Aging reduces the bioavailability of copper and cadmium in soil immobilized by biochars with various concentrations of endogenous metals

Review: Strategies for limiting dietary cadmium in cereals

Cadmium (Cd) is a toxic metal, which in some production areas reaches levels above allowed limits in cereals. Thus, reducing its concentration in cereals is crucial for mitigating health risks and complying with food safety regulations. This review evaluates strategies to reduce Cd accumulation in cereal grains by mitigating soil Cd contamination and its bioavailability to plants. It covers methods for Cd estimation in soil and explores biological, chemical, and genetic approaches to limit Cd uptake by crops. The effectiveness of these strategies depends on genetic factors, soil properties, and crop type. Key approaches include traditional breeding, genome editing, digital and predictive soil mapping, and silicon (Si) and selenium (Se) supplementation. Traditional breeding, enhanced by modern genetic tools, enables the development of high-yielding, low-Cd cultivars but is time-consuming. Genome editing, particularly CRISPR-Cas9, offers precise gene modifications to reduce Cd uptake but faces regulatory constraints. Digital and predictive soil mapping provide high-resolution maps for targeted interventions but require extensive calibration. Silicon supplementation is a promising approach, as it competes with Cd for uptake sites, and limits Cd translocation to edible plant parts. Additionally, Si enhances plant tolerance to abiotic stresses, making it a multifunctional solution. Selenium supplementation can also reduce Cd accumulation while offering health benefits. However, the effectiveness of both Si and Se vary with dosage and crop type. An integrated approach combining these strategies is essential for effective Cd reduction in cereals. Continued research, technological advancements, and supportive policies are crucial for ensuring safe and sustainable cereal production.

Balancing application of plant growth-promoting bacteria and biochar in promoting selenium biofortification and remediating combined heavy metal pollution in paddy soil

Plant-growth-promoting bacteria (PGPB) and biochar have attracted increasing attention for remediating the combined pollution of arsenic (As) and cadmium (Cd) and promoting selenium (Se) biofortification. However, their differing effects on the bioavailability of As, Cd, and Se and their absorption by rice are still poorly understood. In this study, PGP Agrobacterium sp. T3F4 with Se- oxidizing capacity and corn straw biochar were applied to natively polluted paddy soil. Strain T3F4 reduced the bioavailability of As in soil but increased bioavailable Se, decreasing the As content in rice by 16.8% and improving the Se content of rice by 54.5% (p < .05). Application of 2.5% biochar stimulated iron (Fe) plaque formation of the root and immobilized As and Cd in the soil, decreasing the As and Cd absorption of rice by 14.7% and 40.3%, respectively (p < .05). Application of 5.0% biochar achieved similar mitigation effects for As and Cd but also decreased the Se content in rice by 60.6% by reducing bioavailability. This decrease in Se uptake was mitigated when 5.0% biochar was co-applied with strain T3F4. Notably, applying strain T3F4 also alleviated the oxidative stress on rice plants and enhanced soil enzyme activities, contributing to a substantial increase in grain yield in the polluted paddy soil. The adverse effects of 5.0% biochar on soil health and grain yield were mitigated by the co-application of strain T3F4. Our results provide new insights into applying PGPB and biochar for Se biofortification and As and Cd remediation in paddy soil.

Effect of natural aging on biochar physicochemical property and mobility of Cd (II)

This project utilized both field experiment and laboratory analyses to address the gap in understanding regarding the alterations in properties and functions of biochar, and the impact of heavy metal passivation in soil over long-term natural field aging. The study aimed to examine the changes in the physical and chemical characteristics of biochar over an extended period of natural aging. Additionally, it sought to analyze the impact and mechanisms of biochar in reducing of the harmful effects of the heavy metal cadmium (Cd) during the aging process. Both original and aged biochar conformed to the pseudo-second-order kinetics model and the Langmuir model. The aging process enhanced the adsorption of Cd by biochar and mitigated the leaching of Cd2+ into the soil. These findings provide a scientific basis for evaluating biochar's environmental behavior and its potential use in the remediation of soil contaminated with heavy metals.

