Effect of organic acids and soil particle size on heavy metal removal from bulk soil with washing.

To evaluate the effect of soil particle size on heavy metals removal by washing, two soil samples were collected around a lead-zinc mining area (SM) and lead-zinc smelter (SS). The total content of Cd, Pb and Zn in SM and SS were determined. And the effect of soil particle size on Cd removal by low molecular organic acids was studied. The results showed that Cd was the main pollutant and the total content of Cd in SS can reach to 24.8 mg Kg-1. 68.4% of the total Cd in SM existed in the form of residual state, while 54.7% of the total Cd in SS was in weak acid extractable state. About 50.0% of the Cd distributed in < 2 µm soil size fraction. The washing results indicated that citric acid was a highly efficient eluent among the five low molecular weight organic acids (citric acid, malic acid, tartaric acid, oxalic acid and acetic acid). After washing, 40% and 69.6% of the total Cd in SS and SM can be removed by citric acid, respectively. While only 18.7-40.2% and 32.6-68.7% of Cd was removed from different size fractions of SM and SS, respectively. The species of Cd in soil size fractions affected the removal effect of citric acid. The citric acid can easily remove the weak acid extractable and reducible form of Cd in soil. After eluted by citric acid, the bioavailability of Cd in soil decreased markedly, and the highest decreasing rate reached 93%.

Exogenous salicylic acid regulates cell wall polysaccharides synthesis and pectin methylation to reduce Cd accumulation of tomato.

Cadmium (Cd) is harmful to plant growth and can be easily transferred from soil to plants. Plant cell wall plays important role in preventing Cd from entering cells. Salicylic acid (SA) mediated defense response increases plant resistance to heavy metals. In this study, all tomato seedlings were pre-treated with 100 µM SA for 3 d, then seedlings were used to analyze the role of SA in regulating plant cell wall resistance to Cd stress. The results showed that exogenous SA significantly reduced Cd accumulation in tomato plants and changed Cd distribution. By analyzing the cell wall composition, it was found cellulose, hemicellulose, pectin, and lignin were induced by SA. Interestingly, the content of Cd in pectin decreased by SA pretreatment, however it was increased in cellulose. Gene expression analysis showed SA up-regulated the expression level of lignin and cellulose synthase genes, but down-regulated the expression of pectin methylesterase related genes. In addition, SA down-regulated the activity of pectin methylesterase. These results indicated that SA pretreatment up-regulated cell wall polysaccharide synthesis and related gene expression to thicken the cell wall and block Cd from passing through. Furthermore, SA decreased pectin methylesterase activity and content to reduce cell wall Cd accumulation and change the Cd partition ratio.

Rhizosphere iron and manganese-oxidizing bacteria stimulate root iron plaque formation and regulate Cd uptake of rice plants (Oryza sativa L.).

Iron plaque is the amorphous and/or crystalline layer of Fe and Mn (hydr)oxides formed on the root surface of wetland plants. It could adsorb and co-precipitate metal(loid)s at the rhizosphere, thus modulating the uptake and accumulation of metal elements in plants. In this study, the Fe(II)/Mn(II)-oxidizing bacteria Burkholderia sp. D416 (D416) and Pseudomonas sp. YGL (YGL) were isolated from Cd-contaminated rice field, both hydroponic experiment and pot experiment were performed to assess the impact of bacterial inoculation on iron plaque formation, elemental content of the plaque, plant dry mass, antioxidant enzyme activity and Cd content in rice plants. The results revealed that inoculation with D416, YGL, and D416+YGL stimulated iron plaque formation on the root surface of the hydroponic rice. The content of C, N, O, Na, Mg, Al, Si, P, S, Cl, K, Fe and Ca in the root plaque were affected by the bacterial inoculation and varied among different plant growth stages. The pot experiment indicated that inoculation with D416 increased the root dry biomass by 58.89%, and the combined inoculation of D416 and YGL increased the dry biomass of root, shoot and grain by 16.89%, 21.66% and 23.26%, respectively. Importantly, YGL inoculation decreased the Cd translocation from root to shoot and from glume to brown rice grain by 50.00% and 50.27%, respectively, and the Cd content in shoot and brown rice grain were decreased by 20.00% and 34.48%, respectively. Taken together, the elemental content of the iron plaque and Cd content in rice plants varied among different plant growth stages and when plants were inoculated with different bacterial strains. YGL dramatically reduced the Cd content in brown rice grain, thus it could potentially be used to reduce Cd content in rice crop grown in Cd-contaminated soils.

