Using biochar for remediation of soils contaminated with heavy metals and organic pollutants

Sorption/Desorption Behavior and Mechanism of NH4(+) by Biochar as a Nitrogen Fertilizer Sustained-Release Material

Biochar, the pyrolysis product of biomass material with limited oxygen, has the potential to increase crop production and sustained-release fertilizer, but the understanding of the reason for improving soil fertility is insufficient, especially the behavior and mechanism of ammonium sulfate. In this study, the sorption/desorption effect of NH4(+) by biochar deriving from common agricultural wastes under different preparation temperatures from 200 to 500 °C was studied and its mechanism was discussed. The results showed that biochar displayed excellent retention ability in holding NH4(+) above 90% after 21 days under 200 °C preparation temperature, and it can be deduced that the oxygen functional groups, such as carboxyl and keto group, played the primary role in adsorbing NH4(+) due to hydrogen bonding and electrostatic interaction. The sorption/desorption effect and mechanism were studied for providing an optional way to dispose of agricultural residues into biochar as a nitrogen fertilizer sustained-release material under suitable preparation temperature.

Gasified Grass and Wood Biochars Facilitate Plant Establishment in Acid Mine Soils

Heavy metals in exposed mine tailings threaten ecosystems that surround thousands of abandoned mines in the United States. Biochars derived from the pyrolysis or gasification of biomass may serve as a valuable soil amendment to revegetate mine sites. We evaluated the ability of two biochars, produced by gasification of either Kentucky bluegrass seed screenings (KB) or mixed conifer wood (CW), to support the growth of plants in mine spoils from the abandoned Formosa and Almeda Mines in Oregon. To evaluate the potential for plant establishment in mine tailings, wheat was grown in tailings amended with biochar at rates ranging from 0 to 9% (w/w). Both KB and CW biochars promoted plant establishment by increasing soil pH, increasing concentrations of macro- and micronutrients, and decreasing the solubility and plant uptake of heavy metals. Formosa tailings required at least 4% biochar and Almeda soil required at least 2% biochar to promote healthy wheat growth. A complimentary experiment in which mine spoils were leached with simulated precipitation indicated that biochar amendment rates ≥4% were sufficient to neutralize the elution pH and reduce concentrations of potentially toxic elements (Zn, Cu, Ni, Al) to levels near or below concern. These findings support the use of gasified biochar amendments to revegetate acid mine soils.

Feasibility of biochar application on a landfill final cover-a review on balancing ecology and shallow slope stability

Due to the increasing concerns on global warming, scarce land for agriculture, and contamination impacts on human health, biochar application is being considered as one of the possible measures for carbon sequestration, promoting higher crop yield and contamination remediation. Significant amount of researches focusing on these three aspects have been conducted during recent years. Biochar as a soil amendment is effective in promoting plant performance and sustainability, by enhancing nutrient bioavailability, contaminants immobilization, and microbial activities. The features of biochar in changing soil physical and biochemical properties are essential in affecting the sustainability of an ecosystem. Most studies showed positive results and considered biochar application as an effective and promising measure for above-mentioned interests. Bio-engineered man-made filled slope and landfill slope increasingly draw the attention of geologists and geotechnical engineers. With increasing number of filled slopes, sustainability, low maintenance, and stability are the major concerns. Biochar as a soil amendment changes the key factors and parameters in ecology (plant development, soil microbial community, nutrient/contaminant cycling, etc.) and slope engineering (soil weight, internal friction angle and cohesion, etc.). This paper reviews the studies on the production, physical and biochemical properties of biochar and suggests the potential areas requiring study in balancing ecology and man-made filled slope and landfill cover engineering. Biochar-amended soil should be considered as a new type of soil in terms of soil mechanics. Biochar performance depends on soil and biochar type which imposes challenges to generalize the research outcomes. Aging process and ecotoxicity studies of biochar are strongly required.

