Effect of lychee biochar on the remediation of heavy metal-contaminated soil using sunflower: A field experiment

Enhancing remediation efficiency of hyperaccumulators through earthworm addition: Evidence from a pot study on cadmium-contaminated soil

Soil cadmium (Cd) contamination is an urgent environmental problem, which endangers human health through the food chain. Bioremediation attracted extensive attention around the world due to the high cost-efficiency. However, the remediation efficiency of different plant and earthworm species of soil Cd pollution is still unclear, it is thus of great significance to explore the combined effects of different remediation plants and earthworm species to improve the bioremediation capacity. In the present study, we consequently selected three species of Cd hyperaccumulator plants (vetiver, P. vittata and S. emarginatum) and three species of earthworms (E. fetida P1, E. fetida P2, and P. guillelmi) to compare the differences in Cd accumulation among various earthworm-plant combinations. Results indicated that the changes of soil pH and SOM in plant-animal combined application induced the higher soil Cd removal efficiency. The Cd removal efficiency showed highest in combination groups P. vittata-E. fetida P2 and P. vittata-P. guillelmi. Meanwhile, the improvements of biomass of plants and animals also were consistent with the increasing of Cd concentration in both plants and earthworms after combined application. It showed that the Cd concentrations in P. vittata were the highest while the TFs of Cd in S. emarginatum displays significantly more than that in others. In conclusion, the recommended combined system of earthworm-plant (P. vittata-E. fetida P2 and P. vittata-P. guillelmi) to provide reference for soil Cd bioremediation system in practice.

Silica-magnesium coupling in lignin-based biochar: A promising remediation for composite heavy metal pollution in environment

Lignin hydrothermal silica-carbon material served as a backbone for MgCl2 activation to prepare lignin-based silicon/magnesia biochar (ALB/Si-Mg) for Cd2+, Pb2+, Cu2+, and Zn2+ removal from water and soil environment. Characterization studies revealed a 1017.71-fold increase in the specific surface area of ALB/Si-Mg compared to the original lignin biochar (ALB), producing abundant oxygen functional groups (OC-O, Si-O, Mg-O), and mineral matter (Mg2SiO4 and MgO). Crucially, batch adsorption experiments demonstrated that the adsorption capacity of ALB/Si-Mg for Cd2+, Pb2+, Cu2+, and Zn2+ was 848.17, 665.07, 151.84, and 245.78 mg/g, which were 29.09-140.45 times of the ALB. Soil remediation experiments showed that applying ALB/Si-Mg increased soil effective silicon (109.04%-450.2%) and soil exchangeable magnesium (276.41%-878.66%), enhanced plant photosynthesis, and notably reduced the bioavailability of heavy metals in soil as well as the content of heavy metals in Pakchoi, thereby promoting Pakchoi growth and development. The presence of oxygen-containing functional groups on ALB/Si-Mg, along with Mg2SiO4 and MgO nanoparticles, enhanced the adsorption capacity for heavy metals through the promotion of heavy metal precipitation, ion exchange, and complexation mechanisms. This study establishes the groundwork for the coupling of silica and magnesium elements in biochar and the remediation of composite heavy metal environmental pollution.

Effect of organic acids on fermentation quality and microbiota of horseshoe residue and corn protein powder

