Adsorption of Cd and Pb in contaminated gleysol by composite treatment of sepiolite, organic manure and lime in field and batch experiments

Predictive Machine Learning Model to Assess the Adsorption Efficiency of Biochar-Heavy Metals for Effective Remediation of Soil-Plant Environment

Biochar is crucial for agricultural output and plays a significant role in effectively eliminating heavy metals (HMs) from the soil, which is essential for maintaining a soil-plant environment. This work aimed to assess machine learning models to analyze the impact of soil parameters on the transformation of HMs in biochar-soil-plant environments, considering the intricate non-linear relationships involved. A total of 211 datasets from pot or field experiments were evaluated. Fourteen factors were taken into account to assess the efficiency and bioavailability of HM-biochar amendment immobilization. Four predictive models, namely linear regression (LR), partial least squares (PLS), support vector regression (SVR), and random forest (RF), were compared to predict the immobilization efficiency of biochar-HM. The findings revealed that the RF model was created using 5-fold cross-validation, which exhibited a more reliable prediction performance. The results indicated that soil features accounted for 79.7% of the absorption of HM by crops, followed by biochar properties at 17.1% and crop properties at 3.2%. The main elements that influenced the result have been determined as the characteristics of the soil (including the presence of different HM species and the amount of clay) and the quantity and attributes of the biochar (such as the temperature at which it was produced by pyrolysis). Furthermore, the RF model was further developed to predict bioaccumulation factors (BAF) and variations in crop uptake (CCU). The R2 values were found to be 0.7338 and 0.6997, respectively. Thus, machine learning (ML) models could be useful in understanding the behavior of HMs in soil-plant ecosystems by employing biochar additions.

Enhanced multi-metals stabilization: Synergistic insights from hydroxyapatite and peroxide dosing strategies

Sediment contamination by heavy metals is a pressing environmental concern. While in situ metal stabilization techniques have shown promise, a great challenge remains in the simultaneous immobilization of multi-metals co-existing in contaminated sediments. This study aims to address this challenge by developing a practical method for stabilizing multi-metals by hydroxyapatite and calcium peroxide (HAP/CaO2) dosing strategies. Results showed that dosing 15.12 g of HAP/CaO2 at a ratio of 3:1 effectively transformed labile metals into stable fractions, reaching reaction kinetic equilibrium within one month with a pseudo-second-order kinetic (R2 > 0.98). The stable fractions of Nickel (Ni), Chromium (Cr), and lead (Pb) increased by approximately 16.9 %, 26.7 %, and 21.9 %, respectively, reducing heavy metal mobility and ensuring leachable concentrations complied with the stringent environmental Class I standard. Mechanistic analysis indicated that HAP played a crucial role in Pb stabilization, exhibiting a high rate of 0.0176 d-1, while Cr and Ni stabilization primarily occurred through the formation of hydroxide precipitates, as well as the slowly elevated pH (>8.5). Importantly, the proposed strategy poses a minimal environmental risk to benthic organisms exhibits almost negligible toxicity towards Vibrio fischeri and the Chironomus riparius, and saves about 71 % of costs compared to kaolinite. These advantages suggest the feasibility of HAP/CaO2 dosing strategies in multi-metal stabilization in contaminated sediments.

Efficient and safe use of a slow-release Mn material for three sequential crops of rice in Cd-contaminated paddy soils

Traditional passivators reduce the effectiveness of Cd by ion exchange, chemisorption, and complexation in soil. However, traditional passivators have defects such as easy aging and poor durability, which not only reduce the treatment efficiency but also increase the risk of primary soil environmental pollution. For this reason, considering that Mn and Cd have physiological antagonism in rice, sepiolite-supported manganese ferrite (SMF) was prepared in this study to improve passivation persistence. The passivation mechanism, effect and duration of SMF were explored. The results showed that SMF has a dense and small pore structure and that the surface is rough, which provides abundant adsorption sites for Cd2+ adsorption. When the SMF adsorbs Cd2+, ions or functional groups in the material, such as MnOOH*, will exchange with Cd2+ to form Cd(OH)2 and other internal complexes. Indoor pure soil cultivation experiments showed that 0.1 % SMF can reduce the effective Cd content of soil by 41.32 %, demonstrating the efficiency of SMF. The three-crop rice experiments in pots showed that SMF could increase soil pH and continuously increase the content of available Mn in soil. Increasing the content of available Mn reduces the ability of rice to absorb Cd. In addition, the three-cropping rice experiments also indicated that the passivation effect of SMF materials on Cd-contaminated paddy fields was long-lasting and stable and that SMF is a more efficient and safe Cd passivation agent.

