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In this study, we report on the extraction, characterization, and potential applications of colloidal biochar derived from pyrolyzed wood-an untapped source of carbonaceous particles. A series of characterizations was performed on biochar colloids to unravel their colloidal properties and surface chemistry through which it was found that they have a net negative charge and are stable between pH 3 and 10. Moreover, our initial toxicity tests showed that biochar colloids themselves are not toxic and they can be used in remediation applications, which led us to investigate (1) their copper sorption, a model inorganic contaminant, in a scenario that biochar colloids are released into the environment and (2) their potential use in organic pollutants adsorption and degradation. Copper sorption studies showed that biochar colloids have a copper sorption capacity as high as 22 mg g −1 in sub-ppm copper solutions. This increased the acute 48 h lethal concentration (LC 50) of copper for Daphnia magna by 21 ppb, which is comparable to the previously reported effect by dissolved organic matter. Adsorption and degradation of methylene blue (MB), an often-used proxy for organic contaminants in water, were studied by coupling the biochar colloids to positively charged TiO 2 nanoparticles and using it as a photocatalyst. The hybrid MB photodegradation efficiency was 21% higher than that of TiO 2 nanoparticles alone. Enhancement of demethylation is proposed as the main degradation mechanism of MB, as confirmed by liquid chromatography-mass spectroscopy (LC/MS), and the positive impact of biochar colloids is ascribed to their abundant adsorption sites, which may facilitate MB adsorption and its photocatalytic degradation.

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Despite an abundance of short-term studies focusing on biochar's effects on annual plants, the long-term effects of biochar on perennial plants and the effects of the biochar on the mobility and speciation changes of metals/metalloids not limited to main plant nutrients in soils are poorly constrained. This study reports on the amelioration a sloped orthic ferralsol by biochar from Tibouchina wood and the resulting effects on perennial crops and microbiota, including a comprehensive analysis of metals/ metalloids speciation changes. Fields were amended with biochar and urine-amended biochar (2 kg/m 2) and were planted with papaya, banana, and manioc. Soil and plant materials were analyzed using acid digestions, sequential extractions, and 16S rRNA gene sequencing. Biochar applications led to decreased soil acidity, shifted the cation exchange capacity from being Al-influenced to being Mg/K/Ca-dominated, and elevated the concentrations of Mg, K, Ca, Zn, and Ba in soils. The exchangeable/acid-soluble fraction of Ca, P, and S notably increased. The soil microbial biome became more species rich and diverse in the biochar-amended fields. Manioc benefited from biochar applications, demonstrating increased growth, which resulted in generally decreased concentrations of trace elements in most plant parts, however, with an increased total elemental uptake. Urine amendment contributed to higher concentrations of P, S, and K in soils, but did not further increase plant growth. Biochar was shown to be a promising soil amendment for agricultural use of orthic ferralsols of the Brazil's Atlantic forest region, but the accumulation of potentially harmful metals needs to be considered.