The reduction of the As and Pb phytotoxicity of a former mine technosol depends on the amendment type and properties

Predicting the efficiency of arsenic immobilization in soils by biochar using machine learning

Arsenic (As) pollution in soils is a pervasive environmental issue. Biochar immobilization offers a promising solution for addressing soil As contamination. The efficiency of biochar in immobilizing As in soils primarily hinges on the characteristics of both the soil and the biochar. However, the influence of a specific property on As immobilization varies among different studies, and the development and application of arsenic passivation materials based on biochar often rely on empirical knowledge. To enhance immobilization efficiency and reduce labor and time costs, a machine learning (ML) model was employed to predict As immobilization efficiency before biochar application. In this study, we collected a dataset comprising 182 data points on As immobilization efficiency from 17 publications to construct three ML models. The results demonstrated that the random forest (RF) model outperformed gradient boost regression tree and support vector regression models in predictive performance. Relative importance analysis and partial dependence plots based on the RF model were conducted to identify the most crucial factors influencing As immobilization. These findings highlighted the significant roles of biochar application time and biochar pH in As immobilization efficiency in soils. Furthermore, the study revealed that Fe-modified biochar exhibited a substantial improvement in As immobilization. These insights can facilitate targeted biochar property design and optimization of biochar application conditions to enhance As immobilization efficiency.

High-carbon wood ash biochar for mine tailings restoration: A field assessment of planted tree performance and metals uptake

Unique properties of biochar render it appealing for revegetating and decontaminating historic, barren, and chemically complex mine tailings. Bottom ash from bioenergy facilities can contain high levels of charcoal residue, and thus qualify as a type of biochar; the wide availability of this material at low cost makes it of particular interest in the context of tailings remediation. Nevertheless, bottom ash is variable and often contains residual toxic metal/loids that could be phytoabsorbed into plant tissues. We implemented a replicated field trial on historic contaminated metal mine tailings in Northern Ontario (Canada) over a range of high‑carbon wood ash biochar (HCWAB) dosages (0-30 t/ha) to evaluate tree and substrate responses. Sapling survivorship and aboveground biomass growth were quantified over a 4-year period; substrate chemical parameters were measured using acid-digestion and ICP-MS, as well as ion exchange resin probes. To assess elemental composition of sapling tissues, we used electron probe microanalysis combined with laser-ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) on intact samples across the range of dosages applied. Survival and growth of saplings peaked at mid-range ash dosages of 3-6 t/ha. Similarly, substrate ion availability of P, K, and Zn were stable at lower dosages, but increased above 6 t/ha. The trace amounts of toxic metal/loids of concern measured in wood ash (As, Cd, Cu, and Pb) did not result in significantly increased sapling tissue concentrations at low to moderate dosages, but in some cases tissue contaminant levels were elevated at the highest dosage examined (30 t/ha). Our findings highlight the potential for high‑carbon wood ash biochar to be used for metal mine restoration at low to moderate dosages.

Natural ageing of biochar improves its benefits to soil Pb immobilization and reduction in soil phytotoxicity

Amendments are good tools for immobilizing metal(loid) and improving phytoremediation success. However, the amendment effect is variable and depends on multiple parameters, including amendment type and ageing. Such an ageing effect is rarely studied. Our study is one of the first focusing on how biochar storage affects its effect on soil properties and metal(loid) immobilization, when biochar was applied alone or in combination with green manure. To answer this, a 33-day pot incubation experiment was set up using contaminated soil, amended with two biochars (differing in ages: old (Bo) and new (Bn)) and/or two green manures (leaves of clover or poplar) and sown with Phaseolus vulgaris (bioindicator plant). Soil pore waters, plant growth and metal(loid) accumulation were evaluated. Biochar reduced soil acidity (Bn: + 0.75 pH unit, Bo: + 0.72 unit) and Pb mobility (Bn: - 42%, Bo: - 50%), while green manures acidified the soil (- 0.30 pH unit) and immobilized Pb only after 10 days (- 44%). All amendments reduced soil phytotoxicity. Moreover, the biochar stored at room temperature for a few years demonstrated better abilities to improve soil properties, particularly for Pb immobilization, than the biochar freshly prepared. Finally, as mixtures maturated, soil parameters changed until about ten days, then tended to stabilize. Therefore, it can be concluded that (1) biochar storage will affect its chemical properties and ameliorate its effects, (2) biochar can ameliorate soil properties and immobilize metal(loid)s, while green manures tended to have adverse effects at first, and (3) soil/amendment mixtures should be left to mature about two weeks before potential plant implementation.