Effects of soil amendments on Cd and As mobility in the soil-rice system and their distribution in the grain

The accumulation, mobilization, and distribution of toxic metal(loid)s in rice are key factors that affect food security and determine bio-utilization patterns. In this study, five soil amendments with different components were used in paddy fields to study the key factors: organic amendments: (1) polyaspartic acid (OA1) and (2) organic fertilizer (OA2); inorganic amendments: (3) kaolinite (IA1) and (4) magnesium slag (IA2); and organic-inorganic composite amendments: (5) modified biochar/quicklime (OIA). Although the Cd and As exhibited opposite chemical dissolution behaviors, IA1/OIA, can simultaneously reduce their accumulation and transfer coefficients in rice tissues, while other amendments only work for one of them. The in situ distribution in grains showed that IA1/OIA changed the original Cd distribution in the lemma and palea, whereas all amendments reduced Cd accumulation in the germ. In contrast, OA1/IA2 amendments led to more As accumulation in the rice husks and bran than in the endosperm center, and the germ had higher As signals. Because of their similar transport pathways and interactions, the concentrations of Cd and As in the grains were correlated with a variety of mineral elements (Fe, Mo, Zn, etc.). Changes in the Cd/As concentration and distribution in rice were achieved through the improvement of soil properties and plant growth behavior through amendments. The application of OIA resulted in the highest immobilization indices, at 82.17 % and 35.34 % for Cd and As, respectively. The Cd/As concentrations in the rice grains were highly positively correlated with extractable-Cd/As in the soil (Cd: R2 = 0.95, As: R2 = 0.93). These findings reveal the migration and distribution mechanisms of Cd and As in the soil-rice system, and thus provide fundamental information for minimizing food safety risk.

Effects of biochar and zero valent iron on the bioavailability and potential toxicity of heavy metals in contaminated soil at the field scale

Heavy metals (HMs) such as copper, nickel and chromium are toxic, so soil contaminated with these metals is of great concern. In situ HM immobilization by adding amendments can decrease the risk of contaminants being released. A five-month field-scale study was performed to assess how different doses of biochar and zero valent iron (ZVI) affect HM bioavailability, mobility, and toxicity in contaminated soil. The bioavailabilities of HMs were determined and ecotoxicological assays were performed. Adding 5 % biochar, 10 % ZVI, 2 % biochar + 1 % ZVI, and 5 % biochar + 10 % ZVI to soil decreased Cu, Ni and Cr bioavailability. Metals were most effectively immobilized by adding 5 % biochar + 10 % ZVI, and the extractable Cu, Ni, and Cr contents were 60.9 %, 66.1 % and 38.9 % lower, respectively, for soil with 5 % biochar + 10 % ZVI added than unamended soil. The extractable Cu, Ni, and Cr contents were 64.2 %, 59.7 % and 16.7 % lower, respectively, for soil with 2 % biochar + 1 % ZVI added than unamended soil. Experiments using wheat, pak choi and beet seedlings were performed to assess the remediated soil toxicity. Growth was markedly inhibited in seedlings grown in extracts of soil with 5 % biochar, 10 % ZVI, or 5 % biochar + 10 % ZVI added. More growth occurred in wheat and beet seedlings after 2 % biochar + 1 % ZVI treatment than the control, possibly because 2 % biochar + 1 % ZVI simultaneously decreased the extractable HM content and increased the soluble nutrient (carbon and Fe) content of the soil. A comprehensive risk assessment indicated that adding 2 % biochar + 1 % ZVI gave optimal remediation at the field scale. Using ecotoxicological methods and determining the bioavailabilities of HMs can allow remediation methods to be identified to efficiently and cost-effectively decrease the risks posed by multiple metals in soil at contaminated sites.