Effects of EDTA and plant growth-promoting rhizobacteria on plant growth and heavy metal uptake of hyperaccumulator Sedum alfredii Hance.

Phytoremediation is a cost-effective and environment-friendly strategy for decontaminating heavy-metal-contaminated soil. However, the practical use of phytoremediation is constrained by the low biomass of plants and low bioavailability of heavy metals in soil. A pot experiment was conducted to investigate the effects of the metal chelator ethylenediaminetetraacetic acid (EDTA) and EDTA in combination with plant growth-promoting rhizobacteria (Burkholderia sp. D54 or Burkholderia sp. D416) on the growth and metal uptake of the hyperaccumulator Sedum alfredii Hance. According to the results, EDTA application decreased shoot and root biomass by 50% and 43%, respectively. The soil respiration and Cd, Pb, Zn uptake were depressed, while the photosynthetic rate, glutathione and phytochelatin (PC) contents were increased by EDTA application. Interestingly, Burkholderia sp. D54 and Burkholderia sp. D416 inoculation significantly relieved the inhibitory effects of EDTA on plant growth and soil respiration. Compared with the control, EDTA + D416 treatment increased the Cd concentration in shoots and decreased the Pb concentration in shoots and roots, but did not change the Zn concentration in S. alfredii plants. Furthermore, EDTA, EDTA + D54 and EDTA + D416 application increased the cysteine and PC contents in S. alfredii (p < 0.05); among all tested PCs, the most abundant species was PC2, and compared with the control, the PC2 content was increased by 371.0%, 1158.6% and 815.6%, respectively. These results will provide some insights into the practical use of EDTA and PGPR in the phytoremediation of heavy-metal-contaminated soil by S. alfredii.

Adsorption of heavy metals from aqueous solution by UV-mutant Bacillus subtilis loaded on biochars derived from different stock materials.

Two kinds of biochars, one derived from corn straw (CBC) and one from pig manure (PBC), were used as the carriers of a bacterium (B38) to adsorb heavy metals in solution. CBC exhibited high affinity to Hg(II), while PBC showed large adsorption capacity of Pb(II). After loading with B38, the sorption capacity of the co-sorbents were enhanced for Pb(II), but weakened for Hg(II). In a binary system, the overall adsorption capacity to Hg-Pb (CBC+B38, 136.7mg/g; PBC+B38, 181.3mg/g) on co-sorbents was equal to the sum of the single-component values for Hg(II) and Pb(II). Electrostatic interactions and precipitation are the major mechanisms in the adsorption of Hg(II). In contrast, cation-π interactions and precipitation were involved in the sorption process of Pb(II). Moreover, the sorption sites of Hg(II) and Pb(II) partially overlapped on the biochar surface, but were different on co-sorbents. Hence, the co-sorbents have an advantage over the biochar alone in the removal of heavy metal mixtures.

Effects of salicylic acid, Epi-brassinolide and calcium on stress alleviation and Cd accumulation in tomato plants.