This experiment aimed to investigate the impact of malic acid (MA) and citric acid (CA) on the nutritional composition, fermentation quality, rumen degradation rate, and microbial diversity of a mixture of apple pomace and corn protein powder during ensiling. The experiment used apple pomace and corn protein powder as raw materials, with four groups: control group (CON), malic acid treatment group (MA, 10 g/kg), citric acid treatment group (CA, 10 g/kg), and citric acid + malic acid treatment group (MA, 10 g/kg + CA, 10 g/kg). Each group has 3 replicates, with 2 repetitions in parallel, subjected to mixed ensiling for 60 days. The results indicated: (1) Compared to the CON group, the crude protein content significantly increased in the MA, CA, and MA + CA groups (p < 0.05), with the highest content observed in the MA + CA group. The addition of MA and CA effectively reduced the water-soluble carbohydrate (WSC) content (p < 0.05). Simultaneously, the CA group showed a decreasing trend in NDFom and hemicellulose content (p = 0.08; p = 0.09). (2) Compared to the CON group, the pH significantly decreased in the MA, CA, and MA + CA groups (p < 0.01), and the three treatment groups exhibited a significant increase in lactic acid and acetic acid content (p < 0.01). The quantity of lactic acid bacteria increased significantly (p < 0.01), with the MA + CA group showing a more significant increase than the MA and CA groups (p < 0.05). (3) Compared to the CON group, the in situ dry matter disappearance (ISDMD) significantly increased in the MA, CA, and MA + CA groups (p < 0.05). All three treatment groups showed highly significant differences in in situ crude protein disappearance (ISCPD) compared to the CON group (p < 0.01). (4) Good's Coverage for all experimental groups was greater than 0.99, meeting the conditions for subsequent sequencing. Compared to the CON group, the Shannon index significantly increased in the CA group (p < 0.01), and the Simpson index increased significantly in the MA group (p < 0.05). However, there was no significant difference in the Chao index among the three treatment groups and the CON group (p > 0.05). At the genus level, the abundance of Lentilactobacillus in the MA, CA, and MA + CA groups was significantly higher than in the control group (p < 0.05). PICRUSt prediction results indicated that the metabolic functional microbial groups in the CA and MA treatment groups were significantly higher than in the CON group (p < 0.05), suggesting that the addition of MA or CA could reduce the loss of nutritional components such as protein and carbohydrates in mixed ensilage. In conclusion, the addition of malic acid and citric acid to a mixture of apple pomace and corn protein powder during ensiling reduces nutritional losses, improves fermentation quality and rumen degradation rate, enhances the diversity of the microbial community in ensiled feed, and improves microbial structure. The combined addition of malic acid and citric acid demonstrates a superior effect.

Evaluation of iron-modified biochar on arsenic accumulation by rice: a pathway to assess human health risk from cooked rice

Arsenic (As) contamination of rice grain poses a serious threat to human health. Therefore, it is crucial to reduce the bioavailability of As in the soil and its accumulation in rice grains to ensure the safety of food and human health. In this study, mango (Mangifera indica) leaf-derived biochars (MBC) were synthesized and modified with iron (Fe) to produce FeMBC. In this study, 0.5 and 1% (w/w) doses of MBC and FeMBC were used. The results showed that 1% FeMBC enhanced the percentage of filled grains/panicle and biomass yield by 17 and 27%, respectively, compared to the control. The application of 0.5 and 1% FeMBC significantly (p < 0.05) reduced bioavailable soil As concentration by 33 and 48%, respectively, in comparison to the control. The even higher As flux in the control group as compared to the biochar-treated groups indicates the lower As availability to biochar-treated rice plant. The concentration of As in rice grains was reduced by 6 and 31% in 1% MBC and 1% FeMBC, respectively, compared to the control. The reduction in As concentration in rice grain under 1% FeMBC was more pronounced due to reduced bioavailability of As and enhanced formation of Fe-plaque. This may restrict the entry of As through the rice plant. The concentrations of micronutrients (such as Fe, Zn, Se, and Mn) in brown rice were also improved after the application of both MBC and FeMBC in comparison to the control. This study indicates that the consumption of parboiled rice reduces the health risk associated with As compared to cooked sunned rice. It emphasizes that 1% MBC and 1% FeMBC have great potential to decrease the uptake of As in rice grains.

Potential mechanism of biochar enhanced degradation of oxytetracycline by Pseudomonas aeruginosa OTC-T

Extensive use of oxytetracycline (OTC) and the generation of its corresponding resistance genes have resulted in serious environmental problems. Physical-biological combined remediation is an attractive method for OTC degradation because of its high remediation efficiency, stability, and environmental friendliness. In this study, an effective OTC-degrading strain identified as Pseudomonas aeruginosa OTC-T, was isolated from chicken manure. In the degradation experiment, the degradation rates of OTC in the degradation systems with and without the biochar addition were 92.71-100 % and 69.11-99.59 %, respectively. Biochar improved the tolerance of the strain to extreme environments, and the OTC degradation rate increased by 20.25 %, 18.61 %, and 13.13 % under extreme pH, temperature, and substrate concentration conditions, respectively. Additionally, the degradation kinetics showed that biochar increased the reaction rate constant in the degradation system and shortened the degradation period. In the biological toxicity assessment, biochar increased the proportion of live cells by 17.63 % and decreased the proportion of apoptotic cells by 58.87 %. Metabolomics revealed that biochar had a significant effect on the metabolism of the strains and promoted cell growth and reproduction, effectively reducing oxidative stress induced by OTC. This study elucidates how biochar affects OTC biodegradation and provides insights into the future application of biochar-assisted microbial technology in environmental remediation.