Assessing biochar ecotoxicology for soil amendment by root phytotoxicity bioassays

Soil amendment with biochar has been proposed as effective in improving agricultural land fertility and carbon sequestration, although the characterisation and certification of biochar quality are still crucial for widespread acceptance for agronomic purposes. We describe here the effects of four biochars (conifer and poplar wood, grape marc, wheat straw) at increasing application rates (0.5, 1, 2, 5, 10, 20, 50% w/w) on both germination and root elongation of Cucumis sativus L., Lepidium sativum L. and Sorghum saccharatum Moench. The tested biochars varied in chemical properties, depending on the type and quality of the initial feedstock batch, polycyclic aromatic hydrocarbons (PAHs) being high in conifer and wheat straw, Cd in poplar and Cu in grape marc. We demonstrate that electrical conductivity and Cu negatively affected both germination and root elongation at ≥5% rate biochar, together with Zn at ≥10% and elevated pH at ≥20%. In all species, germination was less sensitive than root elongation, strongly decreasing at very high rates of chars from grape marc (>10%) and wheat straw (>50%), whereas root length was already affected at 0.5% of conifer and poplar in cucumber and sorghum, with marked impairment in all chars at >5%. As a general interpretation, we propose here logarithmic model for robust root phytotoxicity in sorghum, based on biochar Zn content, which explains 66% of variability over the whole dosage range tested. We conclude that metal contamination is a crucial quality parameter for biochar safety, and that root elongation represents a stable test for assessing phytotoxicity at recommended in-field amendment rates (<1-2%).

Pyrolytic Treatment and Fertility Enhancement of Soils Contaminated with Heavy Hydrocarbons

Pyrolysis of contaminated soils at 420 °C converted recalcitrant heavy hydrocarbons into "char" (a carbonaceous material similar to petroleum coke) and enhanced soil fertility. Pyrolytic treatment reduced total petroleum hydrocarbons (TPH) to below regulatory standards (typically <1% by weight) within 3 h using only 40-60% of the energy required for incineration at 600-1200 °C. Formation of polycyclic aromatic hydrocarbons (PAHs) was not observed, with post-pyrolysis levels well below applicable standards. Plant growth studies showed a higher biomass production of Arabidopsis thaliana and Lactuca sativa (Simpson black-seeded lettuce) (80-900% heavier) in pyrolyzed soils than in contaminated or incinerated soils. Elemental analysis showed that pyrolyzed soils contained more carbon than incinerated soils (1.4-3.2% versus 0.3-0.4%). The stark color differences between pyrolyzed and incinerated soils suggest that the carbonaceous material produced via pyrolysis was dispersed in the form of a layer coating the soil particles. Overall, these results suggest that soil pyrolysis could be a viable thermal treatment to quickly remediate soils impacted by weathered oil while improving soil fertility, potentially enhancing revegetation.

Long-term impact of biochar on the immobilisation of nickel (II) and zinc (II) and the revegetation of a contaminated site

A field remediation treatment was carried out to examine the long-term effect of biochar on the immobilisation of metals and the revegetation of a contaminated site in Castleford, UK. The extracted concentrations of nickel (Ni) (II) and zinc (Zn) (II) in the carbonic acid leaching tests were reduced by 83-98% over three years. The extracted Ni (II) and Zn (II) concentrations three years after the treatment were comparable to a cement-based treatment study carried out in a parallel manner on the same site. The sequential extraction results indicated that biochar addition (0.5-2%) increased the residue fractions of Ni (II) (from 51% to 61-66%) and Zn (II) (from 7% to 27-35%) in the soils through competitive sorption, which may have resulted in the reduction of leachabilities of Ni (II) (from 0.35% to 0.12-0.15%) and Zn (II) (from 0.12% to 0.01%) in the plots with biochar compared with that without biochar three years after the treatment. The germination of grass in the plots on site failed. Further laboratory pot study suggested that larger amounts of biochar (5% or more) and compost (5% or more) were needed for the success of revegetation on this site. This study suggests the effectiveness and potential of biochar application in immobilising heavy metals in contaminated site in the long term.

Preponderant adsorption for chlorpyrifos over atrazine by wheat straw-derived biochar: experimental and theoretical studies

In competitive sorption, WS750 prefers to adsorb chlorpyrifos over atrazine since that chlorpyrifos has stronger pi–pi interaction with WS750 (23.68 kcal mol⁻¹) and larger lipophilicity (log P= 4.7) than that (22.70 kcal mol⁻¹, log P= 2.7) of atrazine.