Salicylic acid (SA), Epi-brassinolide (EBL) and calcium (Ca) play crucial roles in plant development and mediate plant response to biotic and abiotic stress. This study was aimed to investigate the possible mediatory role of SA, EBL, Ca or their combination in protecting tomato plants from cadmium (Cd) toxicity. According to the results, Cd stress resulted in a significant reduction of plant dry mass, photosynthetic pigment content as well as photosynthetic rate. Exogenous application of SA decreased the malondialdehyde (MDA) level by 39.27% and increased catalase (CAT) activity by 81.17%. SA and EBL treatment significantly increased chlorophyll a (Chl a), chlorophyll b (Chl b) content, photosynthetic rate (Pn) as well as water use efficiency (WUE). SA+EBL (1:1)/Ca+SA+EBL (1:1:1) treatment obviously alleviated Cd-induced growth inhibition, the dry mass of different tomato organs were significantly increased (p < 0.05). Especially in Ca+SA+EBL treated plants, the dry mass of roots, stems and leaves increased by 141.18%, 128.57% and 118.52%, respectively. Besides, SA+EBL and Ca+SA+EBL treatments reduced the MDA level, but increased photosynthetic pigment concentration and photosynthetic efficiency. CAT activity was increased by 62.92% in Ca+SA+EBL treated plants, the WUE was increased by 557.76% in SA+EBL pretreated plants. Moreover, exogenous application of SA, SA+EBL and Ca+SA+EBL significantly decreased Cd accumulation in tomato organs (p < 0.05) compared with Cd-stressed plants. Taken together, our results indicated that exogenous application of SA, EBL and Ca individually or in combination could alleviate Cd toxicity in tomato plants, although the extent varies.

Effects of salicylic acid, Fe(II) and plant growth-promoting bacteria on Cd accumulation and toxicity alleviation of Cd tolerant and sensitive tomato genotypes.

In this study, we investigated the ameliorative effects of salicylic acid (SA), metal ion (Fe(II)), and plant growth-promoting bacteria Burkholderia sp. D54 (B) on two tomato genotypes with different Cd tolerances under Cd stress, viz. Liger (Cd tolerant) and Tabd (Cd sensitive). The plant biomass, Cd accumulation, antioxidative response, pigment content and photosynthetic performance were determined. According to the results, exogenous application of SA, Fe(II) and Burkholderia sp. D54 or their complex effectively reduced Cd accumulation and increased biomass of root, stem and leaves in both Cd sensitive and Cd tolerant genotypes. Among all treatments, SA+Fe+B exerted the best performance. Burkholderia sp. D54 effectively alleviated Cd-induced oxidative toxicity in both tomato genotypes, while SA ameliorated oxidative stress in Cd sensitive genotype. Photosynthetic pigment content and photosynthetic rate of Cd tolerant genotype was increased by all treatments, but only SA and Burkholderia sp. D54 treatment increased pigment contents and photosynthetic performance in Cd sensitive genotypes. All treatments significantly decreased Cd accumulation in both tomato genotypes. The effect of Cd reduction was Fe+SA+B>SA>Fe>B. Taken together, our results indicated that exogenous application of SA, Fe(II) and Burkholderia sp. D54 could alleviate the Cd toxicity in both Cd sensitive and Cd tolerant genotypes, although the extent varies.

Marked changes in biochar's ability to directly immobilize Cd in soil with aging: implication for biochar remediation of Cd-contaminated soil.

To investigate the change in biochar's ability to directly immobilize Cd in soil, a successive wheat cultivation experiment was conducted. Three biochars with different Cd adsorption mechanisms were added to the soils, and a mesh bag was used to separate the soil particles (> 1 µm) from the biochar. The results showed that the ash contents and anionic contents (CO32- and PO43-) of the biochar decreased with the cultivation time, while the oxygen-containing functional group content and CEC of the biochar increased. As a result, the Cd concentration on biochar decreased, by 68.9% for WBC300, while unstable Cd species (acid soluble and reducible fraction of Cd) on biochar increased with successive cultivation, increasing from 3 to 17% for WBC300 in FS. Correspondingly, the ability of biochar to inhibit Cd accumulation in wheat decreased. The results of this study illustrated that the ability of biochar to directly immobilize Cd in soil is not permanent; it gradually decreases with aging in soil. The adsorption mechanism of Cd on biochar changed from precipitation to complexation, and ion exchange processes could be the main reason.

Effects of urease-producing bacteria and eggshell on physiological characteristics and Cd accumulation of pakchoi (Brassica chinensis L.) plants.