Exploring adsorption capacity and mechanisms involved in cadmium removal from aqueous solutions by biochar derived from euhalophyte

Biochar has shown potential as a sorbent for reducing Cd levels in water. Euhalophytes, which thrive in saline-alkali soils containing high concentrations of metal ions and anions, present an intriguing opportunity for producing biochar with inherent metal adsorption properties. This study focused on biochar derived from the euhalophyte Salicornia europaea and aimed to investigate its Cd adsorption capacity through adsorption kinetics and isotherm experiments. The results demonstrated that S. europaea biochar exhibited a high specific surface area, substantial base cation content, and a low negative surface charge, making it a highly effective adsorbent for Cd. The adsorption data fit well with the Langmuir isotherm model, revealing a maximum adsorption capacity of 108.54 mg g-1 at 25 °C. The adsorption process involved both surface adsorption and intraparticle diffusion. The Cd adsorption mechanism on the biochar encompassed precipitation, ion exchange, functional group complexation, and cation-π interactions. Notably, the precipitation of Cd2+ with CO32- in the biochar played a dominant role, accounting for 73.7% of the overall removal mechanism. These findings underscore the potential of euhalophytes such as S. europaea as a promising solution for remediating Cd contamination in aquatic environments.

Activation of endogenous cadmium from biochar under simulated acid rain enhances the accumulation risk of lettuce (Lactuca sativa L.)

Biochar has been widely applied to remediate heavy metal-contaminated soils, but the environmental risk of the endogenous pollutants in biochar remains unclear. Two biochars with different endogenous cadmium (Cd) concentrations were prepared from background soil (BCB) and contaminated soil (BCC), respectively. We studied the effects of simulated acid rain (SAR) on the activation mechanism of endogenous Cd in biochar and Cd uptake of Cd by lettuce from the biochar-amended soils. SAR aging significantly increased Cd bioavailability by 27.5 % and 53.9 % in BCB and BCC, respectively. The activation of Cd from biochar may be due to the decrease of biochar pH and persistent free radicals (PFRs) and the increase of specific surface area (SSA) and O-contained functional groups in biochars. Two biochars at dosages of 2 % and 5 % rates did not change soil pore water Cd, but BCB and BCC at 10 % increased pore water Cd by 17.3 % and 219 %, respectively after SAR aging. SAR aging significantly increased the bioavailability of Cd in BCB and BCC treated soils than those before SAR aging. BCB application enhanced the biomass of lettuce (Lactuca sativa L.) and decreased the uptake of Cd. However, BCC addition at 10 % decreased the biomass of lettuce and increased the accumulation of Cd. In summary, endogenous Cd in biochar from contaminated soils has a potential environmental risk to plants and human health and the negative effects of endogenous pollutants from the biochars should be further investigated.

The molecular markers provide complementary information for biochar characterization before and after HNO3/H2SO4 oxidation

Biochar inevitably goes through long-term aging under biotic and abiotic processes in the environment, which results in various changes in its physicochemical properties. However, the traditional characterization methods based on particle separation cannot effectively monitor biochar in complex matrixes. Molecular markers, especially benzene polycarboxylic acids (BPCAs), can be used to directly identify the source and properties of biochar. In this study, biochars were prepared using corn straw (CS) and pinewood (PW) and were oxidized with HNO₃/H₂SO₄ to simulate the aging processes. Molecular markers of lignin-derived phenols showed that PW has more vanillyl unit and thus more stable than CS. The overall BPCAs content and the relative content of mellitic acid (B6CA) both increased with pyrolysis temperature, indicating increased aromatic condensation/aromaticity. The pristine CS biochar has a higher BPCAs content compared to PW biochar. HNO₃/H₂SO₄ treatment greatly decreased the lignin components and more vanillyl and cinnamyl units were removed from CS biochar than PW biochar. In addition, BPCAs contents decreased by 41-60 mg/g for CS biochar, while increased by 86-133 mg/g for PW biochar after HNO₃/H₂SO₄ oxidation. This is owing to the release of the condensed aromatic structures in CS biochars, but the concentration of the condensed aromatic structures in PW biochars after oxidation. These results showed that PW biochars are more stable than CS biochars. The application of the molecular markers can help understanding the dynamic change of biochar in the environment.

A review of pristine and modified biochar immobilizing typical heavy metals in soil: Applications and challenges

In recent years, the application of biochar in the remediation of heavy metals (HMs) contaminated soil has received tremendous attention globally. We reviewed the latest research on the immobilization of soil HMs by biochar almost in the last 5 years (until 2021). The methods, effects and mechanisms of biochar and modified biochar on the immobilization of typical HMs in soil have been systematically summarized. In general, the HMs contaminating the soil can be categorized into two groups, the oxy-anionic HMs (As and Cr) and the cationic HMs (Pb, Cd, etc.). Reduction and precipitation of oxy-anionic HMs by biochar/modified biochar are the dominant mechanism for reducing HMs toxicity. Pristine biochar can effectively immobilize cationic HMs. The commonly applied modification method is to add substances that can precipitate HMs to the biochar. In addition, we assessed the risks of biochar applications. For instance, biochar may cause the leaching of certain HMs; biochar aging; co-transportation of biochar nanoparticles with HMs. Future work should focus on the artificial/intelligent design of biochar to make it suitable for remediation of multiple HMs contaminated soil.

