the form of MnS and CaMnO₃ with high valence states. These thermochemical behaviors of intrinsic metals also facilitated graphitized structure growth and pore development (for Zn) and persistent free radicals (PFRs) generation (for Mn and Zn) in biochar, and these processes were crucial for imidacloprid degradation in biochar/PDS systems. Moreover, Fe/Zn@PB9/PDS showed better imidacloprid degradation performance, while Mn species in Mn@PB were catalytically inert. In addition, the radical pathway depending on·SO₄^- and·OH was the dominant pathway for imidacloprid degradation in the Fe@PB9/PDS systems, while the·O₂^--mediated ¹O₂ pathway and ¹O₂-based nonradical pathway contributed more in the Zn@PB9/PDS systems. These results reveal the role of intrinsic metals in biochar-based catalysts and provide a reference for the preparation of green and efficient hyperaccumulator-derived biochar catalysts for AOPs.

Effect of temperature on PTEs deportment and ecological risks of the biochars obtained from sewage sludge

Sewage sludge-derived biochars (SSBCs) were obtained at temperatures of 300, 500, and 700 °C to investigate the potentially toxic elements (PTEs) behaviors and assess the environmental acceptability for the possible application in the environment. Results indicated that PTEs exhibited diversely in the distribution of chemical speciation, while all elements tended to be immobilized in biochar matrix and the total amount elevated during the pyrolysis. The risk assessment of biochars implied a low degree of environmental risk for the utilization of SSBCs prepared at high temperatures. In addition, higher pyrolysis temperature alleviated the inhibition on the early seedling growth of Triticum aestivum L., with root elongation more sensitive to the biochar addition. PTEs, especially Cr, contributed much to the phytotoxicity of biochars as revealed by the principle component analysis (PCA) and leaner correlation analysis. Findings from this work illustrated that SSBCs prepared at higher temperatures might be more conductive to a wide range of applications with acceptable environmental risk.

Comfrey (Symphytum spp.) as a feed supplement in pig nutrition contributes to regional resource cycles

In smallholder agriculture, the fast-growing and perennial accumulator plant comfrey (Symphytum spp.) was used to supply pigs with protein and minerals. Comfrey leaves show similar values in dry matter as soybean or blue lupine in crude protein content, but much higher levels of calcium and phosphorus. However, in terms of increased efficiency in animal husbandry, comfrey has been displaced by mainly soybean and cereals. Due to its profile of macro- and micronutrients the use of comfrey could have the potential to re-establish local resource cycles and help remediate over-fertilized soils. The aim of the study was to evaluate whether a modern pig breed accepts a continuous feed supplement of dried comfrey leaves. After an initial adaptation period post-weaning, German Landrace piglets were subjected to either a standard control diet or a diet supplemented with 15% dried comfrey leaves for 4 weeks. Body weight was reduced in comfrey-supplemented piglets compared to controls, which might be attributed to reduced palatability in the experimental setting. Nevertheless, comfrey-supplemented piglets exhibited adequate bone mineralization and intestinal integrity. The microbiome profile in feces and digesta revealed higher diversity in comfrey-supplemented piglets compared to controls, with pronounced effects on the abundances of Treponema and Prevotella. This may be due to described bio-positive components of the comfrey plant, as data suggest that the use of comfrey leaves may promote intestinal health. Digestive tract phosphorus levels were reduced in piglets receiving comfrey supplementation, which may ultimately affect phosphorus levels in manure. Results indicate that comfrey leaves could serve as a feed component in integrated agricultural systems to establish regional nutrient cycles. The trial provides a basis for further work on comfrey as a regionally grown protein source and effective replacement for rock mineral supplements.