Amendment of biochar reduces the release of toxic elements under dynamic redox conditions in a contaminated floodplain soil

Biochar (BC) can be used to remediate soils contaminated with potential toxic elements (PTEs). However, the efficiency of BC to immobilize PTEs in highly contaminated floodplain soils under dynamic redox conditions has not been studied up to date. Thus, we have (i) quantified the impact of pre-definite redox conditions on the release dynamics of dissolved aluminum (Al), arsenic (As), cadmium (Cd), copper (Cu), nickel (Ni), and zinc (Zn) in a highly contaminated soil (CS) (non-treated) and in the same soil treated with 10 g kg(-1) biochar based material (CS+BC), and (ii) assessed the efficacy of the material to reduce the concentrations of PTEs in soil solution under dynamic redox conditions using an automated biogeochemical microcosm apparatus. The impact of redox potential (EH), pH, dissolved organic carbon (DOC), dissolved inorganic carbon (DIC), iron (Fe), manganese (Mn), and sulfate (SO4(2-)) on dynamics of PTEs was also determined. The EH was lowered to +68 mV and afterwards increased stepwise to +535 mV. Significant negative correlation between EH and pH in CS and CS+BC was detected. The systematic increase of EH along with decrease of pH favors the mobilization of PTEs in CS and CS+BC. The material addition seems to have little effect on redox processes because pattern of EH/pH and release dynamics of PTEs was basically similar in CS and CS+BC. However, concentrations of dissolved PTEs were considerably lower in CS+BC than in CS which demonstrates that BC is able to decrease concentrations of dissolved PTEs even under dynamic redox conditions.

Effect of aging process on adsorption of diethyl phthalate in soils amended with bamboo biochar

Biochar is a carbonaceous sorbent and can be used as a potential material to reduce the bioavailability of organic pollutants in contaminated soils. In the present study, the adsorption and desorption of diethyl phthalate (DEP) onto soils amended with bamboo biochar was investigated with a special focus on the effect of biochar application rates and aging conditions on the adsorption capacity of the soils. Biochar amendment significantly enhanced the soil adsorption of DEP that increased with increasing application rates of biochar. However, the adsorption capacity decreased by two aging processes (alternating wet and dry, and constantly moist). In the soil with low organic carbon (OC) content, the addition of 0.5% biochar (without aging) increased the adsorption by nearly 98 times compared to the control, and exhibited the highest adsorption capacity among all the treatments. In the soil with high OC content, the adsorption capacity in the treatment of 0.5% biochar without aging was 3.5 and 3 times greater than those of the treatments of biochar aged by alternating wet and dry, and constantly moist, respectively. Moreover, constantly moist resulted in a greater adsorption capacity than alternating wet and dry treatments regardless of biochar addition. This study revealed that biochar application enhanced soil sorption of DEP, however, the enhancement of the adsorption capacity was dependent on the soil organic carbon levels, and aging processes of biochar.

Effect of biochar on the extractability of heavy metals (Cd, Cu, Pb, and Zn) and enzyme activity in soil

Biochar is a carbon-rich solid material derived from the pyrolysis of agricultural and forest residual biomass. Previous studies have shown that biochar is suitable as an adsorbent for soil contaminants such as heavy metals and consequently reduces their bioavailability. However, the long-term effect of different biochars on metal extractability or soil health has not been assessed. Therefore, a 1-year incubation experiment was carried out to investigate the effect of biochar produced from bamboo and rice straw (at temperatures ≥500 °C) on the heavy metal (cadmium (Cd), copper (Cu), lead (Pb), and zinc (Zn)) extractability and enzyme activity (urease, catalase, and acid phosphatase) in a contaminated sandy loam paddy soil. Three rates (0, 1, and 5%) and two mesh sizes (<0.25 and <1 mm) of biochar applications were investigated. After incubation, the physicochemical properties, extractable heavy metals, available phosphorus, and enzyme activity of soil samples were analyzed. The results demonstrated that rice straw biochar significantly (P < 0.05) increased the pH, electrical conductivity, and cation exchange capacity of the soil, especially at the 5% application rate. Both bamboo and rice straw biochar significantly (P < 0.05) decreased the concentration of CaCl2-extractable heavy metals as biochar application rate increased. The heavy metal extractability was significantly (P < 0.01) correlated with pH, water-soluble organic carbon, and available phosphorus in soil. The 5% application rate of fine rice straw biochar resulted in the greatest reductions of extractable Cu and Zn, 97.3 and 62.2%, respectively. Both bamboo and rice straw biochar were more effective at decreasing extractable Cu and Pb than removing extractable Cd and Zn from the soil. Urease activity increased by 143 and 107% after the addition of 5% coarse and fine rice straw biochars, respectively. Both bamboo and rice straw biochars significantly (P < 0.05) increased catalase activity but had no significant impact on acid phosphatase activity. In conclusion, the rice straw biochar had greater potential as an amendment for reducing the bioavailability of heavy metals in soil than that of the bamboo biochar. The impact of biochar treatment on heavy metal extractability and enzyme activity varied with the biochar type, application rate, and particle size.