Soil cadmium (Cd) contamination resulting from anthropogenic activity poses severe threats to food safety and human health. In this study, a pot experiment was performed to evaluate the possibility of using urease-producing bacterium UR21 and eggshell (ES) waste for improving the physiological characteristics and reducing Cd accumulation of pakchoi (Brassica chinensis L.) plants. UR21 has siderophore and IAA production ability. The application of UR21 and ES individually or in combination could improve the root and shoot length, and fresh and dry weight of pakchoi plants under Cd stress. In Cd + ES + UR21-treated plants, the dry weight of shoot and root were increased by 61.54% and 72.73%, respectively. The chlorophyll a, chlorophyll b, and carotenoid content were increased by 52.19%, 42.95%, and 95.56% in Cd + ES + UR21-treated plants. Meanwhile, the H2O2 and MDA content were decreased while the SOD and POD activity were increased, and an increase of soluble protein level in pakchoi plants was observed under Cd + ES + UR21 treatment. Importantly, eggshell and UR21 alone or in combination induced a decline of Cd content in pakchoi plants, especially that Cd + ES + UR21 treatment decreased Cd content in shoot and root by 26.96% and 42.91%, respectively. Meanwhile, the soil urease and sucrase activities were enhanced. Generally, the combined application of ureolytic bacteria UR21 and eggshell exhibited better effects than applied them individually in terms of alleviating Cd toxicity in pakchoi plants. Our findings may give a unique perspective for an eco-friendly and sustainable strategy to remediate heavy metal-polluted soils.

The potential effectiveness of mixed bacteria-loaded biochar/activated carbon to remediate Cd, Pb co-contaminated soil and improve the performance of pakchoi plants.

Cadmium (Cd) and lead (Pb) are toxic heavy metals that cause severe soil pollution and pose health risks to humans. It is urgent to develop feasible strategies for Pb and Cd remediation. In this study, a bacteria consortium (Enterobacter asburiae G3, Enterobacter tabaci I12 and Klebsiella variicola J2 in a 1:3:3 proportion) with optimal Cd, Pb adsorption ability was constructed and immobilized on biochar (BC)/activated carbon (AC) via physisorption and sodium alginate encapsulation. The effects of mixed bacteria-loaded BC/AC on Cd and Pb remediation were investigated. The results indicated that their application reduced the DTPA-extractable Cd, Pb in soil by 22.05%-55.84% and 31.64%-48.13%, respectively. The residual Pb, Cd were increased while the exchangeable fractions were decreased. Soil urease, catalase and phosphatase activities were enhanced and soil bacterial community was improved, indicating a soil quality improvement. Consequently, the biomass of pakchoi plants was significantly increased. Cd and Pb in the shoots of pakchoi plants were decreased by 28.68%-51.01% and 24.18%-52.87%, respectively. Collectively, the bacteria-loaded BC/AC showed superior performance than free bacteria, BC and AC alone. Our study may provide a better understanding of the development of green and sustainable materials for remediation of heavy metal by the combination of BC/AC and functional bacteria.

Mechanism of selective gold adsorption on ion-imprinted chitosan resin modified by thiourea.

Thiourea-modified chitosan-imprinted resin (IM-TUCS) and a corresponding nonimprinted resin (NIM-TUCS) were synthesized and characterized using adsorption experiments. The adsorption results showed that adsorption reached equilibrium within 4 h. The adsorption data were better fitted using the Langmuir model (R2>0.99), and the gold adsorption capacities of IM-TUCS and NIM-TUCS were 933.2 and 373.7 mg·g-1, respectively. The IM-TUCS adsorbent was more suitable for gold than other coexisting anions and cations. The possible mechanism underlying Au(Ⅲ) adsorption on IM-TUCS was further investigated using X-ray photoelectron spectroscopy (XPS), Fourier-transform infrared spectroscopy (FTIR), and X-ray diffraction analyses. The protonation of the amino group on the resin under low pH conditions promoted Au(Ⅲ) adsorption; O, N and S in the C‒OH, CË­S and C-NH2 groups contained in the IM-TUCS coordinated with Au(III) ions. The cross-linking of the imprinted resin provided holes that could hold Au(III), thus the imprinted resin supported more Au(III). The adsorption capacity of the IM-TUCS for Au(III) was significantly higher than that of the NIM-TUCS, which is attributed to the cross-linking of the imprinted resin. Moreover, the IM-TUCS showed specific recognition capabilities for Au(III). After elution with the eluent, IM-TUCS was reused for four cycles with a gold recovery rate of approximately 93%, revealing its high potential economic value.