Distribution, behaviour, bioavailability and remediation of poly- and per-fluoroalkyl substances (PFAS) in solid biowastes and biowaste-treated soil

Aqueous film-forming foam, used in firefighting, and biowastes, including biosolids, animal and poultry manures, and composts, provide a major source of poly- and perfluoroalkyl substances (PFAS) input to soil. Large amounts of biowastes are added to soil as a source of nutrients and carbon. They also are added as soil amendments to improve soil health and crop productivity. Plant uptake of PFAS through soil application of biowastes is a pathway for animal and human exposure to PFAS. The complexity of PFAS mixtures, and their chemical and thermal stability, make remediation of PFAS in both solid and aqueous matrices challenging. Remediation of PFAS in biowastes, as well as soils treated with these biowastes, can be achieved through preventing and decreasing the concentration of PFAS in biowaste sources (i.e., prevention through source control), mobilization of PFAS in contaminated soil and subsequent removal through leaching (i.e., soil washing) and plant uptake (i.e., phytoremediation), sorption of PFAS, thereby decreasing their mobility and bioavailability (i.e., immobilization), and complete removal through thermal and chemical oxidation (i.e., destruction). In this review, the distribution, bioavailability, and remediation of PFAS in soil receiving solid biowastes, which include biosolids, composts, and manure, are presented.

Migration and transformation of heavy metals in hyperaccumulators during the thermal treatment: a review

The pollution of heavy metals (HMs) in the soil has become one of the important factors affecting the national environment and human health. Phytoremediation, as a technology to deal with HM pollution in soil, has been extensively studied and applied due to its sustainability and environmental friendliness. However, hyperaccumulators polluted by HMs need to be properly treated to avoid secondary pollution to the environment. This paper reviews the migration and transformation of HMs during the incineration, pyrolysis, gasification, and hydrothermal treatment of hyperaccumulators; comprehensively evaluates the advantages and disadvantages of each technology in the treatment of HM-enriched hyperaccumulators; and analyzes the current development status and unsolved problems in detail for each technology. Generally speaking, thermal treatment technology can fix most of the HMs of exchangeable fraction in biochar, reducing its bioavailability and biotoxicity. In addition, the application direction and research focus of the target product are discussed, and it is clarified that in the future, it is necessary to further optimize the reaction conditions and explore the mechanism of HM immobilization to maximize the immobilization of HMs and improve the quality and output of the target product.

Double-edged effects of polyvinyl chloride addition on heavy metal separation and biochar production during pyrolysis of Cd/Zn hyperaccumulator

Pyrolysis is a promising technique to achieve the sustainable utilization of heavy metal hyperaccumulator derived from phytoremediation of contaminated soils. To investigate the feasibility of synergistic treatment of hyperaccumulator and plastic waste (i.e. polyvinyl chloride, PVC), co-pyrolysis of Sedum alfredii and different mass percentages of PVC (5-25 wt%) was conducted at 300-900 °C in the present study. High pyrolysis temperature and low PVC addition amount (5 wt%) effectively promoted the volatilization of Cd and Zn from S. alfredii, while high PVC addition amount (15 wt% and 25 wt%) caused a significant suppression effect at insufficient pyrolysis temperatures. After PVC addition, the yields of biochar increased by 5.18-37.19% as compared with the theoretical values. However, the concentrations of Cd and Zn leached from biochar significantly elevated with increasing PVC addition amount, indicating that the addition of PVC improved the mobility of Cd and Zn in biochar. Moreover, S. alfredii derived biochars showed considerable sorption capacity for Cd (87.6-198.3 mg/g). These results imply that the addition of PVC has double-edged effects on heavy metal separation and biochar production during pyrolysis of Cd/Zn hyperaccumulator, and low PVC addition amount and sufficient pyrolysis temperature are beneficial for the further utilization of biochar.