Enhanced immobilization of cadmium and lead adsorbed on crop straw biochars by simulated aging processes

Aging is an important natural process affecting the physiochemical properties of biochar, while mechanistic understanding of its effect on the adsorbed heavy metals is still lacking. After adsorption of Cd²⁺ and Pb²⁺, biochars produced from wheat straw (WS) and maize straw (MS) at 300 and 500 ^°C (denoted as WS300, WS500, MS300, and MS500, respectively) were subjected to 60 cycles of wet-dry or freeze-thaw aging. The results showed that simulated aging treatment transformed the Cd²⁺ and Pb²⁺ adsorbed on the low-temperature biochars from the readily and potentially bioavailable fractions into the non-bioavailable one, while the fractionation of Cd²⁺ and Pb²⁺ adsorbed on WS500 and Pb²⁺ on MS500 barely changed. Spectroscopic characterization revealed that simulated aging enhanced the complexation of Cd²⁺ and precipitation of Pb²⁺ on the biochars. These findings suggest that heavy metals could be effectively immobilized on low-temperature biochars amended to contaminated soils in the long term.

Contrasting effects of dry-wet and freeze-thaw aging on the immobilization of As in As-contaminated soils amended by zero-valent iron-embedded biochar

Zero-valent iron-embedded biochar (ZVI/BC) is considered as an effective material for arsenic (As) immobilization in soil, but the stability of As after remediation against aging remains unknown. Herein, the effects of dry-wet and freeze-thaw aging on the immobilization of As in two As-contaminated soils amended by ZVI/BC were evaluated. ZVI/BC showed high immobilization capacity for As-contaminated soils with an over 82% decrease of bioavailable As, mainly due to the As-Fe co-precipitation accompanied with ZVI oxidation. The aging of dry-wet and freeze-thaw had an opposite effect on the bioavailability of As. After 35 rounds of dry-wet aging, bioavailable As concentration increased from 1.25-9.50 to 1.83-21.75 mg/kg, because of the oxidation dissolution of ZVI and the formation of mobile reduced As(III). By contrast, the crystallization of amorphous iron with the structural incorporation of sorbed As and the oxidation of As(III) into stable As(V) occurred during the 35 rounds of freeze-thaw aging, leading to the decrease of bioavailable As concentration from 9.50-1.25 to 5.42-0.45 mg/kg. Our results revealed that the stability of soil As after remediation by ZVI/BC varied with the different aging process, which needs more consideration for the long-term soil As immobilization in the different whether areas.

High efficiency electrochemical disinfection of Pseudomons putida using electrode of orange peel biochar with endogenous metals

The electrochemical disinfection efficiency of Pseudomons putida was studied using ruthenium iridium coated titanium (RICT) electrode as anode and carbonized orange peel biochar (OPB) or graphite as the cathode. The results indicated that RICT/OPB system induced 6.5 and 7.0 log of P. putia inactivation after 60 s at 2 V and 45 s at 10 V, respectively. RICT/OPB system showed better efficiency than RICT/graphite system. The energy consumption of OPB cathode (17.5 Wh m^-3 per log) was significantly lower than that of graphite cathode (23.09 Wh m^-3 per log). Both anode and cathode played great roles on the disinfection. The anode absorbed electric energy to generate electrical hole, which can oxidize chloride ions to chlorine free radicals. The continuous porous structure of OPB can provide more adsorption sites and reduce electrolyte transport resistance, resulting in more Cl· production. Moreover, P. putia was much easier adsorbed to the anode surface in the RICT/OPB system because of the stronger electrostatic repulsion between cells and OPB cathode. As a result, P. putia was more easily inactivated by the Cl· produced on the anode. Besides chlorine active species, superoxide radical (O₂·﹣) produced on surface of cathode may also result in P. putia inactivation. The endogenous CuO in OPB can induce persistent free radicals (PFRs) production during pyrosis process. O₂·﹣ can be produced by O₂ activation through the function of Cu₂O/CuO and PFRs existed in OPB cathode. The more superoxide radical production led to the better disinfection effect than the graphite cathode. As a consequence, OPB electrode showed high efficiency electrochemical disinfection of P. putida.