Co-pyrolysis of rice straw with industrial wastes: Waste disposal and environmental remediation.

To reduce waste volumes and recover valuable products, char was synthesized via co-pyrolysis of rice straw (RS) with spent tires, sulfur wastes, and CO2. The inclusion of wastes and CO2 in pyrolysis of RS was hypothesized to enhance the sorption ability of char for various contaminants, including 2,4-dinitrotoluene (DNT), 2,4-dichlorophenol (DCP), lead, barium, chromate (CrO42-), and selenate (SeO42-). Using a lab-scale electrical furnace, the co-pyrolysis was conducted, and the soprtion capacity of char was evaluated via a series of batch sorption experiments. The maximum sorption capacity of spent tire-RS char for DNT was 16.8 ± 0.2 mg g-1, much higher than that of RS biochar (10.1 ± 0.3 mg g-1) due to increasing carbon content from the spent tires. The sorption of DCP to the spent tire-RS char was also enhanced via hydrophobic sorption to carbon residues, although not to the same degree of DNT due to deprotonation of the DCP. Compared with RS biochar, co-pyrolysis with raw sulfur wastes and CO2 enhanced sorption of lead, barium, and chromate, which can be attributed to increased cation and anion exchange capacities resulting from developments of oxygen or sulfur-containing functional groups. Sorption of selenate was strongly affected by pH. The results suggest that co-pyrolysis of agricultural and industrial wastes and CO2 is a promising option for the final waste disposal and the production of valuable char, which can be selectively customized for various types of contaminants as sorbents.

FeS-biochar and Zn(0)-biochar for remediation of redox-reactive contaminants.

To enhance the removal of redox-reactive contaminants, biochars including FeS and Zn(0) were developed via pyrolysis. These biochars significantly promoted the removal of 2,4-dichlorophenol (DCP) by means of sorption and reduction. Compared to direct reduction with FeS and Zn(0), the formation of reduction intermediates and product was enhanced from 21% and 22% of initial DCP concentration to 41% and 52%, respectively. 2,4-Dinitrotoluene (DNT), chromate (CrO4 2-) and selenate (SeO4 2-) were also reductively transformed to reduction products (e.g., 2,4-diaminotoluene [DAT], Cr3+, and selenite [SeO3 2-]) after they sorbed onto the biochars including FeS and Zn(0). Mass recovery as DAT, Cr3+ and selenite was 4-20%, 1-3%, and 10-30% under the given conditions. Electrochemical and X-ray analyses confirmed the reduction capability of the biochars including FeS and Zn(0). Fe and S in the FeS-biochar did not effectively promote the reductive transformation of the contaminants. Contrastingly, the stronger reducer Zn(0) yielded faster reductive transformation of contaminants over the Zn(0)-containing biochar, while not releasing high concentrations of Zn2+ into the aqueous phase. Our results suggest that biochars including Zn(0) may be suitable as dual sorbents/reductants to remediate redox-reactive contaminants in natural environments.

Factors affecting the sorption of halogenated phenols onto polymer/biomass-derived biochar: Effects of pH, hydrophobicity, and deprotonation.

High-performance biochar synthesized via co-pyrolysis of a polymer and rice straw (RS) was evaluated as a sorbent for ionizable halogenated phenols. Compared with RS-derived biochar, the sorption of 2,4-dichlorophenol (DCP), 2,4-dibromophenol (DBP), and 2,4-difluorophenol (DFP) onto polymer/RS-derived biochar was significantly enhanced by the properties of biochar changing due to polymer residues. According to Langmuir sorption isotherm model maximum sorption capacities for DCP, DBP, and DFP were 25.5-27.8, 22.1-26.5, and 11.5-13.3 mg/g, respectively, 3-5 times higher than those of RS-derived biochar. The removal of the polymer residues and increasing aromaticity of polymer/RS-derived biochar at elevated pyrolysis temperatures affected the sorption capacity of halogenated phenols. The surface charge of biochar and deprotonation of the halogenated phenols according to the solution pH were other factors responsible for sorption onto polymer/RS-derived biochar. Competition with other halogenated phenols, Zn2+, and Cu2+ implied that similar sorption mechanisms existed and that surface complexation and electron donor-acceptor interactions were involved in sorption onto polymer/RS-derived biochar. Our results suggest that co-disposal of thermoplastic and biomass wastes through pyrolysis may be an effective option to produce high-performance upgraded biochar as a sorbent for various types of contaminants.

Sorption of Nitro Explosives to Polymer/Biomass-Derived Biochar.

Factors affecting the sorptive removal of nitro explosives (2,4,6-trinitrotoluene [TNT] and hexahydro-1,3,5-trinitro-1,3,5-triazine [RDX]) to polymer/biomass-derived biochar were investigated through batch experiments. Compared with that of rice ( L.) straw (RS)-derived biochar, the sorption of TNT and RDX to polymer/RS-derived biochar was greatly enhanced by >2.5 and 4 times, respectively. The type and amount of polymer did not significantly affect the sorption of nitro explosives to polymer/RS-derived biochar. Pyrolysis temperature did not affect the sorption capacity. Surface treatment with acid or an oxidant did not significantly change the sorption capacity, suggesting that polymer residues may be strongly responsible for the enhancement. Possible polymer residues were identified via gas chromatography mass spectrometry analysis. The toxicity characteristic leaching procedure and Microtox bioassay analyses indicated that polymer/RS-derived biochar did not show possible harmful effects. Our results suggest that polymer/RS-derived biochar can be effectively used as a sorbent to remove nitro explosives both in the natural environment and engineered systems.