Study on removal of copper ions from aqueous phase by modified sepiolite flocs method

With the improvement of environmental protection and standards, the recovery and recycling of copper ions released from industrial wastewater discharge has aroused sufficient research interest. A new adsorbent (ABsep) derived from natural sepiolite (Sep) by modification technology of hexadecyltrimethoxysilane (HDTMS) or tetraethylorthosilicate (TEOS) and flocs separation method for adsorption of Cu²⁺ in wastewater have been investigated in this paper. The changes of crystal structure and physicochemical properties of Sep during modification process showed that HDTMS and TEOS were loaded onto the Sep surface without inserting into the Sep interlayer structure. The adsorption experimental results presented a smaller amount of ABsep (1 g/L) achieved 97.5% removal of 50 mg/L Cu²⁺ at pH 6 and temperature of 298 K within a shorter contact time (50 min). It is interesting the adsorption process of Cu²⁺ on ABsep was spontaneous and exothermic, with physical adsorption dominating, as result of combination of physical adsorption, electrostatic gravitational force, and chemical reaction. Because of good physical-chemical adsorption performance of ABsep surface to Cu²⁺, high removal rates of Cu²⁺ from aqueous phase could be achieved after three adsorption regeneration cycles, this indicated the ABsep was expected to be a promising adsorbent of Cu²⁺ removal for wastewater treatment.

Natural field freeze-thaw process leads to different performances of soil amendments towards Cd immobilization and enrichment

Cadmium (Cd) soil pollution is a global issue affecting crop production and food safety. Remediation methods involving in-situ Cd immobilization have been developed, but their effectiveness can diminish under seasonal freeze-thaw aging processes. In this study, we assessed the field performance of four soil treatments at a seasonally frozen rice paddy. Amendments were applied at 2 wt%, including: (i) sepiolite (a 2:1 clay mineral), (ii) superphosphate, (iii) biochar (produced by rice husk at 500 °C for 2 h), and (iv) joint application of biochar & superphosphate (1:1 mixture by weight). Immobilization performance was determined as DTPA extractable Cd and plant uptake in various organs. Overall, the four treatments significantly reduced Cd bioavailability during the plant growth period, with average DTPA-extractable concentrations decreasing by 43%, 34%, 39% and 45% for the four treatments, respectively, relative to untreated soil (control). Rice grain yields from the superphosphate and the joint application treatments increased by 8.0% and 11.8%, respectively, and Cd accumulation within those grains reduced by 14.3% and 48.9%, respectively. During the winter non-growth period, freeze-thaw aging facilitated Cd mobilization, with DTPA-extractable Cd increasing by 16.9% in the control soil, relative to the initial period. However, this reduced to 10.9%, 14.4%, 7.6% and 5.0%, for the sepiolite, superphosphate, biochar and joint application treatments, respectively. Overall, the joint application of biochar and superphosphate provided the best performance in terms of both long-term Cd immobilization and rice production enhancement, offering a green remediation option for risk management at Cd contaminated rice paddies in seasonally frozen regions.

Effect of humic and calcareous substance amendments on the availability of cadmium in paddy soil and its accumulation in rice

Humic substances (HS) are widely known as important components in soil and significantly affect the mobility of metals due to their large surface area and abundant organic functional groups. Calcareous substances (CSs) are also commonly used as robust and cost-effective amendments for increasing the pH of acidic soils and decreasing the mobility of metals in soils. In this study, we developed a new remediation scheme for cadmium (Cd)-contaminated soil remediation by coupling HS and CS. The results showed that regardless of the addition of fulvic acid (FA), all the CS-containing treatments significantly increased the soil pH by 0.32-0.60, and the concentration of bioavailable Cd decreased in the moderately (field experiment soil, maximum 62%) and highly (pot experiment soil, maximum 57%) Cd-contaminated soils. The Cd content in rice (Oryza sativa L.) tissues significantly decreased after all the treatments. The bioaccumulation factors (BAFs) decreased by over 50% in the roots, stems, leaves and husks in all treatments, while the translocation factors (TFs) only significantly decreased in the highly contaminated soil. Among all treatments, the two HS+CS treatments (FA+CaCO₃ and FA+CaO) had the greatest effect on decreasing the concentration of bioavailable Cd in soil and Cd in brown rice grains. The suggested mechanism for the effectiveness of coupled HS and CS was that CS first mitigated the pH and precipitated Cd, followed by a complexation effect between HS and Cd. Although the Cd in rice grains in both cases was higher than the standard limit, HS+CS remediation can be advocated as a robust, simple and cost-effective scheme for Cd remediation if the additive dose is slightly increased, as this approach can simultaneously improve the pH of acidic soil and adsorb Cd in soil.