5-Aminolevulinic acid mitigates the chromium-induced changes in Helianthus annuus L. as revealed by plant defense system enhancement

Chromium (Cr) in the soil is one of the major pollutants for agricultural production. This study examined the efficiency of sunflower plants to remediate Cr-contaminated soils using a plant growth regulator, 5-aminolevolinic acid (ALA). At six leaf stage, sunflower plants were exposed to soil-applied Cr (0.15 g kg^-1), manganese (Mn, 0.3 g kg^-1) and trisodium (S,S)-ethylenediamine-N,N'-disuccinic acid (EDDS, 2.5 mmol kg^-1), ALA (10 mg L^-1) was sprayed. After ALA treatment, the plants were harvested for further biochemical analyses. Results showed that EDDS and Mn improved the Cr accumulation but restrained plant growth. Conversely, ALA improved the growth of Cr-stressed plants by promoting chlorophyll concentration in the top fully expanded leaves. The bioaccumulation quantity and removal efficiency of sunflowers treated by Cr + EDDS + ALA was improved by 47.92% and 47.94%, respectively, as compared to the Cr treatment. This was further supported by qRT-PCR analysis, where the expression of heavy metal transport genes such as ZIP6 and NRAMP6 and subsequently Cr accumulation in sunflower tissues increased by EDDS, Mn, and ALA application. However, compared with other treatments, ALA ameliorated cellular injury from Cr-stress by uptake or movement of Cr prevention, modulation of antioxidant enzymes, and elimination of reactive oxygen species. Our study suggested that ALA as an ideal option for the phytoremediation of Cr-contaminated soils.

Effects of α-Fe2O3 nanoparticles and biochar on plant growth and fruit quality of muskmelon under cadmium stress

Cadmium pollution in farmland has become a global environmental problem, threatening ecological security and human health. Biochar is effective in remediation of soil pollution. However, high concentrations of biochar can inhibit plant growth, and low concentrations of biochar have limited mitigation effect on cadmium toxicity. Therefore, the combination of low-concentration biochar and other amendments is a promising approach to alleviate cadmium toxicity in plants and improve the safety of edible parts. In this study, muskmelon was selected as the research object, and different concentrations of α-Fe₂O₃ nanoparticles were used alone or combined with biochar to explore the effects of different treatments on muskmelon plants in cadmium-contaminated soil. The results showed that the combined application of 250 mg/kg α-Fe₂O₃ nanoparticles and biochar had a good effect on the repair of cadmium toxicity in muskmelon plants. Compared with cadmium treatment, its application increased plant height by 32.53%, cadmium transport factor from root to stem decreased by 32.95%, chlorophyll content of muskmelon plants increased by 14.27%, and cadmium content in muskmelon flesh decreased by 18.83%. Moreover, after plant harvest, soil available cadmium content in 250 mg/kg α-Fe₂O₃ nanoparticles and biochar combined treatment decreased by 31.18% compared with cadmium treatment. The results of this study provide an effective reference for the composite application of different exogenous amendments and a feasible idea for soil heavy metal remediation and mitigation of cadmium pollution in farmland.

Meeting the heavy-metal safety requirements for food crops by using biochar: An investigation using sunflower as a representative plant under different atmospheric CO2 concentrations

Global warming impacts on plant growth and food safety are emerging topics of concern, while biochar as a soil additive benefits plants. This study investigates (1) sunflower plant growth at various biochar concentrations in a soil-compost growing substrate under both ambient (420 ppm) and elevated (740 ppm) atmospheric CO₂ concentrations, and (2) concentrations of heavy metals in the growing substrates and organs of the plants. The elevated CO₂ concentration benefits the vegetative parts but harms the reproductive parts of the plants. Additionally, the elevated CO₂ concentration inhibits the beneficial effects that biochar confers on the plants at the ambient concentration. The optimum biochar concentration at both CO₂ levels was found to be 15%. At the time of harvest, most of the heavy-metal concentrations in the growing substrate increased. It was demonstrated that biochar can reduce the amount of heavy metals that accumulate in the roots and seeds whose heavy-metal concentrations complied with Singapore food safety regulations, while those for the biochar met the proposed Singapore biochar standard's thresholds. Our results show that the proposed Singapore biochar standard is practical and sound.

