Effects of fresh and field-aged holm-oak biochar on As, Cd and Pb bioaccumulation in different rice growing environments

Controlling As, Cd, and Pb bioaccumulation in rice under different levels of alternate wetting and drying irrigation with biochar amendment: A 3-year field study

One challenging task to produce rice that comply with the increasing demanding regulations, is to reduce, simultaneously, grain bioaccumulation of As, Cd, and Pb. A 3-year field experiment was conducted in a Mediterranean environment, to evaluate the effects on As, Cd, and Pb bioaccumulation in rice grain, of the adoption of two levels of alternate wetting and drying (AWD) irrigation conditions: moderate and intensive (reflooding at -20 kPa and -70 kPa soil matric water potential, respectively), relative to the traditional permanent flood irrigation. Plots were prepared with or without a one-time holm oak biochar application (35 Mg ha-1), in the first year of the study. Arsenic bioaccumulation decreased in rice grain in the AWD systems, both total and inorganic (AsInorg), with the lower values reached in the intensive AWD irrigation (0.131-0.151 mg kg-1 dry weight), when the drying conditions were more intense. For As, biochar contributed to a further reduction in the bioaccumulation in the first two years but lost its efficacy with the field aging after three years of its application. However, the transition to AWD irrigation led to a significant increase in Cd bioaccumulation in rice grain (21-fold increase in the more intensive system, whose values reached up to 0.127 mg kg-1), which can be counteracted by biochar application, to values statistically similar to those of permanent flooding. Contrariwise, the effects on Pb bioaccumulation were not so significant, but decreased with the transition to ADW irrigation, and with biochar application, relatively to the non-amended counterparts. Therefore, the implementation of intensive AWD with biochar represents a potentially fruitful strategy to enhance food safety of rice production, controlling, simultaneously, As, Cd, and Pb bioaccumulation. Nevertheless, new approaches need to be developed to attend the limits established for AsInorg to produce food for infants, even in uncontaminated soils.

Simultaneous immobilization strategy of anionic metalloids and cationic metals in agricultural systems: A review

Concurrent heavy metals remediation in natural environments poses significant challenges due to factors like metal speciation and interactions with soil moisture. This review focuses on strategies for immobilizing both anionic and cationic metals simultaneously in soil-crop systems. Key approaches include water management, biochar utilization, stabilizing agents, nanotechnology, fertilization, and bioremediation. Sprinkler or intermittent irrigation combined with soil amendments/biochar effectively immobilizes As/Cd/Pb simultaneously. This immobilization occurs through continuous adsorption-desorption, oxidation-reduction, and precipitation mechanisms influenced by soil pH, redox reactions, and Fe-oxides. Biochar from sources like wine lees, sewage sludge, spent coffee, and Fe-nanoparticles can immobilize As/Cd/Pb/Cr/Co/Cu/Zn together via precipitation. In addition, biochar from rice, wheat, corn straw, rice husk, sawdust, and wood chips, modified with chemicals or nanoparticles, simultaneously immobilizes As and Cd, containing higher Fe3O4, Fe-oxide, and OH groups. Ligand exchange immobilizes As, while ion exchange immobilizes Cd. Furthermore, combining biochar especially with iron, hydroxyapatite, magnetite, goethite, silicon, graphene, alginate, compost, and microbes-can achieve simultaneous immobilization. Other effective amendments are selenium fertilizer, Ge-nanocomposites, Fe-Si materials, ash, hormone, and sterilization. Notably, combining nano-biochar with microbes and/or fertilizers with Fe-containing higher adsorption sites, metal-binding cores, and maintaining a neutral pH could stimulate simultaneous immobilization. The amendments have a positive impact on soil physio-chemical improvement and crop development. Crops enhance production of growth metabolites, hormones, and xylem tissue thickening, forming a protective barrier by root Fe-plaque containing higher Fe-oxide, restricting upward metal movement. Therefore, a holistic immobilization mechanism reduces plant oxidative damage, improves soil and crop quality, and reduces food contamination.

