Nano-black carbon (BC) is one of the most active fractions in the pyrogenic carbonaceous matter continuum. The majority of recent studies mainly focus on the role of nano-BC in the global carbon cycle. However, based on literature and our recent studies, we suggest that nano-BC may also serve as a super suspending agent, carrier, and redox mediator for sorbates during its migration from terrestrial to water bodies due to its unique properties such as high colloidal stability, strong sorption capacity, and high surface reactivity. The full implications of nano-BC in water/soil environments are far more than we expected. Thus, we call for more detailed investigations on the activity and reactivity of nano-BC in water/soil environments.

The article “Nano-black carbon (biochar) released from pyrogenic carbonaceous matter as a super suspending agent in water/soil environments.

There are thousands of abandoned mine land (AML) sites in the U.S. that need to be restored to reduce wind and water erosion, provide wildlife forage, shade streams, and improve productivity. Biochar created from woody biomass that would normally be burned in slash piles can be applied to soil to improve soil properties and is one method to restore AML soil productive capacity. Using this ‘waste’ biomass for biochar and reclamation activities will reduce wildfire risk, air pollution from burning, and particulates released from burning wood. Biochar has the potential to improve water quality, bind heavy metals, or decrease toxic chemical concentrations, while improving soil health to establish sustainable plant cover, thereby preventing soil erosion, leaching, or other unintended, negative environmental consequences. Using forest residues to create biochar also helps reduce woody biomass and improves forest health and resilience. We address concerns surrounding organic and inorganic contaminants on the biochar and how this might affect its’ efficacy and provide valuable information to increase restoration activities on AMLs using biochar alone or in combination with other organic amendments. Several examples of AML biochar restoration sites initiated to evaluate short- and long-term above- and belowground ecosystem responses are presented.

The transformation of mercury (Hg) into the more toxic and bioaccumulative form methylmercury (MeHg) in soils and sediments can lead to the biomagnification of MeHg through the food chain, which poses ecological and health risks. In the last decade, biochar application, an in situ remediation technique, has been shown to be effective in mitigating the risks from Hg in soils and sediments. However, uncertainties associated with biochar use and its underlying mechanisms remain. Here, we summarize recent studies on the effects and advantages of biochar amendment related to Hg biogeochemistry and its bioavailability in soils and sediments and systematically analyze the progress made in understanding the underlying mechanisms responsible for reductions in Hg bioaccumulation. The existing literature indicates (1) that biochar application decreases the mobility of inorganic Hg in soils and sediments and (2) that biochar can reduce the bioavailability of MeHg and its accumulation in crops but has a complex effect on net MeHg production. In this review, two main mechanisms, a direct mechanism (e.g., Hg-biochar binding) and an indirect mechanism (e.g., biochar-impacted sulfur cycling and thus Hg-soil binding), that explain the reduction in Hg bioavailability by biochar amendment based on the interactions among biochar, soil and Hg under redox conditions are highlighted. Furthermore, the existing problems with the use of biochar to treat Hg-contaminated soils and sediments, such as the appropriate dose and the long-term effectiveness of biochar, are discussed. Further research involving laboratory tests and field applications is necessary to obtain a mechanistic understanding of the role of biochar in reducing Hg bioavailability in diverse soil types under varying redox conditions and to develop completely green and sustainable biochar-based functional materials for mitigating Hg-related health risks.

Biochar is an effective absorbent for remediating heavy metal contaminated soil, but functional optimization is still needed to improve its performance in field application. Here, we characterized the physical structures and surface chemical properties of raw wood biochar and palm biochar (WB and PB) and the corresponding sulfhydryl-modified biochar (SWB and SPB). Their adsorption capacity for Pb was evaluated by combining thermodynamic and kinetic adsorption at 0.01 mol/L KCl and corresponding model simulation. The results demonstrated successful grafting of sulfhydryl groups onto the biochar, which dramatically reduced the specific surface area (SSA) and pore volume of biochar. The pK_a in the surface complexation model (SCM) indicated similar proton affinity between sulfhydryl groups and original functional groups on the biochar. SCM could satisfactorily fit the Pb adsorption behaviors, and model analysis revealed that Pb tended to be adsorbed on low-proton affinity sites at low pH, but high-proton affinity sites became dominant in Pb adsorption with increasing pH and adsorbed almost all Pb ions at pH > 7.0. Besides, the Pb adsorption density of SWB and SPB was improved by 8.86 and 3.64 folds relative to that of WB and PB, respectively. Over 90% of initially added Pb ions were removed in 1440 and 720 min by raw and sulfhydryl-modified biochar, respectively, indicating that sulfhydryl modification accelerated the Pb adsorption of biochar. These results suggest that site density, SSA and pore structure of biochar play crucial roles in heavy metal adsorption, and sulfhydryl modification may improve the performance of biochar in remediating heavy metal contaminated soil.