Effects of ionic liquid [N4444] AOT on rice seedling growth cytomembrane damage and rhizobacteria resistance.

Ionic liquids (ILs) are solvents composed of ions, containing a large asymmetric cation with an anion. With increasing and widespread applications, the toxic effects of ILs have been considerable in recent years. This study explained the effects of the new functional ionic liquids [N4444] bis(2-ethylhexyl) sulfonyl succinate (AOT) on rice seedling and the growth of rhizobacteria. The rice seeds pretreated by [N4444] AOT revealed that it exhibited a significant negative impact on rice seedlings. The inhibition of rice growth increased with increasing concentration. When the concentration of [N4444] AOT increased to 0.25 and 0.5 mL L-1, the germination potential decreased by 40.0% and 86.3%, respectively, compared with the control. The germination potential and germination rate of rice were reduced, and the stress effect of ionic liquid on the root parts was higher than the aerial parts. The biomass of rice seedlings was decreased by 34.8 to 91.2%. Iodinic propane staining showed that by increasing concentration, the root cell cytomembrane damage level was increased and also changed the cell shapes, especially under 0.25 mg L-1 concentration stress. However, rhizobacteria of rice showed strong [N4444] AOT-resistant characteristics when the concentration was reached to 120 mg L-1. The ILs even more promoted the growth of Enterobacter sp. NP1142 and Pantoea sp. BR23. It was indicated that IL [N4444] AOT can be degraded easily by rhizobacteria to eliminate the eco-risk of ILs.

Effect of exogenous silicon and methyl jasmonate on the alleviation of cadmium-induced phytotoxicity in tomato plants.

In the present study, a hydroponic experiment was performed to evaluate the effect of exogenous silicon (Si) and methyl jasmonate (MeJA) on the mitigation of Cd toxicity in tomato seedlings. The results revealed that Cd-stressed plants exhibited growth inhibition, increased lipid peroxidation, and impaired photosynthetic pigment accumulation. However, Si and MeJA applied alone or in combination significantly ameliorated the above-mentioned adverse effects induced by Cd. Among all treatments, Cd+Si+MeJA treatment elevated the dry mass of roots, stems, and leaves by 317.39%, 110.85%, and 119.71%, respectively. The chlorophyll a, chlorophyll b, and carotenoid contents in Cd+Si+MeJA-treated group were dramatically elevated (p < 0.05). Meanwhile, the malondialdehyde content in roots and shoots were reduced by 32.24% and 69.94%, respectively. The Si and MeJA applied separately or in combination also resulted in a prominent decrease of Cd influxes in tomato roots; therefore, a reduction of Cd content in tomato tissues were detected, and the Cd concentration in tomato roots were decreased by 27.19%, 25.18%, and 17.51% in Cd+Si, Cd+MeJA and Cd+Si+MeJA-treated plants, respectively. Moreover, in Cd+Si+MeJA-treated group, the percentage of Cd in cell wall fraction was enhanced while that in organelle fraction was decreased as compared with Cd-stressed plants. Collectively, our findings indicated that Si and MeJA application provide a beneficial role in enhancing Cd tolerance and reducing Cd uptake in tomato plants.

Study of soil microorganisms modified wheat straw and biochar for reducing cadmium leaching potential and bioavailability.