Bioremediation of multi-polluted soil by spent mushroom (Agaricus bisporus) substrate: Polycyclic aromatic hydrocarbons degradation and Pb availability

This study investigates the effect of three spent Agaricus bisporus substrate (SAS) application methods on bioremediation of soil multi-polluted with Pb and PAH from close to a shooting range with respect natural attenuation (SM). The remediation treatments involve (i) use of sterilized SAS to biostimulate the inherent soil microbiota (SSAS) and two bioaugmentation possibilities (ii) its use without previous treatment to inoculate A. bisporus and inherent microbiota (SAS) or (iii) SAS sterilization and further A. bisporus re-inoculation (Abisp). The efficiency of each bioremediation microcosm was evaluated by: fungal activity, heterotrophic and PAH-degrading bacterial population, PAH removal, Pb mobility and soil eco-toxicity. Biostimulation of the native soil microbiology (SSAS) achieved similar levels of PAH biodegradation as SM and poor soil detoxification. Bioaugmented microcosms produced higher PAH removal and eco-toxicity reduction via different routes. SAS increased the PAH-degrading bacterial population, but lowered fungal activity. Abisp was a good inoculum carrier for A. bisporus exhibiting high levels of ligninolytic activity, the total and PAH-degrading bacteria population increased with incubation time. The three SAS applications produced slight Pb mobilization (<0.3%). SAS sterilization and further A. bisporus re-inoculation (Abisp) proved the best application method to remove PAH, mainly BaP, and detoxify the multi-polluted soil.

Lead toxicity in rice: effects, mechanisms, and mitigation strategies--a mini review

Lead (Pb) is a major environmental pollutant that affects plant morpho-physiological and biochemical attributes. Its higher levels in the environment are not only toxic to human beings but also harmful for plants and soil microbes. We have reviewed the uptake, translocation, and accumulation mechanisms of Pb and its toxic effects on germination, growth, yield, nutrient relation, photosynthesis, respiration, oxidative damage, and antioxidant defense system of rice. Lead toxicity hampers rice germination, root/shoot length, growth, and final yield. It reduces nutrient uptake through roots, disrupts chloroplastic ultrastructure and cell membrane permeability, induces alterations in leaves respiratory activities, produces reactive oxygen species (ROS), and triggers some enzyme and non-enzymatic antioxidants (as defense to oxidative damage). In the end, biochar amendments and phytoremediation technologies have been proposed as soil remediation approaches for Pb tainted soils.

Application of laboratory prepared and commercially available biochars to adsorption of cadmium, copper and zinc ions from water

The goal of the presented work was the evaluation and comparison of two biochars (produced from Sida hermaphrodita - BCSH/laboratory produced and from wheatstraw - BCS/commercial available) to adsorb heavy metal ions (Cd(II), Cu(II) and Zn(II)) from water. Kinetics of the sorption as well as sorption isotherms, the influence of solution pH and interfering ions were investigated. Different physico-chemical properties of biochars had the great influence on adsorption capacity. The greater adsorption efficiency was observed for BCSH than for BCS in the case of all investigated metals. The adsorption efficiency of BCSH was correlated with higher content of carbon and oxygen, what is equal with higher content of polar-groups on the BCSH surface e.g., -COOH. Furthermore, the molar ratio of O/C as well as polarity index (which was higher for BCSH) was also important parameters.