Effect of crop straw biochars on the remediation of Cd-contaminated farmland soil by hyperaccumulator Bidens pilosa L

Cadmium (Cd) due to its strong toxicity and high mobility, which poses a considerable threat to soil environment and human health, has aroused widespread concern. Biochar has been used for remediating Cd-contaminated soil recently, however this method has the risk of fixed-Cd re-release. Phytoremediation can make up for its shortcoming. In this study, a pot experiment was carried out, where Bidens pilosa L. (B.pilosa) was as the tested plant and biochars (maize straw biochar and wheat straw biochar with two particle sizes) were as amendments. The mechanism of how biochars promoted B.pilosa Cd accumulation in Cd-contaminated farmland soil was explored. Results showed that the application of 5% wheat straw fine biochar (WF), wheat straw coarse biochar (WC), maize straw fine biochar (MF) and maize straw coarse biochar (MC) increased the total Cd accumulation of B.pilosa to 251.57%, 217.41%, 321.64% and 349.66%, respectively. Biochars amendment significantly promoted B.pilosa growth and increased Cd accumulation by improving soil physical properties, nutrient levels (available nitrogen, available phosphorus (AP), available potassium (AK) and organic matter (OM)) and microbial activity, and changing the nutrients distribution in B.pilosa organs although tissues although DTPA-Cd reduced to some extent. The effect of MF on AP increase was better than MC, while the effect of WF on AK increase was better than WC. Fine-particle was superior to coarse-particle in increasing B.pilosa biomass of aboveground, OM and microbial activity in soil. The changes of N, P and K concentrations in B.pilsosa roots, stems and leaves were closely related to the changes of AN, AP and AK in soil after biochars application. The results indicated that the combination of straw biochars and hyperaccumulators had the synergistic effect. This study can provide data support and meaningful reference values for remediating actual Cd-contaminated soil.

Co-pyrolysis of sewage sludge and organic fractions of municipal solid waste: Synergistic effects on biochar properties and the environmental risk of heavy metals

The introduction of heavy metal-free biomass into the sewage sludge (SS) pyrolysis can effectively improve the biochar properties and reduce the bioavailability and toxicity of heavy metals (HMs) in blended biochar. Herein, this study aimed to understand the biochar properties and associated environmental risks of HMs, by comparing the residual contents from the co-pyrolysis of SS with various organic fractions of municipal solid waste (OFMSW) at 550 °C and pyrolysis alone at different temperatures between 350 and 750 °C. The results indicated that, compared with SS pyrolysis alone, co-pyrolysis of SS with various OFMSW (except PVC) lead to lower biochar yields but with higher pH values (increased between 21.80% and 31.70%) and carbon contents (raised between 33.45% and 48.22%) in blended biochars, and the chemical speciation analysis suggested that co-pyrolysis further promoted the HMs transformation into more stable forms which significantly reduce the associated environmental risk of HMs in the blended biochars (the values of RI lower than 55.80). The addition of PVC, however, impeded biochar properties and compromised HMs immobilization during SS pyrolysis.

Transformation behaviors and environmental risk assessment of heavy metals during resource recovery from Sedum plumbizincicola via hydrothermal liquefaction

Environmentally sound disposal of hyperaccumulator harvests is of critical importance to industrialization of phytoremediation. Herein, transformation behaviors and environmental risk of heavy metals were comprehensively examined during subcritical hydrothermal liquefaction of Sedum plumbizincicola. It is concluded that low temperature liquefaction favored resource recovery of heavy oil and hydrochars in terms of higher energy density, improved carbon sequestration and less energy consumption. Heavy metals were mainly distributed into hydrochars and water soluble phase with less than 10% in heavy oil. All metal elements except As could be accumulated in hydrochars by extending reaction time, whereas more than 96% of As was redistributed into water soluble phase. Prolonged liquefaction time facilitated immobilization of Cd, Cr and As in hydrochars, but fast liquefaction favored Pb stabilization. Liquefaction significantly reduced environmental risk level of Cd, Zn and As, but may mobilize Pb and Mn, especially for Mn to very high risk level at 240 ºC. High temperature with long reaction time tended to inhibit leaching rate of Mn, whereas low liquefaction temperature with short reaction time prevented the leaching of Zn and As from hydrochars. Overall, these findings are essential for downstream upgrading of heavy oil and metals recovery from hydrochars.