Immobilization effect of heavy metals in biochar via the copyrolysis of sewage sludge and apple branches

The excess sludge produced by sewage treatment plants can be recycled into energy through pyrolysis, and the byproduct biochar can be used for soil remediation. However, the heavy metals in sludge are retained in biochar after pyrolysis and may cause secondary pollution during its soil application. Herein, a fast copyrolysis method of activated sludge (AS) and apple branches (AT) was proposed to immobilize heavy metals while improving bio-oil yield. The results showed that the heavy metal release from the copyrolyzed biochar was markedly reduced compared with that from the biochar produced through the pyrolysis of AS alone (78% for Cr and 28% for Pb). The kinetic behavior of ion release from different biochars could be described by a first-order kinetic model. The excellent fixation of heavy metals was attributed to complexation by abundant oxygen-containing surface functional groups (-O-, =O, and -CHO) that were mainly donated by AT. Furthermore, high-temperature pyrolysis was conducive to the fixation of metals, and the release of Pb²⁺ and Cr³⁺ from the biochar pyrolyzed at 600 °C was approximately 2/3 and 1/10 of that from the biochar pyrolyzed at 400 °C, respectively. A growth experiment on Staphylococcus aureus and Escherichia coli revealed that the toxicity of the copyrolyzed biochar was greatly reduced. This work can provide a method for heavy metal fixation and simultaneous resource recovery from organic wastes.

Role of biochar-based free radicals in immobilization and speciation of metals in the contaminated soil-plant environment

The structure of biochar produced at various pyrolysis temperatures influences metal geochemical behavior. Here, the impact of wheat straw-derived biochar (300, 500, and 700 °C) on the immobilization and transformation of metals in the contaminated soil-plant system was assessed. The findings of the sequential extraction revealed that biochar additives had a substantial influence on the speciation of Cr, Ni, Pb, and Zn in the contaminated soil. The lowest F1 (exchangeable and soluble fraction) + F2 (carbonate fraction) accounted for Cr (44%) in WB-300, Ni (43.87%) in WB-500, Pb (43.79%), and Zn (49.78%) in WB-700 with applied amendments of their total amounts. The characterization results indicated that high pyrolysis temperatures (300-700 °C) increased the carbon-containing groups with the potential to adsorb metals from the soil-plant environment. The bioconcentration and translocation factors (BCF and TF) were less than 1, indicating that metal concentration was restricted to maize roots and translocation to shoots. Reactive oxygen species (ROS) intracellularly influence metal interactions with plants. Electron paramagnetic resonance (EPR) was performed to determine hydroxyl radical generation (•OH) in plant segments to assess the dominance of free radicals (FRs). Consequently, the formation of •OH significantly depends on the pyrolysis temperature and the interaction with a contaminated soil-plant environment. Thus, metal transformation can be effectively decreased in the soil-plant environment by applying WB amendments.

Global perspectives for biochar application in the remediation of heavy metal-contaminated soil: a bibliometric analysis over the past three decades

Herein, 7,308 relevant documents on biochar application for the remediation of heavy metal (HM)-contaminated soil (BARHMCS) from 1991 to 2020 were extracted from the Web of Science Core Collection and subjected to bibliometric and knowledge mapping analyses to provide a global perspective. The results showed that (1) the number of publications increased over time and could be divided into two subperiods, i.e., the slow growth period (SGP) and rapid growth period (RGP), according to whether the annual publication number was ≥300. (2) A total of 126 countries, 741 institutions, and 1,021 scholars have contributed to this field. (3) These studies are mainly published in Science of the Total Environment, Chemosphere, etc., and are mainly based on the categories of environmental science, soil science, and environmental engineering. (4) The top five keyword clusters for the SGP were biochar, biochar, sorption, charcoal, and HMs, and those for the RGP were adsorption, black carbon, nitrous oxide, cadmium, and pyrolysis. (5) The main knowledge domains and the most cited references during the SGP and RGP were discussed. (6) Future directions are related to biochar application for plant remediation, the mitigation of climate change through increased carbon sequestration, biochar modification, and biochar for HMs and multiple organic pollutants.

