Effects of biochar, zeolite and mycorrhiza inoculation on soil properties, heavy metal availability and cowpea growth in a multi-contaminated soil

Enhanced remediation of heavy metal-contaminated soils using biochar and zeolite combinations with additives: A meta-analysis

Soil heavy metal (HM) contamination is a major concern in agricultural lands due to its potential to enter the food chain and its adverse health effects. Remediation materials such as biochar (BC) and zeolites (ZE) have been studied for their potential to mitigate risks associated with soil HM contamination. This meta-analysis evaluates changes in the availability of Cd, Cu, Pb, and Zn following the application of BC and ZE to soil, whether applied individually, in combination (BC + ZE), or with additives (BC + ZE + A). Individually, BC reduced the availability of Cd, Cu, Pb, and Zn in soil by 24.0%, 33.0%, 31.3% and 10.1%, respectively; and ZE reduced these levels by 32.4%, 18.8%, 20.3% and 38.9%. Results indicate that, on average, BC + ZE effectively decreases the availability of Cd, Cu, Pb, and Zn in soils by 32.6%, 54.3%, 35.4%, and 18.3%, respectively. The combination with additives, BC + ZE + A, reduced the Cd and Pb availability by 54.2% and 20.9%, respectively. Most studies were undertaken with Cd, representing 59% of observations, followed by Pb, Zn, and Cu, respectively, with 29%, 8%, and 5%. The small number of studies with Pb, Zn and Cu prevented the creation of subgroups involving these three HMs. Notably, the nature of the additive influences the variation in available Cd content in remediated soils. Inorganic additives combined with BC + ZE demonstrated greater effectiveness in Cd remediation, achieving reductions of available content by 86.8%, compared to those containing clay minerals or organic compounds, with reductions of 27.4% and 15.4%, respectively. These findings enhance our understanding of how BC and ZE can be utilized in soil HM remediation and their effectiveness against different metals.

The effects of amendments on cd and pb under different fertilizer application conditions

Fertilizer application is a common agricultural practice that enhances soil fertility but can also increase heavy metal mobility in contaminated soils. This study used a pot experiment with four vegetables (water spinach, Chinese cabbage, lettuce, and garland chrysanthemum) to evaluate the impact of BC, ZE, and their combination (CO) on Cd and Pb levels under different fertilization schemes. Results showed that CO treatment significantly enhanced enzyme activities, increasing urease by 2.6-31.6% and catalase by 1.37-14.24% under varying fertilizer conditions. However, sucrase activity increased only with compound fertilizers. The use of compound fertilizers alone raised Cd and Pb levels in vegetable shoots by 0.65 mg·kg⁻¹ and 12.76 mg·kg⁻¹, respectively, while the CO amendment effectively mitigated these increases. BCR sequential extraction indicated that BC and CO shifted Cd and Pb into more stable soil fractions, reducing their bioavailability. Specifically, CO reduced Pb accumulation in shoots by 24.8-49.7%, with BC showing particular efficacy in reducing Cd levels. These findings highlight that BC and ZE, particularly in combination, offer an effective strategy for remediating heavy metal-contaminated soils in agricultural systems, especially when chemical fertilizers are used.

Synthesis and Characterization of NaX Zeolite from Coal Gangue and Its Efficacy in Cd and Pb Remediation in Water and Soil

Alkaline fusion is a pivotal process influencing the cost of synthesizing zeolite from coal gangue. This study examined the effects of alkaline fusion temperature (X 1), treatment duration (X 2) and the NaOH/coal gangue weight ratio (X 3) on the composition and properties of the products, as well as their adsorption capacities for Cd2+ (q e,Cd) and Pb2+ (q e,Pb). Response surface methodology (RSM) was employed to analyze the interactions among these factors, and the adsorption mechanisms for Cd2+ and Pb2+ were investigated using X-ray diffraction, scanning electron microscopy-EDS, Fourier transform infrared, X-ray photoelectron spectroscopy, and N2 adsorption-desorption techniques. The results reveal that (1) under optimized conditions-X 1 of approximately 800 °C, X 2 of around 2.8 h and X 3 of 1.2-the maximum q e,Cd and q e,Pb for the synthesized NaX zeolite can reach 181.3 and 419.9 mg/g, respectively. (2) The RSM models indicate that increasing the X 3 value can lower the required X 1. For q e,Cd and q e,Pb of 150 and 350 mg/g, respectively, with X 2 fixed at 2 h, increasing X 3 from 0.976 to 1.134 and from 0.900 to 1.289 enables a reduction in X 1 from 800 to 600 °C. (3) NaX zeolite primarily adsorbs Cd2+ and Pb2+ through ion exchange, allowing these ions to enter the zeolite's cage structure. Pb2+ can also precipitate as hydrocerussite (Pb3(CO3)2(OH)2) within the zeolite channels, while Cd2+ has a more significant impact on the [SiO4] and [AlO4] tetrahedra. (4) The synthesized NaX zeolite effectively reduces the exchangeable Cd content in contaminated soil from 3.51 to below 1.5 mg/kg. The remediation performance of the NaX zeolite for Cd and Pb in water and soil can be further enhanced by optimizing its Si/Al ratio and pore structure.

