Sorption of four hydrophobic organic contaminants by biochars derived from maize straw, wood dust and swine manure at different pyrolytic temperatures

Release of polycyclic aromatic hydrocarbons from biochar fine particles in simulated lung fluids: Implications for bioavailability and risks of airborne aromatics

Airborne carbonaceous fine particles, such as soot and biochar, represent a significant fraction of air particulate matter and have received widespread concern due to their health effects. Atmospheric carbonaceous particles can contain high concentration of polycyclic aromatic hydrocarbons (PAHs), and may pose significant health risks when carried into respiratory system from inhalation of particulates. In this study, the bioaccessibility of two PAH compounds, phenanthrene and pyrene, bound to biochar fine particles was assessed by examining their release in two simulated lung fluids: Gamble's solution and artificial lysosomal fluid (ALF). We observed that only 0.47 to 0.75% of biochar-bound PAHs were released in the simulated lung fluids, most likely due to the physical entrapment of PAH molecules in the micropore regimes of biochar, resulting in strong desorption hysteresis, even though apparent desorption equilibrium was reached within 30 min, well within the average clearance time of particulate matter in lung system. The inorganic and organic salts in the simulated lung fluids were found to inhibit the release of PAHs by exerting the pore blockage effect and salting-out effect. Moreover, the low molecular weight organic acids (LMWOAs) in the lung fluids further inhibited PAH release by increasing the micropore volume and surface area of biochar fine particles. When taking into account the inhibited release, the estimated carcinogenic risks of biochar-bound PAHs are typically low, even under extreme conditions wherein both biochar concentrations and PAH loadings on biochar are very high. An important implication is that contaminant bioavailability needs to be taken into account when assessing the risks of the contaminants bound to airborne carbonaceous materials.

Impact of percentage and particle size of sugarcane biochar on the sorption behavior of clomazone in Red Latosol

Biochar is a carbonaceous material that has excellent potential as a fertilizer and soil conditioner. However, there is a lack of information concerning the effects of the amount and particle size of this pyrogenic material on the soil sorption capacity. In this work, evaluation was made of changes in clomazone (CMZ) sorption in a Red Latosol following soil conditioning using different percentages (0.25, 0.5, and 1% w/w) of sugarcane biochar in three particle sizes (<106, 106-508, and 508-610 µm). The conditioned soils presented apparent sorption coefficients (Kd) up to 1300 times higher than that of pure soil, besides changes in the behavior of CMZ sorption. The biochar particle size and percentage influenced sorption of the herbicide as well as its retention in the amended soil during desorption processes. Both sorption and desorption Freundlich constants were linearly correlated with the external surface area of the biochar present in the soil.

Nanopore-filling effect of phenanthrene sorption on modified black carbon

Black carbon was produced by slow pyrolysis under an oxygen-limited condition at 500 °C, and was modified by some chemical methods (oxidation, hydrolysis, activation, and surface recombination). The modified samples were characterized by using elemental analysis, Fourier transformed infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS) surface analysis, Boehm titration, cation exchange capacity(CEC)analysis, CO₂ adsorption analysis, and then used to investigate the sorption behavior of phenanthrene. The results showed that the activation of ZnCl₂ gave a maximum nanopore volume of 96.5 μL/g and a specific surface area of 241 m²/g, while the oxidation of NaClO gave a minimum nanopore volume of 63.3 μL/g and a specific surface area of 158 m²/g. The FTIR, XPS, and Boehm titration analysis showed that the new oxygen-containing functional groups were introduced during the oxidation treatments of H₂O₂ and NaClO. The sorption of phenanthrene on all samples was typically nonlinear, and the nonlinear factor (n) was negatively correlated with V_o, especially with V_o at 0-1.1 nm. The sorption parameter (log K_OC) was positively correlated with nanopore volume (V_o) and specific surface area (SSA). Moreover, the model analysis showed that the nanopore filling was the main sorption mechanism, and molecular sieve effect was observed in the sorption of phenanthrene.