Effects of UV-modified biochar derived from phytoremediation residue on Cd bioavailability and uptake in Coriandrum sativum L. in a Cd-contaminated soil

Biochar has been applied widely as an amendment in the remediation of contaminated soil to immobilize the heavy metals. However, the role of ultraviolet (UV) irradiation modified biochar derived from the residues of phytoremediation plants in the contaminated soil not investigated yet. In this study, the UV-modified biochars were obtained from Brassica napus L. and Lolium perenne L. by pyrolysis at 600 °C. They were applied in a pot experiment to investigate their effect on Cd bioavailability and uptake in Coriandrum sativum L. in a Cd-contaminated soil at four addition rate (0%, 0.2%, 0.4%, and 0.6%). The results showed that the Cd was effectively stabilized in the biochar with environmentally acceptable leaching toxicity. The specific surface area and carboxyl functional group of biochar were greatly increased after UV modification. The application of biochar progressively increased the soil pH and electrical conductivity (EC). Furthermore, the CaCl₂-extractable Cd was significantly reduced by 18.4-51.4% with biochar amendments. The concentration of Cd in shoots and roots was significantly reduced by biochars. In conclusion, the UV-modified biochar obtained from phytoremediation residue could effectively deal with hazardous waste and repair Cd-contaminated soil.

A review on the thermal treatment of heavy metal hyperaccumulator: Fates of heavy metals and generation of products

Phytoremediation is perceived as a promising technique for remediation of heavy metal (HM) contaminated soils, while the harvested HM-enriched hyperaccumulator biomass should be appropriately disposed. Recently, various thermal treatments of hyperaccumulator have drawn increasing attention. After thermal treatment, the hyperaccumulator was converted to bio-oil, bio-gas, biochar, or ash in accordance with the corresponding conditions, and the HMs were separated, immobilized, or trapped. The migration and transformation of HMs during the thermochemical conversion processes are critical for the safe disposal and further utilization of HM hyperaccumulator. This paper provides a systematic review on the migration and transformation of typical HMs (Cd, Ni, Mn, As, and Zn) in hyperaccumulator during various thermochemical conversion processes, and special emphasis is given to the production and application of targeted products (e.g. biochar, hydrochar, bio-oil, and syngas). Besides, future challenges and perspectives in the thermal treatment of hyperaccumulator are presented as well. The distribution and speciation of HMs were influenced by thermal technique type and reaction conditions, thereby affecting the utilization of the derived products. This review suggests that speciation and availability of HMs in hyperaccumulator are tunable by varying treatment techniques and reaction conditions. This information should be useful for the selective conversion of hyperaccumulator into green and valuable products.

Comparison of ashing and pyrolysis treatment on cadmium/zinc hyperaccumulator plant: Effects on bioavailability and metal speciation in solid residues and risk assessment

Phytoremediation of metal(loid)s contaminated sites is widely used, while there is scarce of investigation on the metal-enriched biomass waste safely disposal which resulted in risks of causing secondary pollution to the soil and water bodies and even to human health. Thus, this study compared the effects of ashing and pyrolysis treatments on cadmium (Cd) and zinc (Zn) hyperaccumulation plant Sedum plumbizincicola. Chemical speciation, the Toxicity Characteristic Leaching Procedure (TCLP), and diethylenetriamine pentaacetic acid (DTPA) extraction were employed to characterize the bioavailability and leachability of Cd and Zn in the solid residues after pyrolysis and ashing. The risk assessment code (RAC) and potential ecological risk index (RI) were subsequently used to evaluate the risk of the solid residues to the environment. The results showed that both ashing and pyrolysis treatments could transform the bioavailable Cd and Zn in S. plumbizincicola into a more stable form, and the higher the temperature the greater the stablility. Pyrolysis converted a maximum of 80.0% of Cd and 70.3% of Zn in S. plumbizincicola to the oxidisable and residual fractions, compared with ashing which achieved only a ∼42% reduction. The pyrolysis process minimised the risk level of Cd and Zn to the environment based on the RAC and RI assessments. The results of the TCLP test, and DTPA extraction confirmed that the leaching rate and the bioavailable portion of Cd and Zn in the biochars produced by pyrolysis were invariably significantly (p < 0.05) lower than the solid residues produced by ashing, and reached the lowest at 650 °C. In other words, pyrolysis was better than ashing for thermal treatment of the metal-enriched hyperaccumulator plant, in view of minimising the bioavailability and leachability of Cd and Zn from the solid residues to the environment. This study provides fundamental data on the choice of treatments for the disposal of metal-enriched plant biomass.