Effects of remediation agents on rice and soil in toxic metal(loid)s contaminated paddy fields: A global meta-analysis

Toxic metal(loid)s contamination of paddy soil is a nonnegligible issue and threatens food safety considering that it is transmitted via the soil-plant system. Applying remediation agents could effectively inhibit the soil available toxic metal(loid)s and reduce their accumulation in rice. To comprehensively quantify how remediation agents impact the accumulation of Cd/Pb/As in rice, rice growth and yield, the accumulation of available Cd/Pb/As in paddy soil, and soil characteristics, 50 peer-reviewed publications were selected for meta-analysis. Overall, the application of remediation agents exhibited significant positive effects on rice plant length (ES = 0.05, CI = 0.01-0.08), yield (ES = 0.20, CI = 0.13-0.27), peroxidase (ES = 0.56, CI = 0.18-0.31), photosynthetic rate (ES = 0.47, CI = 0.34-0.61), and respiration rate (ES = 0.68, CI = 0.47-0.88). Among the different types of remediation agents, biochar was the most effective in controlling the accumulation of Cd/Pb/As in all portions of rice, and was also superior in inhibiting the accumulation of Pb in rice grains (ES = -0.59, 95 % CI = -1.04-0.13). This study offers an essential contribution for the remediation strategies of toxic metal(loid)s contaminated paddy fields.

Innovative accumulative risk assessment of co-exposure to Cd, As, and Pb in contaminated rice based on their in vivo bioavailability and in vitro bioaccessibility

The consumption of cadmium (Cd), arsenic (As), and lead (Pb) co-contaminated rice exposes humans to multiple heavy metals simultaneously, with relative bioavailability (RBA) and bioaccessibility (BAc) being important determinants of potential health risks. This study evaluated the relationship between in vivo RBA and in vitro BAc of Cd, As, and Pb in rice and their cumulative risk to humans. A total of 110 rice samples were collected in Zhejiang Province, China, and 10 subsamples with varying concentration gradients were randomly selected to measure RBA using a mouse model (liver, kidney, femur, blood, and urine as endpoints) and BAc using four in vitro assays (PBET, UBM, SBRC, and IVG). Our results indicated that Cd-RBA varied from 21.2 % to 67.5 %, As-RBA varied from 23.2 % to 69.3 %, and Pb-RBA varied from 22.2 % to 68.9 % based on mouse liver plus kidneys. The BAc values for Cd, As, and Pb in rice varied according to the assay. Compared to Cd and As, Pb exhibited a lower BAc in the gastric (GP) and intestinal (IP) phases. According to the relationship between the BAc and RBA values, IVG-GP (R2 = 0.92), SBRC-IP (R2 = 0.73), and UBM-GP (R2 = 0.80) could be used as predictors of Cd-, As-, and Pb-RBA in rice, respectively. The health risks associated with co-exposure to Cd, As, and Pb in contaminated rice for both adults and children exceeded the acceptable threshold, with Cd and As being the primary risk factors. The noncarcinogenic and carcinogenic risks were markedly reduced when the RBA and BAc values were incorporated into the risk assessment. Due to the risk overestimation inherent in estimating the risk level based on total metal concentration, our study provides a realistic assessment of the cumulative health risks associated with co-exposure to Cd, As, and Pb in contaminated rice using in vivo RBA and in vitro BAc bioassays.

Release behavior of fertilizers and heavy metals from iron-loaded sludge biochar in the aqueous environment

In this study, the release behavior of fertilizers (NH4+-N, PO43- and K) and heavy metals (Mn, Zn, Ni, Cu, Pb and Cr) from iron-loaded sludge biochar (ISBC) was investigated to evaluated the feasibility and risks of ISBC as a slow release fertilizer. Their release capacity was significantly enhanced with decreasing initial pH, increasing solid-liquid ratio (RS-L) and rising temperature (p < 0.05). When the initial pH, RS-L and temperature were separately 5 (fertilizers)/1 (heavy metals), 1:5 and 298 K, the final concentrations of NH4+-N, PO43-, K, Mn, Zn and Ni were 6.60, 14.13, 149.4, 53.69, 72.56, and 1.01 mg L-1, while the maximum concentrations of Cu, Pb and Cr were 0.94, 0.77, and 0.22 mg L-1, respectively. Due to the tiny difference between the R2 values, revised pseudo-first-order and pseudo-second-order kinetics models described their release behavior well, suggesting that physical and chemical interactions played an important role. Activation energies greater than 40 kJ mol-1 indicated that the rate-controlling steps of the release of NH4+-N, PO43- and Ni were chemical reactions, while chemical reactions and diffusion together determined the release rates of K, Mn, Zn, Cu, Pb and Cr because their activation energies were in the range of 20-40 kJ mol-1. The increasingly negative ΔG and positive ΔH and ΔS suggested that their release was a spontaneous (except Cr) and endothermic process with an increase of randomness between the solid-liquid interface. The release efficiency of NH4+-N, PO43- and K were in the ranges of 28.21%-53.97%, 2.09%-18.06% and 39.46%-66.14%, respectively. Meanwhile, the pollution index and evaluation index of heavy metals were in the ranges of 33.31-227.4 and 4.64-29.24, respectively. In summary, ISBC could be used as a slow-release fertilizer with low risk when the RS-L was less than 1:40.