Elaboration, characteristics and advantages of biochars for the management of contaminated soils with a specific overview on Miscanthus biochars

Biochars are products that are rich in carbon obtained by pyrolysis processes that consist in introducing a biomass (such as wood or manure) in a closed container and heating it with little or no available air. This paper reports the impacts of pyrolysis parameters on biochar characteristics. A preliminary examination of the scientific literature revealed that the type of feedstock, the temperature, the heating rate and the gas flow were the major parameters influencing the biochar characteristics. This review highlights the multitude of biochars that can be made and shows the importance of characterizing them before their use in soils. Then we assess how the input of biochars in soils can affect soil parameters. A review of the literature showed modifications on: i) the physical properties of soils (i.e. the modification in soil structure and water retention), ii) the chemical properties of soils (i.e. the modification of pH, cation exchange capacity, nutrient availability, the organic matter content) and iii) the biological properties (i.e. the changes in microbial and faunal communities). All these modifications can lead to an increase in crop productivity, which confirms the value of biochars as a soil amendment. Moreover, biochars can also provide an advantage for soil remediation. Indeed, biochars efficiently reduce the bioavailability of organic and inorganic pollutants. In addition, this review focuses on a specific plant that can be used to produce biochars: Miscanthus, a non-wood rhizomatous C4 perennial grass. Miscanthus presents advantages for biochar production due to: i) its lignocellulosic content, ii) its silicon content, which can mitigate environmental stresses (notably for plants grown on contaminated sites) and iii) the greater surface area of the Miscanthus biochars compared to the biochars produced with other feedstock.

Immobilization of Cd(II) in acid soil amended with different biochars with a long term of incubation

Biochars derived from bamboo, coconut shell, pine wood shavings, and sugarcane bagasse were applied into Ultisol to investigate their effects on Cd(II) immobilization. After 360 days of incubation, the physical/chemical properties of the Ultisol were improved by the addition of different biochars. As a result, the maximum adsorption capacities of soil for Cd(II) were increased from 8.02 to 9.07-11.51 mmol/kg, and bamboo biochar showed the highest effect on Cd(II) immobilization. The Langmuir model (R(2) > 0.983) fitted the data better than the Freundlich model (R (2) were 0.902-0.937). Column leaching experiments suggested that biochar can also increase the immobilization of Cd(II) under leaching conditions. Biochar mainly increased the weak/unstable binding force of Cd(II) by soil, such as ion exchange, electrostatic attraction, physical adsorption, and carbonate precipitation. In addition, a significant enhancement of surface complexation was also observed.

Effect of biochar on leaching of organic carbon, nitrogen, and phosphorus from compost in bioretention systems

Compost is used in bioretention systems to improve soil quality, water infiltration, and retention of contaminants. However, compost contains dissolved organic matter, nitrate, and phosphorus, all of which can leach out and potentially contaminate ground and surface waters. To reduce the leaching of nutrients and dissolved organic matter from compost, biochar may be mixed into the bioretention systems. Our objective was to test whether biochar and co-composted biochar mixed into mature compost can reduce the leaching of organic carbon, nitrogen, and phosphorus. There was no significant difference between the effects of biochar and co-composted biochar amendments on nutrient leaching. Further, biochar amendments did not significantly reduce the leaching of dissolved organic carbon, nitrate, and phosphorus as compared to the compost only treatment. The compost-sand mix was the most effective in reducing nitrate and phosphorus leaching among the media.

Sludge-Derived Biochar for Arsenic(III) Immobilization: Effects of Solution Chemistry on Sorption Behavior