Effects of biochar application on the loss characteristics of Cd from acidic soil under simulated rainfall conditions

Biochar is widely used for immobilizing heavy metals in soil as a kind of high-effective passivator. This research conducted incubation and simulated rainfall experiments to study the effects of biochar application on the loss characteristics of runoff and sediment, as well as the transportation of the Cd during the water erosion process. Two rainfall intensities (60 and 120 mm h^-1) and five biochar application rates (0%, 1%, 3%, 5%, and 7%) were considered in the experiment. The result showed that slaking had a greater effect than mechanical stirring in aggregate breakdown of the soil, and the addition of biochar generally increased the sensitivity of the soil to wet stirring, while had no obvious influence on the resistance to slaking. The H₂O and CaCl₂ extractable Cd in soil significantly decreased with the increase of biochar application rate. The runoff yields decreased with the increase of biochar application rate at both the two rainfall intensities, while the eroded sediment generally decreased at the 120 mm h^-1 rainfall intensity. The addition of biochar tended to increase the loss of the middle-sized (1-0.05 mm) aggregates at the 60 mm h^-1 rainfall intensity, whereas reduced their loss at the 120 mm h^-1 rainfall intensity. Biochar application could significantly reduce the concentration of Cd in the runoff and decreased the total loss amount of Cd (sediment+runoff) in most of the cases. Excessively high level (7%) of biochar application may aggravate soil erosion and result in more Cd loss.

Jack Bean Development in Multimetal Contaminated Soil Amended with Coffee Waste-Derived Biochars

Coffee waste-derived biochar was found to immobilize heavy metals in contaminated soil, although there are few studies involving these materials. Given the large amount of waste generated in the coffee industry, this presents a relevant opportunity to contribute to the circular economy and environmental sustainability. Therefore, the objective of this study was to evaluate the effects of the application of biochars derived from coffee grounds and coffee parchment in the remediation of a Cd, Zn and Pb contaminated soil and at the development of jack beans (Canavalia ensiformis) in this area’s revegetation. The biochars were pyrolyzed at 700 °C, and the treatments were: contaminated soil (CT); contaminated soil + calcium carbonate (CaCO3); contaminated soil + 5% (weight (w)/weitght (w)) coffee ground biochar and contaminated soil + 5% (w/w) coffee parchment biochar. These treatments were incubated for 90 days, followed by the cultivation of jack beans for 60 days. Soil samples, soil solution and plants were analyzed for nutrients and heavy metals. The addition of coffee grounds and coffee parchment biochars significantly reduced the contents of heavy metals in the soil compared to the Control (32.13 and 42.95%, respectively, for Zn; 26.28 and 33.06%, respectively, for Cd and 28.63 and 29.67%, respectively, for Pb), all of which had a superior performance than the CaCO3 treatment. Thus, following the observed reduction in the soil soluble fraction of metals, its uptake by the plants was also reduced, especially limiting Cd and Pb accumulation in plant dry matter. In addition, coffee parchment biochar promoted a greater accumulation of nutrients in the shoots, i.e., for K and P (1450 and 21.5 mg pot−1, respectively, dry matter basis) compared to the control (54.4 and 9.3 mg pot−1, respectively). Therefore, coffee parchment biochar use in association with jack beans may represent a viable tool for the remediation of metal contamination concomitantly with revegetation of the contaminated area.

Biochar, Ochre, and Manure Maturation in an Acidic Technosol Helps Stabilize As and Pb in Soil and Allows Its Vegetation by Salix triandra

Past mining extraction activities still have a negative impact in the present time, the resulting metal(loid) contaminated soils affecting both the environment and human health. Assisted phytostabilization technology, combining soil conditioner application to immobilize metal(loid)s and plant growth to reduce erosion and leaching risks, is a useful strategy in the restoration of metal(loid) contaminated lands. However, contaminants will respond differently to a particular amendment, having their own specific characteristics. Therefore, in multi-contaminated soils, soil conditioner combination has been suggested as a good strategy for metal(loid) immobilization. In the present study, in a mesocosm experiment, organic (biochar and manure) and inorganic (ochre) amendments were evaluated in single and combined applications for their effect on metal(loid) stabilization and Salix triandra growth improvement, in an arsenic and lead highly contaminated soil. Specifically, the effects of these amendments on soil properties, metal(loid) behavior, and plant growth were evaluated after they aged in the soil for 6 months. Results showed that all amendments, except biochar alone, could reduce soil acidity, with the best outcomes obtained with the three amendments combined. The combination of the three soil conditioners has also led to reducing soil lead availability. However, only ochre, alone or combined with the other soil fertilizers, was capable of immobilizing arsenic. Moreover, amendment application enhanced plant growth, without affecting arsenic accumulation. On the contrary, plants grown on all the amended soils, except plants grown on soil added with manure alone, showed higher lead concentration in leaves, which poses a risk of return of lead into the soil when leaves will shed in autumn. Considering that the best plant growth improvement, together with the lowest increase in lead aerial accumulation, was observed in manure-treated soil, the addition of manure seems to have potential in the restoration of arsenic and lead contaminated soil.

