Assisted phytostabilisation of As, Pb and Sb-contaminated Technosols with mineral and organic amendments using Douglas fir (Pseudotsuga menziesii (Mirb.) Franco)

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.

Stabilization of arsenic and antimony Co-contaminated soil with an iron-based stabilizer: Assessment of strength, leaching and hydraulic properties and immobilization mechanisms

Soils with relatively high concentrations of arsenic (As) and antimony (Sb) in mining areas would impose significant risks to human health and ecosystem. A new stabilizer PFSC composed of polymerized ferric sulfate (PFS) and calcium hydroxide (Ca(OH)₂) is proposed to stabilize the soil with co-existed As and Sb sampled at an abandoned arsenic factory site. The effects of stabilizer dosage on the properties of the stabilized soil including leached concentrations of As and Sb, unconfined compressive strength (UCS), and hydraulic conductivity (k_w) were investigated. The mechanisms of As and Sb immobilization in the soils were interpreted by Tessier's sequential extraction procedure (SEP), scanning electron microscope (SEM), and X-ray diffraction (XRD) results. The results showed increasing PFSC dosage was effective for reducing leached concentrations of As and Sb. When the PFSC dosage increased from 2% to 10%, the UCS and k_w increased from 84 to 206 kPa and decreased from 6.48 × 10^-8 to 6.33 × 10^-9 m s^-1, respectively. Tessier's SEP results showed that the leachable As and Sb fractions decreased from 12% to 5.6% and 7.5% to 3.8%, while the Fe-Mn oxides bound fractions increased from 22.3% to 29.4% and 13.2% to 19.5%. The SEM images and XRD patterns of untreated and PFSC stabilized contaminated soils indicated that hematite and calcite (CaCO₃) were the main products of PFSC stabilization processes. Adsorption on ferrihydrite, entrapment in hematite lattices, and co-precipitate with calcite might were the main mechanisms of As and Sb immobilization.

Effects of Soil Amendments on Soil Pb Bioavailability and Pb Absorption by a low-Pb Accumulator Kumquat Grown in Two Types of Pb-Contaminated Soils

A pot experiment was conducted to investigate the effects of 0.5% and 1% alkaline, clay mineral and phosphorus amendments, as well as 2% and 5% organic amendments, on lead (Pb) soil bioavailability and Pb absorption by the low-Pb accumulator kumquat (Citrus japonica Thunb.) 'Cuipi' in two typical Pb-contaminated soils, Jiyuan and Yangshuo, from northern and southern China, respectively. The diethylenetriaminepentaacetic acid-extractable Pb soil concentration and Pb accumulation in kumquat significantly decreased with amendment additions. High amendment doses had greater inhibitory effects than low doses but no significant effects on the kumquat's biomass in the two typical soils. Alkaline, but not clay mineral, amendments greatly increased the soil pH level. Organic amendments effectively reduced Pb accumulation owing to their strong adsorptive capacities. Thus, using organic amendments combined with a low-Pb accumulator kumquat forms a suitable farming practice for producing safe fruit in the two common types of Pb-contaminated soils in China.

Application of biochar, compost and ZVI nanoparticles for the remediation of As, Cu, Pb and Zn polluted soil

Here we tested the capacity of zero valent iron nanoparticles (nZVI) combined with two organic amendments, namely, compost and biochar, to immobilize metal(oid)s such as As, Cu, Pb, and Zn. In addition, the effects of the amendments on the development of Brassica juncea L., a plant widely used for phytoremediation purposes, were also examined. To perform the experiments, pots containing polluted soil were treated with nZVI, compost-biochar, or a blend of compost-biochar-nZVI. Metal(oid)s availability and soil properties were evaluated after 15 and 75 days, and the height and weight of the plants were measured to determine development. The compost-biochar amendment showed excellent capacity to immobilize metals, but As availability was considerably increased. However, the addition of nZVI to the mixture corrected this effect considerably. In addition, soil treatment with nZVI alone led to a slight increase in Cu availability, which was not observed for the mixture with organic amendments. With respect to soil properties, the CEC and pH were enhanced by the compost-biochar amendment, thereby favoring plant growth. Nevertheless, the nanoparticles reduced the concentration of available P, which impaired plant growth to a certain extent. In conclusion, Fe-based nanoparticles combined with organic amendments emerge as powerful approaches to remediate soils contaminated by metals and metalloids.

Effects of pH Conditions and Application Rates of Commercial Humic Substances on Cu and Zn Mobility in Anthropogenic Mine Soils

We studied the effects of commercial humic substances derived from leonardite at different rates (0, 0.25, 2, 10 g kg−1) and pH (4.5, 6.0, 8.0) on Cu and Zn mobility, to evaluate their use for remediation of metal contaminated mine soils and to optimize their application conditions. We conducted a single-step extraction experiment and analyzed extracts for metal concentrations, soluble organic carbon and their E4/E6 ratio (ratio of absorption at 465 to 665 nm). Metal speciation in a soil solution was simulated by the non-ideal competitive adsorption-Donnan (NICA-Donnan) model. Increasing the amount of humic substances and the pH caused higher release rates of soluble organic carbon with a lower humic/fulvic acids ratio. This led to a higher mobility of metals (up to 110 times Cu concentration in control and 12 times for Zn) due to the formation of soluble metal-humic complexes. Speciation modeling predicted that increasing rates of humic substances would result in a higher proportion of Cu and Zn associated with fulvic acids, more mobile than the humic acids fraction. Application of commercial leonardite humic substances at 2–10 g kg−1 and with pH levels similar to or below natural soil could be useful for assisted-phytoextraction of contaminated anthropogenic soils. High rates of humic substances in more alkaline conditions could entail a considerable risk of metal leaching to groundwater, toxicity and transfer to the trophic chain.