Redox and catalytic properties of biochar-coated zero-valent iron for the removal of nitro explosives and halogenated phenols.

A novel biochar-coated zero-valent iron [Fe(0)], which was synthesized with rice straw and Fe(0), was applied to remove nitro explosives (2,4,6-trinitrotoluene and hexahydro-1,3,5-trinitro-1,3,5-triazine) and halogenated phenols (2,4-dibromophenol and 2,4-difluorophenol) from contaminated waters. Due to the presence of biochar on the outside, the removal of nitro explosives and halogenated phenols was significantly enhanced via sorption. The sorbed contaminants were further transformed into reductive products, indicating that the inner Fe(0) played the role of a reductant in the biochar-coated Fe(0). Compared to direct reduction with Fe(0), the reductive transformation with biochar-coated Fe(0) was markedly enhanced, suggesting that the biochar in biochar-coated Fe(0) may act as an electron transfer mediator. Further experiments showed that the surface functional groups of biochar were involved in the catalytic enhancement of electron transfer. Our results suggested that biomass could be used to synthesize a novel sorbent and catalyst for treating redox-sensitive contaminants in natural and engineered systems.

Polymer/biomass-derived biochar for use as a sorbent and electron transfer mediator in environmental applications.

Co-pyrolysis of polymer and biomass wastes was investigated as a novel method for waste treatment and synthesis of enhanced biochar. Co-pyrolysis of rice straw (RS) with polypropylene (PP), polyethylene (PE) or polystyrene (PS) increased the carbon content, cation exchange capacity (CEC), surface area and pH of the biochar. As a result, the sorption of 2,4-dinitrotoluene (DNT) and Pb to polymer/RS-derived biochar was markedly enhanced. The increased aromaticity and hydrophobicity may be responsible for enhancing the DNT sorption to the polymer/RS-derived biochar. In contrast, increasing CEC, higher pH, and the newly developed surface area may account for the enhancement in Pb sorption. The addition of polymer to RS did not significantly change the catalytic role of biochar during the reduction of DNT by dithiothreitol. Our results suggest that co-pyrolysis of RS and polymer can improve the biochar properties to enhance the sorption of DNT and Pb.

Reductive removal of 2,4-dinitrotoluene and 2,4-dichlorophenol with zero-valent iron-included biochar.

In order to remediate organic contaminants in natural waters and soils, a novel zero-valent iron [Fe(0)]-included biochar was synthesized via slow pyrolysis. 2,4-Dinitrotoluene (DNT) and 2,4-dichlorophenol (DCP) were removed in water via sorption to the Fe(0)-included biochar. Compared to sorption control without Fe(0), the sorbed DNT and DCP were further transformed to reduction products by Fe(0)-included biochar. Compared to the reduction control with Fe(0), the presence of biochar promoted the reductive transformation of DNT and DCP. Increasing the pyrolysis temperature resulted in enhancing the removal of DNT and DCP, suggesting that the aromaticity of biochar may be responsible for the removal. The yields of the reduction products also indicated that unlike the direct reduction by Fe(0), different reduction pathways existed in the reduction of DNT and DCP with Fe(0)-included biochar. The results suggest that Fe(0)-included biochar is a viable option to immobilize and transform redox-sensitive organic contaminants in natural environments.

Microbial reduction of nitrate in the presence of zero-valent iron and biochar.

The denitrification of nitrate (NO3(-)) by mixed cultures in the presence of zero-valent iron [Fe(0)] and biochar was investigated through a series of batch experiments. It was hypothesized that biochar may provide microbes with additional electrons to enhance the anaerobic biotransformation of nitrate in the presence of Fe(0) by facilitating electron transfer. When compared to the anaerobic transformation of nitrate by microbes in the presence of Fe(0) alone, the presence of biochar significantly enhanced anaerobic denitrification by microbes with Fe(0). Graphite also promoted the anaerobic microbial transformation of nitrate with Fe(0), and it was speculated that electron-conducting graphene moieties were responsible for the improvement. The results obtained in this work suggest that nitrate can be effectively denitrified by microbes with Fe(0) and biochar in natural and engineered systems.

Sorption of halogenated phenols and pharmaceuticals to biochar: affecting factors and mechanisms.