Sustainable biochar effects on the remediation of contaminated soil: A 2-crop season site practice near a lead-zinc smelter in Feng County, China

It is of great significance to explore the remediation pattern in actual heavy metal (HM) contaminated sites. The field trial was carried out to research the remediation effect of biochar near a lead-zinc smelter in Feng County, China, under the rotation condition of different crops. This kind of cultivation mode is very representative in northern of China. And the pattern of production and restoration is suitable for scarce land resources and large food demand in China. The changes of soil physiochemical properties with the biochar addition, crop growth and the accumulated HMs by crops were focused on. The results showed the biochar application was excellent in improving soil nutrient elements and crop growth. The contents of TK were more obvious than those of TN and TP, with an increase of 2.6%-28.2% compared with the controls (without the addition of biochar). The yield of first season crops, i.e., soybean and corn, increased by 30%-42% and 34%-61%, respectively, and the second season crops, i.e., rape and wheat, with the increment of 25%-41% and 9%-29%, respectively. The availability forms of Cd and Pb decreased by 1.07-10.0% and 2.92-8.35%, respectively. While the improvement on the status of the HMs accumulated by crops was disappointing. The contents of HMs accumulated by crops increased to varying degrees (e.g., Pb and Zn in root, Cu and Pb in grain, and Cd in stems and leaves). Moreover, the concentrations of HMs in seeds of crops were higher than the limited levels given by the Chinese directive. Considering the results of the study and food safety, it is suggested to change the nature of the land around the smelter into woodland or construction land to prohibit the cultivation of food crops in this area.

Biochar promotes arsenic sequestration on iron plaques and cell walls in rice roots

The iron (Fe) cycle in the rice-soil system affects arsenic (As) uptake by rice. The effect of Fe on As uptake can be influenced by the addition of biochar, but has not been thoroughly investigated. In this study, the effects of maize straw-derived biochar (MB) on Fe and As translocation were determined by analysing the Fe and As concentrations in pore water, dithionite-citrate-bicarbonate (DCB) extracts, and rice plants. As-contaminated soils were supplemented with 0 or 1% biochar and 0 or 90 mg kg^-1 P, and rice plants were grown for 70 d. Results indicated that biochar addition increased the concentrations of Fe and As in pore water, while P did not affect them. Additionally, biochar promoted the accumulation of Fe and As in roots. However, the rice biomass increased by 28% upon biochar addition, indicating that the rice plants became more tolerant to As toxicity with biochar. Specifically, biochar increased the root triphenyl tetrazolium chloride (TTC) reductive intensity, reduced the root H₂O₂ concentration, and promoted iron plaque (IP) formation. Moreover, the positive correlation between IP/DCB-extractable As and crystalline Fe on the rice root surface indicated that crystalline Fe appeared to be the determinant species of IP and played a central role in As segregation. In addition, biochar increased both crystalline Fe formation on the root surface and the Fe content in the cell wall, which enhanced As sequestration. Overall, rice could effectively tolerate As stress under biochar treatment since As could be retained on the root surface and root cell wall with MB.

Modification of naturally abundant resources for remediation of potentially toxic elements: A review

Water and soil contamination due to potentially toxic elements (PTEs) represents a critical threat to the global ecosystem and human health. Naturally abundant resources have significant advantages as adsorbent materials for environmental remediation over manufactured materials such as nanostructured materials and activated carbons. These advantages include cost-effectiveness, eco-friendliness, sustainability, and nontoxicity. In this review, we firstly compare the characteristics of representative adsorbent materials including bentonite, zeolite, biochar, biomass, and effective modification methods that are frequently used to enhance their adsorption capacity and kinetics. Following this, the adsorption pathways and sites are outlined at an atomic level, and an in-depth understanding of the structure-property relationships are provided based on surface functional groups. Finally, the challenges and perspectives of some emerging naturally abundant resources such as lignite are examined. Although both unamended and modified naturally abundant resources face challenges associated with their adsorption performance, cost performance, energy consumption, and secondary pollution, these can be tackled by using advanced techniques such as tailored modification, formulated mixing and reorganization of these materials. Recent studies on adsorbent materials provide a strong foundation for the remediation of PTEs in soil and water. We speculate that the pursuit of effective modification strategies will generate remediation processes of PTEs better suited to a wider variety of practical application conditions.

