Effect of nZVI/biochar nanocomposites on Cd transport in clay mineral-coated quartz sand: Facilitation and rerelease.

The development and application of nano-biochar synthesized by ball milling technology is still challenging in the field of environmental remediation because of its higher activity with pollutants. The purpose of this study was to investigate the transport behavior of two kinds of biochar nanoparticles (nanobiochar (NBC) and nZVI-modified nanobiochar (nZVI-NBC)) and Cd2+ in clay mineral (kaolinite, illite, and montmorillonite)-coated quartz sand columns. The interaction between biochar nanoparticles and Cd2+ in saturated porous media was studied in cotransport experiments. Then, the effect of biochar nanoparticles on the release of Cd2+ in contaminated media was explored by elution experiments. The cotransport experiments showed that the mobility of Cd2+ was enhanced by two kinds of biochar nanoparticles, while the transport of biochar was limited due to the presence of Cd2+. The elution experiments showed that the transport of biochar nanoparticles can be inhibited by Cd2+ previously immobilized in the sand column, and Cd2+ can be rereleased by biochar nanoparticles. The rerelease ability of nZVI-NBC to Cd2+ is weaker than that of NBC because nZVI is more easily retained in the sand column after oxidation, thus strengthening the immobilization of Cd2+. In general, the recoveries of NBC, nZVI-NBC and Cd2+ in saturated porous media were reduced by the presence of clay minerals. The experimental results describing the stability of biochar nanoparticles in sand columns are consistent with those predicted by the Derjaguin-Landau-Verwey-Overbeek theory. The transport behavior of Cd2+ and biochar nanoparticles in sand columns can be well simulated by advection-dispersion-reaction. These findings reveal the interaction between biochar nanoparticles and heavy metals in the soil environment and provide new insights into the transport and fate of environmental remediation materials and pollutants in the underground environment.

Co-transport of citrate-modified biochar nanoparticles and released plant-available silicon in saturated porous media: Effect of LMWOAs and solution chemistry.

Citrate-modified biochar nanoparticles (CBCNPs) represent a promising amendment with plant-available silicon (PASi) releasing capacity. However, the co-transport behavior with released PASi remain poorly understood. This study investigated their co-transport in saturated porous media under various solution chemistry and low molecular weight organic acids (LMWOAs). Experimental and two-site kinetic model results revealed that higher ionic strength caused favorable aggregation and size-selective, hindering CBCNPs transport. Divalent Ca2+ ions retained CBCNPs more effectively than K+ due to stronger charge screening and cation bridging. The pH buffering capacity of CBCNPs resulted in consistent transport behavior across a broad pH range (4-8). XDLVO calculation clarified the impact mechanisms of IS, ion types and pH on CBCNPs transport. Furthermore, LMWOAs exhibited a time-dependent blocking effect on CBCNPs transport. Oxalic acid (OA) and citric acid (CA) facilitated CBCNPs transport though mechanisms beyond XDLVO, including steric hindrance, competitive adsorption, and surface hydrophilicity. The presence of LMWOAs significantly hindered PASi co-transport, with the inhibitory effect ranked as acetic acid (AA) ≈ CA > OA > absence of organic acids. The inhibition is attributed to the blocking effect and formation of Si-organic acid complexes, as evidenced by breakthrough curves and density functional theory calculations. This study provides novel insights into the co-transport of CBCNPs with released PASi through mutual mechanisms, indicating both potential environmental benefits and risks.

Nanoscale Biochar for Fertilizer Quality Optimization in Waste Composting: Microbial Community Regulation.

Conventional composting faces challenges of nitrogen loss, product instability, and limited humic substance formation. This study investigated the effects of nanoscale biochars (nano-BCs) derived from rice straw (nano-RSB) and corn stover (nano-CSB) on manure composting. A randomized design with five treatments was used: control, regular biochars (RSB and CSB), and nano-BCs. Nano-BCs, especially nano-CSB, significantly improved compost maturity and reduced phytotoxicity, achieving a 146.20 % germination index. They increased total nitrogen (55.09-63.64 %) and phosphorus (10.25-12.33 %) retention, reduced NH4+-N loss, and promoted nitrification. Nano-CSB showed the highest final NO3--N content (8.63 g/kg). Bacterial richness and diversity increased by 25-30 % in nano-BC treatments, with selective enrichment of beneficial species. The unique properties of nano-BCs, including high surface area and microporous structure, improved nutrient retention and compost quality. Nano-BCs offers a promising solution for sustainable waste management and high-quality compost production in agriculture, significantly enhancing nutrient retention and microbial community regulation during composting process.

Co-transport of biochar nanoparticles (BC NPs) and rare earth elements (REEs) in water-saturated porous media: New insights into REE fractionation.

The present study investigated the co-transport behavior of three REEs3+ (La3+, Gd3+, and Yb3+) with and without biochar nanoparticles (BC NPs) in water-saturated porous media. The presence of REEs3+ enhanced the retention of BC NPs in quartz sand (QS) due to decreased electrostatic repulsion between BC NPs and QS, enhanced aggregation of BC NPs, and the contribution of straining. The distribution coefficients (KD) in packed columns in the co-transport of BC NPs and three REEs3+ were much smaller than in batch experiments due to the different hydrodynamic conditions. In addition, we, for the first time, found that REE fractionation in the solid-liquid phase occurred during the co-transport of REEs3+ in the presence and absence of BC NPs. Note that the REE fractionation during the co-transport, which is helpful for the tracing application during earth surface processes, was driven by the interaction of REEs3+ with QS and BC NPs. This study elucidates novel insights into the fate of BC NPs and REEs3+ in porous media and indicates that (i) mutual effects between BC NPs and REE3+ should be considered when BC was applied to REE contaminated aquatic and soil systems; and (ii) REE fractionation provides a useful tool for identifying the sources of coexisting substances.