The feasibility of using biochar as a sorbent to remove nine halogenated phenols (2,4-dichlorophenol, 2,4-dibromophenol, 2,4-difluorophenol, 2-chlorophenol, 4-chlorophenol, 2-bromophenol, 4-bromophenol, 2-fluorophenol, and 4-fluorophenol) and two pharmaceuticals (triclosan and ibuprofen) from water was examined through a series of batch experiments. Types of biochar, synthesized using various biomasses including fallen leaves, rice straw, corn stalk, used coffee grounds, and biosolids, were evaluated. Compared to granular activated carbon (GAC), most of the biochar samples did not effectively remove halogenated phenols or pharmaceuticals from water. The increase in pH and deprotonation of phenols in biochar systems may be responsible for its ineffectiveness at this task. When pH was maintained at 4 or 7, the sorption capacity of biochar was markedly increased. Considering maximum sorption capacity and properties of sorbents and sorbates, it appears that the sorption capacity of biochar for halogenated phenols is related to the surface area and carbon content of the biochar and the hydrophobicity of halogenated phenols. In the cases of triclosan and ibuprofen, the sorptive capacities of GAC, graphite, and biochars were also significantly affected by pH, according to the point of zero charge (PZC) of sorbents and deprotonation of the pharmaceuticals. Pyrolysis temperature did not affect the sorption capacity of halogenated phenols or pharmaceuticals. Based on the experimental observations, some biochars are good candidates for removal of halogenated phenols, triclosan, and ibuprofen from water and soil.

Factors affecting sorption of nitro explosives to biochar: pyrolysis temperature, surface treatment, competition, and dissolved metals.

The application of rice straw-derived biochar for removing nitro explosives, including 2,4-dinitrotoluene (DNT), 2,4,6-trinitrotoluene (TNT), and hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX), from contaminated water was investigated through batch experiments. An increase in the pyrolysis temperature from 250 to 900°C in general led to higher pH, surface area, cation exchange capacity (CEC), point of zero charge, and C:O ratio of biochar. The maximum sorption capacity estimated by a mixed sorption-partition model increased when pyrolysis temperatures were elevated from 250 to 900°C, indicating that C content and aromaticity of biochar were strongly related to the sorption of nitro explosives to biochar. Surface treatment with acid or oxidant increased the sorption capacity of biochar for the two strong π-acceptor compounds (DNT and TNT) but not for RDX. However, the enhancement of sorption capacity was not directly related to increased surface area and CEC. Compared with single-sorption systems, coexistence of explosives or cationic metals resulted in decreased sorption of each explosive to biochar, suggesting that sorption of nitro explosives and cationic metals to electron-rich portions in biochar was competitive. Our results suggest that π-π electron donor acceptor interactions are main sorption mechanisms and that changing various conditions can enhance or reduce the sorption of nitro explosives to biochar.

Recovery and electrochemical performance in lithium secondary batteries of biochar derived from rice straw.

Renewable biomass has attracted great attention for the production of biooil, biogas, and biochar, a carbon residual applicable for carbon sequestration and environmental remediation. Rice straw is one of the most common biomasses among agricultural wastes in South Korea. As part of our advanced and environmentally friendly research, we applied biochar derived from rice straw as the anode material for lithium-ion batteries (LIBs). Porous carbons with a high surface area were prepared from rice straw. Such porous carbons have exhibited particularly large reversible capacity and hence proven to be a candidate anode material for high-rate and high-capacity LIBs. Rice straw-derived biochars were synthesized at four different temperatures: 400, 550, 700, and 900 °C. The surface was modified by using HCl and H2O2 on the 550 °C biochar in order to increase the surface area. The resulting biochar was characterized by X-ray diffraction (XRD) and field-emission scanning electron microscopy (FE-SEM). The surface area was measured by Brunauer-Emmett-Teller (BET) method. The electrochemical characterizations were investigated by galvanostatic charge-discharge (CD) curves, cyclic voltammetry (CV), and electrochemical impedance spectroscopy (EIS). All samples exhibited reversible capacities of below 200 mAh g(-1). The surface-modified biochars exhibited improved cycle performance. Surface modification using HCl showed better cycle performance than H2O2. However, the capacities of the treated 550 °C biochar were similar to those of non-surface-modified biochar.

Sorptive removal of nitro explosives and metals using biochar.

The feasibility of using biochar as a sorbent to remove nitro explosives and metals from contaminated water was investigated through batch experiments. Biochar, synthesized using various biomasses, showed a porous structure and a high surface area and includes embedded carbonate minerals. Compared with granular activated carbon, biochar was competitive as a sorbent for removing Cd, Cu, Pb, and Zn from water according to the maximum sorption capacities of the metals. Some biochars also effectively sorbed nitro explosives from water. Correlation analysis between maximum sorption capacities and properties of biochar showed that the sorption capacity of biochar for cationic toxic metals is related to cation exchange capacity and that the sorption capacity of explosives is proportional to surface area and carbon content. Results from X-ray photoelectron spectroscopy and Fourier transform infrared spectroscopy analyses and laboratory experiments suggest that surface functional groups may be responsible for the sorption of cationic metals to the biochar surface. In contrast, carbon contents may account for the sorption of explosives, possibly through π-π electron donor-acceptor interactions. Our results suggest that biochar can be an attractive and alternative option in environmental remediation of nitro explosives and metals through sorption and immobilization and that appropriate selection of biochar may be necessary according to the types of contaminant and the properties of biochar.