The application of crops straw and biochar in trace metals remediation from the contaminated environment attracted more and more attention during the past decade. Although there has been some review work on the mechanism of trace metals stabilization by crops straw, the effects and mechanisms of interaction among soil indigenous-microbes and crops-straw for trace metal adsorption and stabilization is still unclear. In this study, the dynamic effects along with potential mechanisms of wheat-straw (WS), wheat-straw biochar (WBC) and biologically modified wheat-straw (BMWS) were conducted to investigate the adsorption, leaching behaviour, chemical fractions and bioavailability of cadmium (Cd). The results showed that the biosorption capacity (qe) was most elevated in the BMWS treatment (14.42 mg g-1) as compared to WBC (6.28 mg g-1) and WS (4.20 mg g-1). The application of BMWS, WBC and WS at the rate of 3% significantly reduced Cd concentration in leachate to 53, 45 and 21% respectively, as compared to control. The addition of BMWS reduced the exchangeable Cd fraction resulted an increase in organic matter and carbonate bound Cd fraction in the soil. The DTPA extractable Cd was significantly decreased by 31.2 and 28.6% with the application of BMWS and WBC at 3% w/w respectively as compared to control. The research results may provide a novel perceptive for the development of functional materials and strategies for eco-friendly and sustainable trace metal remediation in contaminated soil and water by combination of straw and soil-indigenous microorganisms.

Prospects and applications of plant growth promoting rhizobacteria to mitigate soil metal contamination: A review.

The rapid increase in world population has generated the issues of hunger, poverty, food insecurity and malnutrition. To meet the challenge of increased food production of better quality, the farmers were compelled to use more chemical fertilizers, especially in developing countries. The higher use of chemical fertilizers interrupts the food chain through eutrophication, the polluting air and soil by incorporating metals. Trace metals have a deleterious effect on soil microbial and plant growth. To minimize metal toxicity and maximize the production of food, there are different approaches that can lead to lessen the use of chemical fertilizers. Plant growth promoting rhizobacteria (PGPR) are capable to enhance the plant growth and can remediate metal contaminated soils. PGPR has the ability to improve food production with diverse attributes e.g. producing siderophores that promote rhizosphere trace metal sequestration and production of organic and inorganic acids thus affecting trace metal bioavailability and plant induced systemic tolerance (IST) to limit the crop metal accumulation. In this review paper, we have discussed the biological approach which is environmentally friendly and cost-effective mean for metal polluted soils and gives some new insights for safety use of PGPR in trace metal contaminated fields.

Hydrogen sulfide decreases Cd translocation from root to shoot through increasing Cd accumulation in cell wall and decreasing Cd2+ influx in Isatis indigotica.

Hydrogen sulfide (H2S), a small gaseous signalling molecule, plays a pivotal role in the plant response to heavy metal stress. Here, we revealed a novel mechanism of Isatis indigotica resistance to cadmium (Cd) stress, in which H2S promotes Cd accumulation in the root and decreases the long-distance transport of Cd from the root to shoot. Cd significantly inhibited Isatis indigotica growth and induced the endogenous H2S level. Application of NaHS (a H2S donor) alleviated the effects of Cd. NaHS restriction of the translocation factor of Cd, elevated the Cd content in roots and depressed the Cd content in shoots. Cd stress decreased the cellulose and pectin contents in the cell wall, but NaHS restored the effect of Cd on the cell wall components. The Cd2+ fluxes were detected by noninvasive microtest technology (NMT). The data showed that NaHS pretreatment decreased the Cd2+ influx and proportion of the Cd content in organelles. We analyzed the effect of NaHS on the metallothionein and phytochelatin (PC) contents in roots and found that the PC and metallothionein1A (MT1A) contents were induced by NaHS. Additionally, the chemical forms of Cd2+ were changed by NaHS. Thus, H2S alters the content of cell wall component, improves Cd accumulation in the cell wall, depresses Cd2+ transmembrane movement, induces the synthesis of metallothioneins and decreases the toxicity of intracellular Cd. Our finding has great value to reduce the loss of Isatis indigotica resulted by heavy metals stress.

Dynamic changes in atrazine and phenanthrene sorption behaviors during the aging of biochar in soils.