Biochar nanoparticles with different pyrolysis temperatures mediate cadmium transport in water-saturated soils: Effects of ionic strength and humic acid.

Biochar is advocated as an environment-friendly and cost-effective material for removing both heavy metals and organic contaminants in soil remediation. However, our understandings on the cotransport potential of contaminants with the nanoscale biochar downward along soil profiles (e.g., potential environmental risks towards groundwater) remain largely unknown. This study investigated the effects of wheat straw-derived biochar nanoparticles pyrolyzed at 350 °C and 500 °C (BNP350 and BNP500) on the transport of cadmium (Cd(II)) in water-saturated soil packed columns. Different ionic strengths (ISs) without/with humic acid (HA) were tested to mimic the scenarios during soil remediation. BNPs could act as a vehicle mediating Cd(II) transport in soils. At a low IS (1.0 mM KCl), compared to the limited transport of individual Cd(II), BNP500 enhanced (69 times) Cd(II) transport (Cd(II) mass recovery (M) = 7.59%) in soils, which was greater than that by BNP350 (54 times, M = 5.92%), likely due to the higher adsorption of Cd(II) onto BNP500. HA further increased the Cd(II) transport by BNPs (M = 8.40% for BNP350 and M = 11.95% for BNP500), which was mainly due to the increased mobility of BNPs carrying more absorbed Cd(II). In contrast, at a high IS (10 mM KCl), BNP500 dramatically inhibited the transport of Cd(II) (M = 12.9%), decreasing by about 61.6%, compared to the BNPs absence (M = 33.6%). This is because a large amount of BNP500-Cd(II) was retained in soils at a high IS. This inhibition effect of Cd(II) transport by BNPs was reinforced with the presence of HA. Our findings suggest that the pyrolysis temperature of biochar should be carefully considered when applying biochar for in-situ remediation of soils contaminated by heavy metals such as Cd(II) under various organic matter and IS conditions.

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.

Contrasting effects of biochar nanoparticles on the retention and transport of phosphorus in acidic and alkaline soils.

Land application of biomass-derived biochar has been increasingly recommended as a beneficial soil amendment for nutrients (such as N, P) retention. However, the small-scale biochar particles, especially those in the nano-scale range, may carry nutrients downward the soil profile, reducing nutrition retention and posing a potential risk to the groundwater. In this study, column experiments were conducted to investigate the retention and transport of phosphorus (P) in two acidic and two alkaline soils as affected by wood chip-derived biochar nanoparticles (NPs). In acidic paddy and red soils, biochar NPs facilitated the retention of P, increasing by about 24% and 16%, respectively, compared to the biochar absence. It is because biochar NPs stabilize soil Fe/Al oxides and dissolved organic carbon (DOC), thereby reducing the release of Fe/Al oxides- and DOC-associated P. In contrast, in alkaline huangmian and chao soils, retention of P was reduced in the presence of biochar NPs, decreasing by about 23% and 18%, respectively. It was mainly due to the increased transport of Fe/Al oxides-associated P in effluents. Moreover, biochar NPs could also act as a P carrier, mediating the retention of P. The diffusive gradients in thin films provided in-suit measurement of labile P in soil profiles, showing much lower labile P from retained P in acidic soils than that from alkaline soils though the labile P with biochar NPs presence was increased in all soils. Our findings indicate that biochar NPs have contrasting effects on the retention of P in acidic and alkaline soils, implying the cautious land applications of biochar for nutrients retention in soils with different acidities.

Transport and retention of biochar nanoparticles in a paddy soil under environmentally-relevant solution chemistry conditions.

Land application of biochar has been increasingly recommended as a powerful strategy for carbon sequestration and soil remediation. However, the biochar particles, especially those in the nanoscale range, may migrate or carry the inherent contaminants along the soil profile, posing a potential risk to the groundwater. This study investigated the transport and retention of wood chip-derived biochar nanoparticles (NPs) in water-saturated columns packed with a paddy soil. The environmentally-relevant soil solution chemistry including ionic strength (0.10-50 mM), electrolyte type (NaCl and CaCl2), and natural organic matter (0-10 mg L-1 humic acid) were tested to elucidate their effects on the biochar NPs transport. Higher mobility of biochar NPs was observed in the soil at lower ionic strengths, with CaCl2 electrolyte being more effective than NaCl in decreasing biochar NPs transport. The retained biochar NPs in NaCl was re-entrained (∼57.7%) upon lowering transient pore-water ionic strength, indicating that biochar NPs were reversibly retained in the secondary minimum. In contrast, negligible re-entrainment of biochar NPs occurred in CaCl2 due to the primary minimum and/or particle aggregation. Humic acid increased the mobility of biochar NPs, likely due to enhanced electrosteric repulsive interactions. The transport behaviors of biochar NPs can be well interpreted by a two-site kinetic retention model that assumes reversible retention for one site, and irreversible retention for the other site. Our findings indicated that the transport of wood chip biochar NPs is significant in the paddy soil, highlighting the importance of understanding the mobility of biochar NPs in natural soils for accurately assessing their environmental impacts.