Rapid expansion of cultivated bamboo negatively impacts on biodiversity and soil microbial community. As such, it is important to properly manage and use bamboo to prevent and control such issues. This study focuses on optimizing pyrolysis conditions to produce bamboo biochar for agricultural soil amendment, particularly soil potassium (K) and water holding capacity. Bamboo chips were pyrolyzed under nitrogen gas at 400, 600, and 800 °C for 1 and 2 h of retention. A total of six biochar products were created: 400-1 (i.e., 400 °C in 1 h), 400-2, 600-1, 600-2, 800-1, and 800-2. The 600 °C bamboo biochar products were observed to have the greatest potential in increasing soil K and water holding capacity. The 600-1 product had the highest potassium content (4.87%), with a water holding capacity of 3.73 g g⁻¹, while the 600-2 product had the second-highest potassium content (4.13%) and the highest water holding capacity (4.21 g g⁻¹) and cation exchange capacity. The K release in 600 °C products was larger and slower than that of the 400 °C and 800 °C products, respectively. The results also indicated that the physicochemical characteristics of bamboo biochar, such as yield, pH, surface area, water holding capacity, and K content, were significantly impacted by temperature, retention time, or a combination of these parameters. The outcomes from this study are a valuable reference for bamboo biochar production targeting agricultural soil amendment, particularly when it is directed at increasing soil K and water holding capacity.

The article “Enhancing soil water holding capacity and provision of a potassium source via optimization of the pyrolysis of bamboo biochar”.

Biochar can enhance crop production and sequester carbon, but there have been few studies with tree crops. Rubber plantations cover more than 8 million hectares in Southeast Asia, so we assessed the feasibility of biochar application in these plantations with a pot trial. Rubber seedlings were planted in soil with four concentrations (0, 1.25%, 2.5% and 5%, w/w) of biochar combined with two concentrations of compound fertilizer (0 kg/ha and 300 kg/ha). Soil properties and seedling growth were measured, and a leaching experiment was conducted in the rainy season. Our results show that biochar increased pH, water content (27.4–65.1%), total carbon (25.4–53.6%), nitrate nitrogen, and available phosphorus in the soil, and decreased bulk density (3.2–23.9%). Biochar treatment reduced leaching of ammonium nitrogen and ortho-P. Biochar increased seedling nutrient uptake (C, N, P and K), with 2.5% and 5% biochar showing the largest effects, but seedling biomass was the highest with 1.25%, and declined in 2.5% and 5%. Our results suggest that biochar addition is an effective way to improve rubber plantation soils, sequester more carbon and decrease nutrient leaching, but the optimum application rate under field conditions needs further research.

Irrigation water quality plays a vital role in sustaining crop productivity and feeding a growing world population. In many countries, continued agricultural water reuse can lead to greater water-soluble salt concentrations, and in particular Na; finding means by which irrigation water Na, and thus sodium adsorption ratios (SAR), can be reduced would reduce the rate at which soil sodification occurs. Four biochars, containing a variety of organic functional groups and electrochemistries, were examined for their potential to sorb and remove Na from simulated irrigation water, and subsequently reduce water SAR. Two batch experiments examined the role that wheat straw biochar, lodgepole pine biochar, Kentucky bluegrass biochar, and hemp biochar played in terms of sorbing sodium over time or application rate. Of the four biochars examined, hemp biochar had the lowest oxidation–reduction potential (ORP; ~ 0–100 mV), sorbed the greatest Na amount (up to 923 mg kg⁻¹), and released Ca and Mg (up to 115 and 63 mg kg⁻¹, respectively) into solution, all of which led to a significant reduction in water SAR (from 8.8 to 7.3; 17% decrease). Sodium sorption onto hemp biochar better fit a Langmuir versus a Freundlich isotherm, yet followed a pseudo-second-order model better than a pseudo-first-order kinetic model. The data suggest that Na ions formed a monolayer on the hemp biochar surface, influenced by associations with π electrons, but given time the Na ions may diffuse into biochar pores or more slowly interact with biochar-borne π electrons. Hemp biochar shows promise in reducing the SAR of Na-impacted waters. Future investigations should focus on additional laboratory, greenhouse, and field trials with hemp biochar and other biochars designed to have similar or superior properties for sorbing excess irrigation water Na and improving crop growth.

Biochar as an organic amendment improves soil attributes, with a potentially significant effect on soil chemical fertility and quality. The main objective of this study was to quantify the effect of biochar addition on nutrients, carbon sequestration and microbial activity and understand the mechanisms of controlling biochar effects in calcareous soils. Maize residue biochars produced at 200, 400 and 600 °C were added at 5 and 10 g kg⁻¹ rates to sandy loam and clayey texture calcareous soils. The soil properties measured were pH and electrical conductivity (EC), plant-available potassium (K) and available phosphorus (P), total nitrogen (TN), C sequestration; and the fluorescein diacetate (FDA) hydrolysis activity. Addition of raw material and biochars increased pH (0.15–0.46 units), EC (0.14–0.38 dS m⁻¹), TN (63–120%), K (12–41%) and FDA activity (27–280%), but tended to decrease plant-available P (23–86%). Increasing pyrolysis temperature increased soil C pool index (CPI), but decreased the FDA and the changes depended largely upon the application rate and soil texture. The positive effects of biochar addition and its pyrolysis temperature on soil C sequestration potential were more pronounced at high than low application rate and in sandy loam than clayey soils. Nevertheless, the effect of biochar addition and pyrolysis temperature on the FDA activity was higher at high than low application rates, but lower in sandy loam than clayey soils. Although biochar application may successfully improve soil processes and attributes and have a high potential for C sequestration, its effects are controlled by soil texture, pyrolysis temperature and application rate.