Bioaugmentation and bioaugmentation-assisted phytoremediation of heavy metal contaminated soil by a synergistic effect of cyanobacteria inoculation, biochar, and purslane (Portulaca oleracea L.)

In recent decades, soil contamination with heavy metals has become an environmental crisis due to their long-term stability and adverse biological effects. Therefore, bioremediation is an eco-friendly technology to remediate contaminated soil, which the efficiency requires further research. This study was designed to comparatively investigate two strategies: bioaugmentation by using a cyanobacterial species (Oscillatoria sp.) and bioaugmentation-assisted phytoremediation by using Oscillatoria sp. and purslane (Portulaca oleracea L.) for the bioremediation of soil contaminated by heavy metals (Cr (III), Cr (VI), Fe, Al, and Zn). Various quantities of biochar (0.5, 2, and 5% (w/w)) were used as an amendment in the experiments to facilitate the remediation process. The results of the bioaugmentation test showed that applying biochar and cyanobacteria into contaminated soil significantly increased the chlorophyll a, nitrogen, and organic carbon contents. In contrast, the extractable fractions of Cr (III), Cr (VI), Zn, Al, and Fe declined compared with those of the control treatment. The highest reduction content (up to 87 %) in the extractable portion was obtained for Cr (VI). The development of longer root and hypocotyl lengths and vigour index from lettuces and radish seeds grown in the remediated soil confirmed the success of remediation treatments. Moreover, the findings of the bioaugmentation-assisted phytoremediation test displayed a reduction in the bioavailable fraction of Cr (III), Cr (VI), Zn, Al, and Fe. Cr (III) presented the highest reduction (up to 90 %) in metal bioavailability. With cyanobacteria inoculation and biochar addition, the shoot and root lengths of purslane grew 4.6 and 3-fold while the heavy metal accumulation decreased significantly. Besides, these treatments enhanced the tolerance index (TI) quantities of purslane whereas diminished its bioaccumulation coefficient (BAC) and bioconcentration factor (BCF) values. For all heavy metals (except Zn), translocation factor (TF) and BAC values were found to be less than 1.0 at all treatments, indicating the successful phytoextraction by the purslane. These results suggest that the purslane can be considered an excellent phytoextracting agent for soils contaminated with heavy metals.

Biochar-assisted Fenton-like oxidation of benzo[a]pyrene-contaminated soil

In the present study, the biochar derived from sunflower husks was used as a mediator in the heterogeneous Fenton process. The physical and chemical characteristics were studied in terms of specific surface area, elemental contents, surface morphology, surface functional groups, thermal stability, and X-ray crystallography. The main aim was to evaluate the effectiveness of biochar in a heterogeneous Fenton process catalyzed by hematite toward the degradation of benzo[a]pyrene (BaP) in Haplic Chernozem. The Fenton-like reaction was performed at a pH of 7.8 without pH adjustment in chernozem soil. The effects of operating parameters, such as hematite dosage and H₂O₂ concentrations, were investigated with respect to the removal efficiency of BaP. The overall degradation of 65% was observed at the optimized conditions where 2 mg g^-1 hematite and 1.25 M H₂O₂ corresponded to the H₂O₂ to Fe ratio of 22:1. Moreover, the biochar amendment showed an increment in the removal efficiency and promotion in the growth of spring barley (Hordeum sativum distichum). The BaP removal was reached 75 and 95% after 2.5 and 5% w/w addition of biochar, respectively. The results suggested that the Fenton-like reaction's effectiveness would be greatly enhanced by the ability of biochar for activation of H₂O₂ and ejection of the electron to reduce Fe(III) to Fe(II). Finally, the presence of biochar could enhance the soil physicochemical properties, as evidenced by the better growth of Hordeum sativum distichum compared to the soil without biochar. These promising results open up new opportunities toward the application of a modified Fenton reaction with biochar for remediating BaP-polluted soils.