Composites Based on Natural Zeolites and Green Materials for the Immobilization of Toxic Elements in Contaminated Soils: A Review

Soil contamination by toxic elements is a global problem, and the remediation of contaminated soils requires complex and time-consuming technology. Conventional methods of soil remediation are often inapplicable, so an intensive search is underway for innovative and environmentally friendly ways to clean up ecosystems. The use of amendments that stabilize the toxic elements in soil by reducing their mobility and bioavailability is one of the simplest and most cost-effective ways to remediate soil. This paper provides a summary of studies related to the use of composites based on natural zeolites and green materials for the immobilization of toxic elements in contaminated soils and highlights positive examples of returning land to agricultural use. The published literature on natural zeolites and their composites has shown that combinations of zeolite with biochar, chitosan and other clay minerals have beneficial synergistic effects on toxic element immobilization and soil quality. The effects of zeolite properties, different combinations, application rates, or incubation periods on toxic elements immobilization were tested in laboratory scale or field experiments, whereas the mobility of toxic elements in soil was evaluated by chemical extractions of toxic elements transferred to the plants. This review highlights the excellent potential of natural zeolites to be used as single or combined sustainable green materials to solve environmental pollution problems related to the presence of toxic elements.

Unlocking the potential of co-application of steel slag and biochar in mitigation of arsenic-induced oxidative stress by modulating antioxidant and glyoxalase system in Abelmoschus esculentus L

This study investigates our hypothesis that how effect of arsenic stress on okra (Abelmoschus esculentus L.) can be alleviated through the use of waste materials such as steel slag (SS) and corncob biochar (BC). Different growth variables, biochemical parameters, oxidative stress markers, enzymatic and non-enzymatic antioxidants and glyoxylase enzyme activities were assessed. When okra was exposed to As, there was a noticeable decrease in seedling length, biomass, relative water content, various biochemical attributes, however, electrolyte leakage and lipid peroxidation in okra were enhanced. The supplementation of SS and BC-either individually or in combination-improved the growth parameters and reduced oxidative stress markers. Application of SS and BC also lowered As accumulation in roots and shoots of okra mitigating adverse effects of As exposure. Additionally, the activities of antioxidant and glyoxalase enzyme increased when SS and BC were present, concurrently reducing methylglyoxal content. Arsenic-induced stress led to oxidative damage, an enhancement in both enzymatic and non-enzymatic antioxidants, induced the synthesis of thiol and phytochelatins in roots and shoots. These may play a vital function in alleviating oxidative stress induced by As. Superoxide dismutase, catalase, ascorbate peroxidase, and glutathione reductase activities were significantly enhanced in As-treated plants. These enhancement were further amplified when SS and BC were amended to As-treated okra. Therefore, synergistic application of SS and BC effectively protects okra against oxidative stress induced by As by increasing both antioxidant defense and glyoxalase systems. Both SS, an industrial byproduct, and BC, generated from agricultural waste, are cost-effective, environmentally friendly, safe, and non-toxic materials which can be used for crop production in As contaminated soil.

Potential role of apple wood biochar in mitigating mercury toxicity in corn (Zea mays L.)

Mercury (Hg) is a very toxic decomposition-resistant metal that can cause plant toxicity through bioaccumulation and oxidative damage. Biochar, derived from organic waste and agricultural garbage, is an on-site modification technique that can improve soil health in heavy metals-polluted regions. The present experiment was designed to explore the role of apple biochar in the management of mercury toxicity in corn (Zea mays cv. 'PL535'). Different levels of biochar derived from apple wood (0%, 2.5%, 5.0%, and 7.5% w/w) along with different Hg concentrations (0, 20, 40, and 60 mg/L) were used in the experiment that was based on a completely randomized design. Based on the results, HgCl2 at all rates reduced root and shoot dry weight and length, tolerance index, chlorophyll a and b content, the Hill reaction, and dissolved proteins and increased shoot and root Hg content (up to 72.57 and 717.56 times, respectively), cell death (up to 58.36%), MDA level (up to 47.82%), H2O2 (up to 66.33%), dissolved sugars, and proline. The results regarding enzymatic and non-enzymatic antioxidants revealed increases in total phenol and flavonoids content (up to 71.27% and 86.71%, respectively), DPPH free radical scavenging percentage, and catalase (CAT) and ascorbate peroxidase (APX) activity (up to 185.93% and 176.87%, respectively), in corn leaves with the increase in the Hg rate applied to the culture medium. The application of biochar to the substrate of the Hg-treated corns reduced Hg bioavailability, thereby reducing Hg accumulation in the roots (up to 76.88%) and shoots (up to 71.79%). It also reduced the adverse effect of Hg on the plants by increasing their shoot and root dry weight, photosynthesizing pigments, Hill reaction, and APX activity and reducing cell death, H2O2 content, and MDA content. The results reflected the capability of apple wood biochar at all rates in reducing Hg bioavailability and increasing Hg fixation in Hg-polluted soils. However, it was most effective at the rate of 7.5%.