Recycling sewage sludge by pyrolysis has attracted increasing attention for pollutant removal from wastewater and soils. This study scrutinized As(III) sorption behavior on sludge-derived biochar (SDBC) under different pyrolysis conditions and solution chemistry. The SDBC pyrolyzed at a higher temperature showed a lower As(III) sorption capacity and increasingly nonlinear isotherm due to loss of surface sites and deoxygenation-dehydrogenation. The Langmuir sorption capacity on SDBC (3.08-6.04 mg g) was comparable to other waste-derived sorbents, with the highest As(III) sorption on SDBC pyrolyzed at 400°C for 2 h. The As(III) sorption kinetics best fit with the pseudo-second-order equation, thus suggesting the significance of the availability of surface sites and initial concentration. Sorption of As(III) was faster than that of Cr(VI) but slower than that of Pb(II), which was attributed to their differences in molar volume (correlated to diffusion coefficients) and sorption mechanisms. The X-ray photoelectron spectra revealed an increase of oxide oxygen (O) with a decrease of sorbed water, indicative of ligand exchange with hydroxyl groups on SDBC surfaces. The As(III) sorption was not pH dependent in acidic-neutral range (pH < 8) due to the buffering capacity and surface characteristics of the SDBC; however, sorption was promoted by increasing pH in the alkaline range (pH > 8) because of As(III) speciation in solution. An increasing ionic strength (0.001-0.1 mol L) facilitated As(III) sorption, indicating the predominance of ligand exchange over electrostatic interactions, while high concentrations (0.1 mol L) of competing anions (fluoride, sulfate, carbonate, and phosphate) inhibited As(III) sorption. These results suggest that SDBC is applicable for As(III) immobilization in most environmentally relevant conditions.

A novel approach in organic waste utilization through biochar addition in wood/polypropylene composites

In an attempt to concurrently address the issues related to landfill gas emission and utilization of organic wastes, a relatively novel idea is introduced to develop biocomposites where biochar made from pyrolysis of waste wood (Pinus radiata) is added with the same wood, plastic/polymer (polypropylene) and maleated anhydride polypropylene (MAPP). Experiments were conducted by manufacturing wood and polypropylene composites (WPCs) mixed with 6 wt%, 12 wt%, 18 wt%, 24 wt%, and 30 wt% biochar. Though 6 wt% addition had similar properties to that of the control (composite without biochar), increasing biochar content to 24 wt% improved the composite's tensile/flexural strengths and moduli. The biochar, having high surface area due to fine particles and being highly carbonised, acted as reinforcing filler in the biocomposite. Composites having 12 wt% and 18 wt% of biochar were found to be the most ductile and thermally stable, respectively. This study demonstrates that, WPCs added with biochar has good potential to mitigate wastes while simultaneously producing biocomposites having properties that might be suited for various end applications.

Combination of biochar amendment and mycoremediation for polycyclic aromatic hydrocarbons immobilization and biodegradation in creosote-contaminated soil

Soils impregnated with creosote contain high concentrations of polycyclic aromatic hydrocarbons (PAH). To bioremediate these soils and avoid PAH spread, different bioremediation strategies were tested, based on natural attenuation, biochar application, wheat straw biostimulation, Pleurotus ostreatus mycoremediation, and the novel sequential application of biochar for 21 days and P. ostreatus 21 days more. Soil was sampled after 21 and 42 days after the remediation application. The efficiency and effectiveness of each remediation treatment were assessed according to PAH degradation and immobilization, fungal and bacterial development, soil eco-toxicity and legal considerations. Natural attenuation and biochar treatments did not achieve adequate PAH removal and soil eco-toxicity reduction. Biostimulation showed the highest bacterial development but low PAH degradation rate. Mycoremediation achieved the best PAH degradation rate and the lowest bioavailable fraction and soil eco-toxicity. This bioremediation strategy achieved PAH concentrations below Spanish legislation for contaminated soils (RD 9/2005). Sequential application of biochar and P. ostreatus was the second treatment most effective for PAH biodegradation and immobilization. However, the activity of P. ostreatus was increased by previous biochar application and PAH degradation efficiency was increased. Therefore, the combined strategy for PAH degradation have high potential to increase remediation efficiency.

Efficiency of biochar and compost (or composting) combined amendments for reducing Cd, Cu, Zn and Pb bioavailability, mobility and ecological risk in wetland soil

Biochar and compost (or composting) combined amendments had higher efficiency for remediation of heavy metals polluted soils.