Remediation of Pb-Contaminated Soil Using a Novel Magnetic Nanomaterial Immobilization Agent

The management of heavy metal contaminated soil has received extensive research attention. In this study, a novel immobilization agent (SiO₂@Fe₃O₄@C-COOH) was combined with traditional immobilization agents (TIAs), i.e., CaO, organic matter (OM), and calcium superphosphate (CSP), and used to remediate Pb-contaminated soil. The immobilization effects of Pb in soil was evaluated through pot experiments involving wheat cultivation. The results indicated that SiO₂@Fe₃O₄@C-COOH delivered a higher Pb immobilization efficiency than did TIAs such as CaO, OM, and CSP. The application of SiO₂@Fe₃O₄@C-COOH in combination with TIAs (CaO, OM, and CSP) synergistically enhanced the Pb immobilization efficiency of the soil to 85.10%. Further, joint application in a 54.19% reduction of Pb content in wheat roots, a 65.78% reduction in stems, and a 47.96% in leaves. Thus, the combined application of SiO₂@Fe₃O₄@C-COOH and TIAs significantly reduced the bioavailability of Pb, achieved the purpose of Pb stabilization and soil remediation, and has the potential for wide-spread application in the remediation of Pb-contaminated soils.

Effects and mechanisms of Cd remediation with zeolite in brown rice (Oryza sativa)

The effects of zeolite amendment on the Cd absorption and accumulation of two varieties of rice, ZY-18 (Zheyou 18, hybrid indica) and DL-5 (Donglian 5, conventional indica), planted in a slightly Cd-contaminated paddy soil, and the associated mechanisms were studied based on a pot experiment. The results showed that zeolite addition significantly reduced the Cd availability in the soil by the increasing of soil pH and the Cd sorption capacity of soil, as well as the reduction of Eh, dissolved organic carbon (DOC) and the water-soluble Fe/Mn in the soil. The decrease in the amount of Fe and Cd in iron plaques on rice roots treated with zeolite resulted in the reduction of Cd uptake by rice roots. Zeolite application decreased the Cd contents in the roots, stems and leaves, and finally the brown rice through the inhibition of uptake and translocation of Cd by rice plant. The Cd concentration in rice grains of ZY-18 was more decreased than that of DL-5, which was attributed to its higher pH and lower Eh, DOC, and Cd availability in the soil, higher mounts of iron plaque and Cd in plaques on rice roots, and lower root-to-grain transfer factor (TF_root-grain) of ZY-18.

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

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

A phytoremediation coupled with agro-production mode suppresses Fusarium wilt disease and alleviates cadmium phytotoxicity of cucumber (Cucumis sativus L.) in continuous cropping greenhouse soil

Cadmium (Cd) contamination and continuous cropping obstacle often coexist in greenhouse soil and seriously restrict cucumber production. In this study, hyperaccumulator Sedum alfredii Hance was intercropped with spring cucumber (Cucumis sativus L.), then rotated with low accumulator water spinach and autumn cucumber under rational water regime, composited amendment was applied to soil before transplanting autumn cucumber. The results showed that, compared with conventional crop rotation system (Chinese cabbage and cucumber rotation), superposition management practice suppressed Fusarium wilt disease by 28.4 and 57.4% and increased yield by 35.2 and 383% for spring and autumn cucumbers, respectively. Meanwhile, photosynthetic characteristics, antioxidant system and fruit quality were significantly improved. Furthermore, this mode modified soil microbial community structure, enhanced soil enzyme activities, and simultaneously reduced soil total and phytoavailable Cd by 30.3 and 47.7%, respectively. These results demonstrated a feasible technical system to achieve phytoremediation coupled with argo-production in Cd contaminated greenhouse soil with continuous cropping obstacle and provided useful information for further revelation of interaction mechanisms between multicropping and comprehensive biofortification measurements.