Sustainable mitigation of heavy metals from effluents: Toxicity and fate with recent technological advancements

Increase in anthropogenic activities due to rapid industrialization had caused an elevation in heavy metal contamination of aquatic and terrestrial ecosystems. These pollutants have detrimental effects on human and environmental health. The majority of these pollutants are carcinogenic, neurotoxic, and are very poisonous even at very low concentrations. Contamination caused by heavy metals has become a global concern for which the traditional treatment approaches lack in providing a cost-effective and eco-friendly solution. Therefore, the use of microorganisms and plants to reduce the free available heavy metal present in the environment has become the most acceptable method by researchers. Also, in microbial- and phyto-remediation the redox reaction shifts the valence which makes these metals less toxic. In addition to this, the use of biochar as a remediation tool has provided a sustainable solution that needs further investigations toward its implementation on a larger scale. Enzymes secreted by microbes and whole microbial cell are considered an eco-efficient biocatalyst for mitigation of heavy metals from contaminated sites. To the best of our knowledge there is very less literature available covering remediation of heavy metals aspect along with the sensors used for detection of heavy metals. Systematic management should be implemented to overcome the technical and practical limitations in the use of these bioremediation techniques. The knowledge gaps have been identified in terms of its limitation and possible future directions have been discussed.

Co-microwave pyrolysis of electroplating sludge and municipal sewage sludge to synergistically improve the immobilization of high-concentration heavy metals and an analysis of the mechanism

To improve the harmless treatment of high-concentration heavy metals (HMs) in electroplating sludge (ES), this study tried to combine the microwave pyrolysis technology and the addition of municipal sewage sludge (MS) to synergistically improve the immobilization of high-concentration HMs in ES. The results showed that the immobilization rate of HMs was less than 75% in ES pyrolysis biochar. Notably, the immobilization rate of HMs up to 98.00% in co-pyrolysis biochar. Finally, it was found by various characterizations that the organic carbon and inorganic minerals in MS played an important role in the immobilization of HMs through physical and chemical effects. HMs reacted with inorganic minerals to form HMs crystalline minerals (e.g., CuCl, Cu₂NiSnS₄, and NiSi₂, ZnS) to realize the immobilization of HMs. The addition of organic carbon was conducive to the formation of biochar with higher carbon crystallinity (I_D/I_G = 0.96) and larger specific surface area (52.50 m² g^-1), thereby enhancing the physical adsorption to HMs. Meanwhile, the complexation reaction between HMs and functional groups such as -OH, Si-O-Si could also further improve the immobilization of HMs. Therefore, this study provided a technical and theoretical basis for the harmless disposal of waste containing multiple HMs with high-concentrations.

Influence of biochar and EDTA on enhanced phytoremediation of lead contaminated soil by Brassica juncea

Phytoremediation technology is an eco-friendly technology for the treatment of the polluted environment. Conversely, the natural and synthetic amendments have been revealed to improve the heavy metal phytoextraction from polluted soils with hyperaccumulation and/or non-hyper accumulating plants. This study evaluated the synergistic effect of biochar (BC) and EDTA to enhance phytoextraction of heavy metal lead (Pb) from artificially polluted soil by Brassica juncea. The BC and EDTA amendment enhanced the growth and survival of B. juncea under Pb stress environment. BC and EDTA significantly increased the biomass of B. juncea and significantly increased the total chlorophyll content in the combined amendment of BC and EDTA (22.2 mg/g) compared to the individual amendment of BC (12.8 mg/g) and EDTA (12.2 mg/g) respectively. The combined use of EDTA and biochar showed enhanced Pb uptake (60.2 mg/g) compared to control (10.0 mg/g). The order of Pb uptake was found to be BC + EDTA (60.2 mg/g) ˃ EDTA (23.5 mg/g) ˃ BC (22. 0 mg/g) ˃ control (10.0 mg/g). The maximum activity of SOD (35.2 ± 1.2 U/mg), POD (47.0 ± 1.8 U/mg) and CAT (28.0 ± 1.0 U/mg) was obtained in the mixed application of EDTA and BC. The obtained results revealed that the combined use of BC and EDTA was the most advantageous option for the treatment of Pb contaminated soil as compared to individual amendments.