The objective of this study was to elucidate the dynamic changes in the properties of biochar-amended soil and their sorption capacity for typical organic contaminants with increasing contact time between biochar and soil. To do so, biochars that were produced from pig manure at two temperatures were added to two soils, and the sorption behaviors of atrazine and phenanthrene (Phen) on soil-biochar mixtures aged for different times were investigated. Soils freshly amended with biochars showed a dramatic increase in the sorption of atrazine (up to 23.4 times at C e = 0.01 S w) and Phen (up to 3.12 times at C e = 0.01 S w) compared to the bare soils without biochars. The physicochemical properties of soil-biochar mixtures changed with aging time, which in turn affected the sorption capacity. After the biochar produced at 300 °C (BC300) was aged in soil, the sorption of atrazine and Phen by black soil (BS) and fluvo-aquic soil (FS) both increased by different extents, except the sorption of Phen on BS. However, after the biochar produced at 700 °C (BC700) was aged in soil, the sorption of atrazine on the two soils decreased markedly, which was sill 56.3% higher than that on the original soil, while an opposite trend was observed for Phen on the two soils. The complex change patterns could be due to the different dominant sorption mechanisms for different biochars and chemicals.

Aging effect of minerals on biochar properties and sorption capacities for atrazine and phenanthrene.

Biochars that were produced from pig manure at two different temperatures were incubated with three different minerals to examine the effects of soil minerals on biochar properties and sorption capacities. Biochars freshly mixed with minerals showed dramatic decreases in the sorption of atrazine (maximum decrease by 70.9% at Ce = 0.5 Sw) and phenanthrene (maximum decrease by 69.5% at Ce = 0.5 Sw) compared to unmixed biochars. Following the incubation, minerals were tightly attached to the biochar surface and insert into inner pores, thus changing the elemental composition and surface area of biochars in a manner dependent on the types of biochars and minerals involved. The changes in biochar properties in turn affected biochar sorption capacities. The sorption of both atrazine and phenanthrene by the pig manure biochar produced at 300 °C (BC300) decreased after aging due to an increase in surface hydrophilicity. In contrast, the sorption of atrazine and phenanthrene by BC700 increased after aging with minerals, which could be attributed to the increase in surface area caused by the minerals. However, the sorption capacities of the aged BC700 were still lower than those of the fresh BC700.

Effect of aging in field soil on biochar's properties and its sorption capacity.

Due to its high sorption capacity for different kinds of contaminants, biochar is advocated as a novel remediation strategy for contaminated soils. However, it is not clear how long this extraordinary sorption capacity will be maintained after the biochar is applied to the soil. In this study, a commercial biochar was applied to an agricultural soil, and the sorption of atrazine and phenanthrene on biochar amended soils with different aging periods ranging from 0 to 2 y was investigated. The application of fresh biochar in soil led to an obvious enhancement of the sorption coefficients (Kd) of atrazine and phenanthrene (by 3.13 and 2.93 times at Ce = 0.01 Sw, respectively) compared with the untreated soil. The surface area of biochar first increased and then decreased with aging time. Correspondingly, the sorption of atrazine and phenanthrene on the biochar amended soils first increased and then decreased markedly. Based on the changing trend of the Kd values with aging time, it could be predicted that the sorption capacity of biochar amended soils will decrease to the level of the untreated soil after 2.5 y.

Desorption of atrazine in biochar-amended soils: Effects of root exudates and the aging interactions between biochar and soil.

The effects of wheat root exudates and the aging interactions between biochar and soil on atrazine desorption from biochar-amended soil were carefully examined. Compared with CaCl2 solution, wheat root exudates significantly increase the desorption of atrazine from biochar, mainly by promoting the desorption of atrazine adsorbed on biochar with specific forces. Wheat root exudates were effectively separated into three components with different electrical properties, namely, anionic, neutral, and cationic components. Mainly due to the carboxyl-containing compounds, the anionic component was the main active component in the wheat root exudates that enhances the desorption of atrazine from the biochar. Additionally, wheat root exudates can increase the desorption of atrazine from biochar-amended soil. The promotion of atrazine desorption by root exudates was more obvious in soils with low organic matter contents, where atrazine was mainly adsorbed by biochar. The aging interaction between the biochar and soil increased the total desorption rate and rapid desorbing fraction of the atrazine in the soil, most likely due to the reduction of the biochar sorption capacity in the aged biochar-amended soil.