Effect of cornstalk biochar on phytoremediation of Cd-contaminated soil by Beta vulgaris var. cicla L

Cadmium (Cd) contamination is the most common and extensive heavy metal pollution in the farmland of China. Phytoremediation is considered as a promising measure for Cd-contaminated soil remediation, but the remediation efficiency still needs to be enhanced. Biochar as an effective amendment medium is widely manufactured and studied for the soil remediation of heavy metals. In this study, a greenhouse pot trial was conducted to investigate the effects of cornstalk biochar on Cd accumulation of Beta vulgaris var. cicla L. (Beta vulgaris) in Cd contaminated soil. The Cd availability, speciation and nutrients in soil, biomass and Cd chemical forms in the Beta vulgaris root were studied to explore the mechanism that how the cornstalk biochar promoted Cd accumulation in Beta vulgaris. Biochar amendment reduced the DTPA-extractable Cd concentration and stimulated the growth of root. Compared to the Beta vulgaris without biochar treatment, the results of 5% biochar amendment showed that the root dry weight of Beta vulgaris increased to 267%, Cd accumulation in Beta vulgaris increased to 206% and the Cd concentration in leaves and roots increased by 36% and 52%, respectively. Additionally, after 5% biochar was applied to soil, the total content of organic matter-bound Cd and residual Cd increased by 38%, while the content of Fe-Mn oxides-bound Cd decreased by 40%. Meanwhile, Cd may mainly bind to the root cell wall and the ratio of NaCl-extracted Cd to HAc-extracted Cd increased to 166% with 5% biochar amendment. According to our study, Cd in soil can be removed by Beta vulgaris and phytoremediation efficiency can be improved with biochar amendment. The combination of phytoremediation and biochar amendment is a promising strategy for the Cd-contaminated soil remediation.

A critical review of the production and advanced utilization of biochar via selective pyrolysis of lignocellulosic biomass

Biochar is a carbon-rich product obtained from the thermo-chemical conversion of biomass. Studying the evolution properties of biochar by in-situ modification or post-modification is of great significance for improving the utilisation value of lignocellulosic biomass. In this paper, the production methods of biochar are reviewed. The effects of the biomass feedstock characteristics, production processes, reaction conditions (temperature, heating rate, etc.) as well as in-situ activation, heteroatomic doping, and functional group modification on the physical and chemical properties of biochar are compared. Based on its unique physicochemical properties, recent research advances with respect to the use of biochar in pollutant adsorbents, catalysts, and energy storage are reviewed. The relationship between biochar structure and its application are also revealed. It is suggested that a more effective control of biochar structure and its corresponding properties should be further investigated to develop a variety of biochar for targeted applications.

Phytoremediation of heavy metals under an oil crop rotation and treatment of biochar from contaminated biomass for safe use

The disposal of contaminated plants limits the use of phytoremediation. Therefore, the disposal of contaminated sunflower was investigated after determining the phytoremediation of heavy metals under an oil crop rotation of sunflower (Helianthus annuus L)-sesame (Sesamum indicum L.). In the field experiment, the extraction efficiency of sunflower-sesame rotation was 0.07% for lead (Pb); 1.37% for zinc (Zn); 1.10% for copper (Cu); and 6.12% for cadmium (Cd). Contaminated sunflower stems were pyrolyzed at different temperature. The biochar produced at 300 °C was extracted in a two-step process (acid-extraction from biochar and metals precipitation in alkaline condition). At pH = 1, 65.67% of the Cd and much potassium (K) were extracted. After acid-extraction, adjust the pH of filtrate to 10, metals were precipitated and then separated from the K-enriched solution. Therefore, pyrolysis can process contaminated residues, and the biochar extracts can be reutilized as fertilizer to off-site crop production. Thus, an oil crop-rotation system, in addition to creating economic benefits, can be used by local farmers in contaminated soils.