Feasibility of biochar manufactured from organic wastes on the stabilization of heavy metals in a metal smelter contaminated soil

The main objectives of the current study were to evaluate the potential effects of biochar derived from sugar cane bagasse (SC-BC) and orange peel (OP-BC) on improving the physicochemical properties of a metal smelter contaminated soil, and determining its potentiality for stabilizing Pb and As in soil. To achieve these goals, biochar was produced in a small-scale biochar producing plant, and an incubation experiment was conducted using a silt loam metal-contaminated soil treated with different application rates of biochar (0-10% w/w). The obtained results showed that, the addition of SC-BC and OP-BC increased significantly the soil aggregate stability, water-holding capacity, cation exchange capacity, organic matter and N-status in soil. SC-BC considerably decreased the solubility of Pb to values lower than the toxic regulatory level of the toxicity characteristics leaching procedure extraction (5 mg L(-1)). The rise in soil pH caused by biochar application, and the increase of soil organic matter transformed the labile Pb into less available fractions i.e. "Fe-Mn oxides" and "organic" bound fractions. On the other hand, As was desorbed from Fe-Mn oxides, which resulted in greater mobility of As in the treated soil. We concluded that SC-BC and OP-BC could be used successfully for remediating soils highly contaminated with Pb. However, considerable attention should be paid when using it in soil contaminated with As.

Geochemical and spectroscopic investigations of Cd and Pb sorption mechanisms on contrasting biochars: engineering implications

Biochars prepared from nut shells, plum stones, wheat straws, grape stalks and grape husks were tested as potential sorbents for Cd and Pb. Mechanisms responsible for metal retention were investigated and optimal sorption conditions were evaluated using the RSM approach. Results indicated that all tested biochars can effectively remove Cd and Pb from aqueous solution (efficiency varied between 43.8% and 100%). The removal rate of both metals is the least affected by the biochar morphology and specific surface but this removal efficiency is strongly pH-dependent. Results of variable metal removal combined with different optimized conditions explain the different metal sorption mechanisms, where the predominant mechanism is ion exchange. In addition, this mechanism showed very strong binding of sorbed metals as confirmed by the post-desorption of the fully metal-loaded biochars. Finally, these biochars could thus also be applicable for metal contaminated soils to reduce mobility and bioavailability of Cd and Pb.

Biochar amendment affects leaching potential of copper and nutrient release behavior in contaminated sandy soils

Copper (Cu) contamination to soil and water is a worldwide concern. Biochar has been suggested to remediate degraded soils. In this study, column leaching and chemical characterization were conducted to assess effects of biochar amendment on Cu immobilization and subsequent nutrient release in Cu-contaminated Alfisol and Spodosol. The results indicate that biochar is effective in binding Cu (30 and 41%, respectively, for Alfisol with and without spiked Cu; 36 and 43% for Spodosol) and reducing Cu leaching loss (from ∼47 to 10% for the Cu-spiked Alfisol and from 48 to 9% for the Cu-spiked Spodosol). Copper was likely retained on biochar surfaces through complexation, as suggested by Fourier-transform infrared spectra. Biochar amendment converts a portion of Cu from available pool to more stable forms, thus resulting in decreased activities of free Cu and increased activity of organic Cu complexes in leachate. Reduction of >0.45-μm solids and nanoparticles concentrations in leachate was also observed. In addition, biochar application rate was correlated negatively with P, Ca, Mg, Zn, Mn, and NH-N concentration ( < 0.05) but positively with K and Na concentration ( < 0.05) in leachates. These results documented the potential of biochar as an effective amendment for Cu immobilization and mitigation of leaching risk for some nutrients.

Emission reduction of 1,3-dichloropropene by soil amendment with biochar

Soil fumigation is an important treatment in the production chain of fruit and vegetable crops, but fumigant emissions contribute to air pollution. Biochar as a soil amendment has shown the potential to reduce organic pollutants, including pesticides, in soils through adsorption and other physicochemical reactions. A laboratory column study was performed to determine the effects of soil applications of biochar for reducing emissions of the fumigant 1,3-dichloropropene (1,3-D). The experimental treatments comprised of unamended and amended with biochar at doses of 0, 0.5, 1, 2, and 5% (w/w) in the top 5 cm soil layer. The unamended treatment resulted in the highest emission peak flux at 48 to 66 μg m s. Among the biochar amendment treatments, the highest peak flux (0.83 μg m s) was found in the biochar 0.5% treatment. The total emission loss was 35.7 to 40.2% of applied for the unamended treatment and <0.1 to 2.9% for the biochar-amendment treatments. A germination bioassay with cucumber seeds showed that ≥7 d of aeration would be needed to avoid phytotoxicity before replanting in biochar-containing fumigated soil. The results indicate that treatments with 0.5% or more biochar amendment reduced emission peak flux by >99.8% and showed total 1,3-D emission loss by >92% compared with that without biochar. The amendment of surface soil with biochar shows a great potential for reducing fumigant emissions.

