Arsenic (As) is toxic to human and is often found in drinking water in India and Bangladesh, due to the natural abundance of arsenides ores. Different removal procedures such as precipitation, sorption, ion exchange and membrane separation have been employed for removal of As from contaminated drinking water (CDW), however, there is a critical need for low-cost economically viable biochar modification methods which can enhance As sorption. Here we studied the effectiveness of zero-valent iron (ZVI)-biochar complexes produced by high temperature pyrolysis of biomass and magnetite for removing As from CDW. Batch equilibration and column leaching studies show that ZVI-biochar complexes are effective for removing As from CDW for the studied pH range (pH ∼7-7.5) and in the presence of competing ions. XPS As 3d analysis of ZVI-biochar complexes exposed to As in the batch and column studies show primarily As, indicating simultaneous oxidation of Fe° to Fe and reduction of As to As. SEM-EDS and XRD analyses show isomorphous substitution of As for Fe in neo-formed α/γ-FeOOH on biochar surfaces, which is attribute to co-precipitation. This study also demonstrates the efficacy of pyrolyzing biomass with low-cost iron ores at 900 °C to rapidly produce ZVI-biochar complexes, which have potential to be used for treatment of As CDW.

The capacity of biochars to adsorb ionic contaminants is strongly influenced by biochar surface chemistry. We studied the effects of biomass feedstock type, pyrolysis temperature, reaction media pH, and AlCl pre-pyrolysis feedstock treatments on biochar anion exchange capacity (AEC), cation exchange capacity (CEC), point of zero net charge (PZNC), and point of zero salt effect (PZSE). We used the relationship between PZNC and PZSE to probe biochar surfaces for the presence of unstable (hydrolyzable) surface charge functional groups. The results indicate that biochars produced at ≤500°C have high CECs and low AEC, PZSE, and PZNC values due to the dominance of negative surface charge arising from carboxylate and phenolate functional groups. Biochars produced at ≥700°C have low CEC and high AEC, PZSE, and PZNC values, consistent with a dominance of positive surface charge arising from nonhydrolyzable bridging oxonium (oxygen heterocycles) groups. However, biochars produced at moderate temperatures (500-700°C) have high PZSE and low PZNC values, indicating the presence of nonbridging oxonium groups, which are rapidly degraded under alkaline conditions by OH attack on the oxonium α-C. Biochars treated with AlCl have high AEC, PZSE, and PZNC values due to variably charged aluminol groups on biochar surfaces. The results provide support for the presence of both hydrolyzable and nonhydrolyzable oxonium groups on biochar surfaces. They also demonstrate that biochars produced at high pyrolysis temperatures (>700°C) or those receiving pre-pyrolysis treatments with AlCl are optimized for anionic contaminant adsorption, whereas biochars produced at low pyrolysis temperatures (400°C) are optimized for cationic contaminant adsorption.

Proximate analysis is widely used to determine moisture, volatile matter (VM), fixed carbon (FC) and ash content of biochars. The original ASTM D1762-84 method was developed to assess quality of hardwood charcoal for use as fuel. We have developed a modified proximate analysis method to assess quality of diverse biochars for use as soil amendments. We determined that a N 2 purge is necessary during both moisture and VM determination to avoid errors associated with sample oxidation. We assessed a range of boundary temperatures (350-950°C) for separating VM and FC, and determined that 800°C is the minimum temperature required to distinguish between VM and FC in biochars. Furthermore, correlation between VM/FC and molar H/C org ratios suggests that VM/FC ratios are a useful measure of biochar stability. Use of the proposed modified method is encouraged to reduce variance in analytical results among studies.