Pyrolysis of various phytoremediation residues for biochars: Chemical forms and environmental risk of Cd in biochar

Various phytoremediation residues (PMRs), including Brassica napus L. (BN), Pennisetum sinese (PS) and Lolium perenne L.(LP), were pyrolyzed at 400, 500, 600 and 700 °C, respectively. A series of sequential and single extractions were employed to analyze the chemical speciation and potential environmental risk of Cadmium (Cd) in different phytoremediation residues-derived biochars (PMBs). The results showed that the exchangeable Cd fraction decreased but the residual Cd fraction increased, indicating the inhibition of bioavailability of Cd and low potential ecological risk index of PMBs. When the temperature was over 600 °C, the Cd in biochar was acceptable to the environment and the leaching concentration of Cd extracted by the three extraction methods (distilled water, SPLP and TCLP) were all under the standard limit. Findings from this study illustrated that the treatment of pyrolysis was feasible for the three kinds of PMRs at 600 °C with acceptable environment risk.

Effect of pyrolysis temperature on characteristics, chemical speciation and environmental risk of Cr, Mn, Cu, and Zn in biochars derived from pig manure

The comprehensive analysis of environmental risk for heavy metals in pig manure was essential for optimization of pyrolysis conditions and scientific utilization of pig manure biochars as soil amendment. However, in previous studies, the selected pyrolysis temperature points were limited and temperature interval was large, it's was difficult to accurately verify the effect of pyrolysis temperature on chemical speciation and environmental risk of heavy metals. Therefore, in this study, pig manure was pyrolyzed at 300-700 °C with a small interval of 50 °C to study the effect of pyrolysis temperature on characteristics and environmental risk of Cr, Mn, Cu and Zn in pig manure biochar. Results indicated that the characteristics of biochars (>500 °C) were relatively stable. The biochar obtained at 700 °C exhibited the largest surface area (8.28 m² g^-1) and pore volume (25.17 m³ kg^-1), secondly is the biochar derived at 500 °C. The total percentages of exchangeable and acid fraction and reducible fraction decreased from 16.98% to 9.43% for Cr, 85.60% to 65.55% for Mn, 57.26% to 10.61% for Cu, 37.90% to 13.78% for Zn, respectively, suggesting that exchangeable and acid fraction and reducible fraction of Cr, Mn, Cu and Zn in pig manure were transformed into oxidizable and residual fractions after pyrolysis. The leaching rates, risk assessment code and potential ecological risk index values significantly decreased after pyrolysis and presented lower value at 500 and 700 °C. Biochars derived at 300-700 °C conditions posed no phytotoxicity with germination index >80%. Correlation analyses revealed that larger surface area, pore volume and pH values of biochars may help to immobilize heavy metals and reduce bioavailability. These findings demonstrated that bioavailability and toxicity of Cr, Mn, Cu and Zn in pig manure biochar were greatly reduced after pyrolysis and the optimum temperature was 500 °C considering energy cost.

Environmental risk assessment in livestock manure derived biochars

Livestock-manure-derived biochar is one of major products obtained from the pyrolysis of livestock manure. This study quantitatively assesses the pollution level and ecological risks associated with heavy metals in livestock manure and the biochar produced by its pyrolysis. The geo-accumulation index (GAI) values of heavy metals in livestock manure were significantly decreased (P < 0.05) and indicated to be at the grade of uncontaminated expected for Zn in pig-manure-derived biochar (PMB, 0.94, 800 °C) via pyrolysis. Therefore, Zn should be paid more attention in PMB. The risk factors (E r i ) result shows that heavy metals in biochars were significantly decreased (P < 0.05) with increasing pyrolysis temperature. Potential ecological risk index values revealed that the integrated risks from the heavy metals were significantly decreased (P < 0.05) after pyrolysis. Similarly, the risk assessment code values indicated that the risks from the heavy metals in livestock-manure-derived biochars were significantly decreased (P < 0.05) after pyrolysis. In summary, pyrolysis represents an effective treatment method for livestock manure and can provide an effective method to reduce the risks of environmental pollution.