Mechanisms of water interaction with pore systems of hydrochar and pyrochar from poplar forestry waste

The aim of this study was to understand the water-surface interactions of two chars obtained by gasification (pyrochar) and hydrothermal carbonization (hydrochar) of a poplar biomass. The two samples revealed different chemical compositions as evidenced by solid state (13)C NMR spectroscopy. In fact, hydrochar resulted in a lignin-like material still containing oxygenated functionalities. Pyrochar was a polyaromatic system in which no heteronuclei were detected. After saturation with water, hydrochar and pyrochar were analyzed by fast field cycling (FFC) NMR relaxometry. Results showed that water movement in hydrochar was mainly confined in very small pores. Conversely, water movement in pyrochar led to the conclusion that a larger number of transitional and very large pores were present. These results were confirmed by porosity evaluation derived from gas adsorption. Variable-temperature FFC NMR experiments confirmed a slow-motion regime due to a preferential diffusion of water on the solid surface. Conversely, the higher number of large pores in pyrochar allowed slow movement only up to 50 °C. As the temperature was raised to 80 °C, water interactions with the pore surface became weaker, thereby allowing a three-dimensional water exchange with the bulk liquid. This paper has shown that pore size distribution was more important than chemical composition in affecting water movement in two chemically different charred systems.

A three-year experiment confirms continuous immobilization of cadmium and lead in contaminated paddy field with biochar amendment

Heavy metal contamination in croplands has been a serious concern because of its high health risk through soil-food chain transfer. A field experiment was conducted in 2010-2012 in a contaminated rice paddy in southern China to determine if bioavailability of soil Cd and Pb could be reduced while grain yield was sustained over 3 years after a single soil amendment of wheat straw biochar. Contaminated biochar particles were separated from the biochar amended soil and microscopically analyzed to help determine where, and how, metals were immobilized with biochar. Biochar soil amendment (BSA) consistently and significantly increased soil pH, total organic carbon and decreased soil extractable Cd and Pb over the 3 year period. While rice plant tissues' Cd content was significantly reduced, depending on biochar application rate, reduction in plant Pb concentration was found only in root tissue. Analysis of the fresh and contaminated biochar particles indicated that Cd and Pb had probably been bonded with the mineral phases of Al, Fe and P on and around and inside the contaminated biochar particle. Immobilization of the Pb and Cd also occurred to cation exchange on the porous carbon structure.

Biochar and Glomus caledonium influence Cd accumulation of upland kangkong (Ipomoea aquatica Forsk.) intercropped with Alfred stonecrop (Sedum alfredii Hance)

Both biochar application and mycorrhizal inoculation have been proposed to improve plant growth and alter bioaccumulation of toxic metals. A greenhouse pot trial was conducted to investigate growth and Cd accumulation of upland kangkong (Ipomoea aquatica Forsk.) intercropped with Alfred stonecrop (Sedum alfredii Hance) in a Cd-contaminated soil inoculated with Glomus caledonium and/or applied with biochar. Compared with the monocultural control, intercropping with stonecrop (IS) decreased kangkong Cd acquisition via rhizosphere competition, and also decreased kangkong yield. Gc inoculation (+M) accelerated growth and Cd acquisition of stonecrop, and hence resulted in further decreases in kangkong Cd acquisition. Regardless of IS and +M, biochar addition (+B) increased kangkong yield via elevating soil available P, and decreased soil Cd phytoavailability and kangkong Cd concentration via increasing soil pH. Compared with the control, the treatment of IS + M + B had a substantially higher kangkong yield (+25.5%) with a lower Cd concentration (-62.7%). Gc generated additive effects on soil alkalinization and Cd stabilization to biochar, causing lower DTPA-extractable (phytoavailable) Cd concentrations and post-harvest transfer risks.