Biochars have been suggested to have P capture potential from effluent streams and to recycle the captured P to agricultural soils. However, most biochars have low P sorption capacity. The objective of this study was to engineer biochar for enhanced P sorption affinity. Biochar was produced from corn stover biomass pre-treated with FeSO4 (ISIB) using autothermal (air-blown) pyrolysis at 500°C. Point of zero charge (pHZPC) shifted from 8.48 to 4.31 indicating that Fe treatment increased dominance of acid functional groups. Batch equilibration isotherm study showed that ISIB had 11-12 times more P sorption capacity (3,763 versus 46,300 mg kg -1 , and 6,704 versus 48,821 mg kg -1 for non-oxidized and oxidized conditions, respectively) while P desorption rate was ~1/3 relative to the control biochar. A column leaching study also shows that ISIB was effective for removing P from simulated agricultural effluent. XRD and SEM-EDS analyses showed the P sorption was predominately through inner-sphere surface complexation followed by surface precipitation and that P is preferentially sorbed by hematite (α-Fe2O3) relative to magnetite (Fe III 2O3 + Fe II O) or maghemite . This study demonstrates that ISIB can be produced by pyrolyzing corn stover with FeSO4, and the resulting ISIB is effective for adsorption and recycling of P. When loaded with P, the ISIB can potentially be used as a slow-release P fertilizer.

Copper (Cu) contamination to soil and water is a worldwide concern. Biochar has been suggested to remediate degraded soils. In this study, column leaching and chemical characterization were conducted to assess effects of biochar amendment on Cu immobilization and subsequent nutrient release in Cu-contaminated Alfisol and Spodosol. The results indicate that biochar is effective in binding Cu (30 and 41%, respectively, for Alfisol with and without spiked Cu; 36 and 43% for Spodosol) and reducing Cu leaching loss (from ∼47 to 10% for the Cu-spiked Alfisol and from 48 to 9% for the Cu-spiked Spodosol). Copper was likely retained on biochar surfaces through complexation, as suggested by Fourier-transform infrared spectra. Biochar amendment converts a portion of Cu from available pool to more stable forms, thus resulting in decreased activities of free Cu and increased activity of organic Cu complexes in leachate. Reduction of >0.45-μm solids and nanoparticles concentrations in leachate was also observed. In addition, biochar application rate was correlated negatively with P, Ca, Mg, Zn, Mn, and NH-N concentration ( < 0.05) but positively with K and Na concentration ( < 0.05) in leachates. These results documented the potential of biochar as an effective amendment for Cu immobilization and mitigation of leaching risk for some nutrients.

Nitrogen (N) is an essential macronutrient for plant growth; however, excessive use of N fertilizers and complexities of the N cycle in soil cause negative environmental impacts. This imposes several challenges in controlling the N availability timing and losses. The objective of this study was to develop a biochar-based slow-release fertilizer (SRF) to reduce N loss and increase N use efficiency in crop production. We provided a laboratory-based assessment of several H3PO4 activated (5 and 15%) biochar-based SRFs, produced from different combinations of biochar to urea (1:2, 1:3, 1:4, and 1:6), calcium lignosulfonate (5%), and paraffin wax (10%). Characterization analyses (SEM–EDS, XRD, FTIR, and XPS) of developed SRFs suggest successful urea grafting onto biochar through both the urea amine N and carbonyl CO modes, without urea crystal structure disruption. The SRFs were more efficient than uncoated urea (control): (1) urea released in aqueous medium was 61–90% in 4320 min for the SRFs versus 99.6% in 12 min for the control; (2) cumulative N leached from soil columns was 68–71% after 41 leaching events for SRF versus 99.9% after four leaching events for the control; and (3) NH3-N volatilization from soil was 0.2–0.9% for the SRFs versus 2% for the control. Inclusively, our results suggest that the developed SRFs are effective for reducing N loss from soil and provide larger quantities of NH4 +-N to plants for a longer time (improved N use efficiency). We attribute this to that the developed SRFs are optimal for synchronizing with plant N uptake for providing better sustainability in modern agriculture.

The organic O content of biochar is useful for assessing biochar stability and reactivity. However, accurately determining the organic O content of biochar is difficult. Biochar contains both organic and inorganic forms of O, and some of the organic O is converted to inorganic O (e.g., newly formed carbonates) when samples are ashed. Here, we compare estimates of the O content for biochars produced from pure compounds (little or no ash), acid-washed biomass (little ash), and unwashed biomass (range of ash content). Novelty of this study includes a new method to predict organic O content of biochar using three easily measured biochar parameters- pyrolysis temperature, H/C molar ratio, and %biochar yield, and evidence indicating that the conventional difference method may substantially underestimate the organic O in biochar and adversely impact the accuracy of O:C ratios and van Krevelen plots. We also present evidence that acid washing removed 17% of the structural O from biochars and significantly changes O/C ratios. Environmental modelers are encouraged to use biochar H:C ratios.