Effect of potentially toxic metals (PTMs) on the thermal decomposition of phytoremediation plant wastes: Thermokinetic and gas evolution analysis by TG-DTG-MS

Thermal conversion is a promising approach for the disposal of phytoremediation biowastes. The thermal characteristics of potherb mustard biowaste contaminated by potentially toxic metals were studied in this work. Thermogravimetric (TG) and derivative thermogravimetric (DTG) analyses of four feedstocks with different Cd/Zn content were conducted in an inert environment. Evolved gases were identified online by mass spectrometry (MS) coupled with TG. The TG-DTG profiles of the four feedstocks presented similar decomposition stages, whereas the samples with low Cd/Zn had a faster degradation rate, suggesting that the presence of Cd/Zn had little effect on thermal decomposition mechanism. The maximum evolution of H₂ and CH₄ occurred at 500-650 °C, while CO was released at a low temperature. PTMs could catalyze the H₂ production during the pyrolysis of biowaste. This investigation revealed the effects of different Cd/Zn contents on thermal conversion, providing a practical reference for the thermal treatment of phytoremediation biowastes.

Short-term leachability of salts from Atriplex-derived biochars

Disposal of concentrate from brackish water desalination is limited by concerns of salt leaching into fresh water sources. Prevention of salt from leaching can greatly increase desalination concentrate management costs, as there are few options to recover those costs. An option for concentrate disposal is to grow halophyte crops using the concentrate as irrigation water. The purpose of this study was to determine how much of the Ca, Mg, Na, and K taken up by Atriplex species during growth can be stabilized by pyrolysis, thus preventing those cations from returning into the soil solution after harvest. Results show that salts in the biomass are concentrated into the biochars and that pyrolysis conditions affect the amount of salt that can be leached with water. Ca was the most retained of the cations: 100% and 94% were retained in A. canescens biochars and A. lentiformis 600 °C biochar, respectively. Mg retention was similar: 100% and 93% retained for A. canescens and A. lentiformis 600 °C biochars, respectively. Increasing pyrolysis temperature increased the retention of Ca and Mg; trends for K and Na were inconsistent. Soil application of these biochars will still be problematic due to low retention of Na against short-term leaching, thus limiting the potential for on-site sequestration to enable land utilization for crop production. With more understanding of the salt formation and retention mechanisms within biomass, higher amounts of salt might be sequestered longer, while taking advantage of the carbon sequestration and soil quality benefits of biochars.

Foliar litters: Sources of contaminants in phytoremediation sites by returning potentially toxic metals (PTMs) back to soils

Phytoremediation is regarded as one of the most cost-effective and environmentally friendly strategies for potentially toxic metals (PTMs) contaminated soil remediation. However, uncertainties still existed about the contribution of foliar litter on metal accumulation and mobility in phytoremediation sites. Thus, in this study, fallen leaves, decomposed leaves, and soils at different depths (i.e., 0-5 and 5-10 cm) were collected from a phytoremediation site near a Zn smelter factory. Metals content and mobility were evaluated. Results indicated that upper-layer soils (0-5 cm) were higher in the electrical conductivity (EC) and soil organic matter (SOM) content than the deeper-layer soils (5-10 cm). However, the pH was relatively lower in the upper-layer soils. Fallen leaves were sources of metals in the phytoremediation site, and significantly high amounts of Cd (16.08 ± 0.21 mg kg^-1) and Zn (1130.30 ± 60.10 mg kg^-1) were found in the decomposed leaves. Metals in the upper-layer soils demonstrated higher accumulation and mobility than the deeper-layer soils. Moreover, the accumulated metals in leaves would gradually return to the soil as the contents of extractable metals increased with the rising decomposition degree of leaves (i.e., cold-water extraction < 80 °C hot water extraction < 1 M HCl extraction). Results from this research are helpful for the guidance of phytoremediation site management.