Influence of aromatic structure and substitution of carboxyl groups of aromatic acids on their sorption to biochars

In order to clarify the influence of aromatic structure and COOH substitution of aromatic acids on their sorption to biochars, benzoic acid (BA), phthalic acid (PA), hemimellitic acid (HA), 2-biphenylcarboxylic acid (2-BA), 1-naphthoic acid (1-NA) and naphthalene were selected as model sorbates. Batch experiments on sorption of them to wheat straw-derived biochars at 300 °C (WS300) and 700 °C (WS700) were conducted. Results showed that WS700 with higher specific surface area and pore volume had faster and higher sorption of aromatic acids than WS300. Sorption affinity of aromatic acids decreased with increasing number of aromatic rings (BA > 1-NA > 2-BA), and was weakened by COOH substitution (BA > PA > HA). This was likely due to the π-electron delocalization into additional ring, reduced contact area of nonplanar aromatic structure on biochar surfaces, size exclusion of larger molecules in smaller pores of biochars and decreased hydrophobicity of aromatic acids by COOH substitution that abated the sorption. Dissociation of COOH substitution of aromatic acids also weakened their sorption to biochars due to the lower hydrophobicity of anionic species, and electrostatic repulsion between anionic species and negatively charged surface of biochars.

Sorption of tetracycline on H3PO4 modified biochar derived from rice straw and swine manure

Currently, the information about the sorption of tetracycline (TC) on animal manure derived biochar was rare although plant residue derived biochar showed high sorption of TC). Therefore, this study explored the sorption of TC on swine manure derived biochar, and compared with rice straw derived biochar simultaneously. Also, H₃PO₄ was adopted to modify both types of biochar. The sorption kinetic and isotherm data showed H₃PO₄ modification enhanced the sorption of TC on both types of biochar (especially swine-manure-biochar), and indicated the chemisorptions including H-bonding and π-π electron donor acceptor interaction might be the primary mechanism. Moreover, the strengthened electrostatic attraction between TC and biochars might largely explain the enhanced sorption capacity of TC along with pH increasing from 5.0 to 9.0. At the same conditions, swine manure derived biochar demonstrated lower sorption capacity of TC than rice straw biochar, but still could be good material for the sorption of TC.

Speciation of phosphorus in plant- and manure-derived biochars and its dissolution under various aqueous conditions

Phosphorus (P) in biochar serves as both a P source for plant growth and a contributor to water eutrophication, thus prioritizing the efficient management of P in biochar. This study employed solid- and solution- state ³¹P-nuclear magnetic resonance and X-ray diffraction analyses to explore the impact of feedstock and heating treatment temperature (HTT) on P species of biochars. The effects of ambient temperature, coexisting anions, pH and nutrient solution on P release were also investigated to study the effect of various environmental factors on P release from biochars. P species in both plant- and manure- derived biochars were dominated by inorganic orthophosphate and pyrophosphate (mainly calcium-bound-phosphates). The HTT of biochar showed a negative impact upon its pyrophosphate content. Compared with plant biochars, manure biochars contained higher P but had a lower release degree. Release of P from biochars was controlled by diffusion-dissolution process and was enhanced by higher ambient temperature, co-existing anions, and both acidic and alkaline conditions but inhibited by coexisting Hoagland nutrients. Anion-induced increase in P release was more significant for plant biochars than manure biochars. These findings help to adjust favorable environmental conditions for the full utilization of P in biochars.

Effect of minerals on the stability of biochar

The stability of biochar is a major determinant of its potential for carbon sequestration. In previous studies, the biochar stability was evaluated by analyzing the oxidation resistance property of biochar, for which, K₂Cr₂O₇/H₂O₂ oxidation and thermogravimetric analysis (TGA) were widely used to measure the high oxidation resistance of biochars. In this study, rice straw (RI)- and swine manure (SW)-derived biochars produced at different heating treatment temperatures (HTTs) and their corresponding de-ashed biochars and iron-biochars were selected to investigate the effect of minerals on these biochars stability. There was a significantly positive correlation between carbon remaining values (CRVs) obtained after K₂Cr₂O₇/H₂O₂ oxidation and ash (mineral) contents of biochars (p < 0.05). Moreover, the decreasing CRVs of most biochars were observed after de-ashing treatment. These K₂Cr₂O₇/H₂O₂ oxidation results indicated that endogenous minerals in biochars can protect biochars from chemical oxidation. On the other hand, the R₅₀ (C recalcitrance index) values of iron-biochars (33.3-57.4%) were lower than those of corresponding biochars (38.6-60.8%), indicating that Fe-bearing mineral formed in biochars can promote the thermal decomposition of biochars. In H₂O₂ oxidation, the different change trend of CRVs between biochars produced at 450 °C and biochars produced at 600 °C after iron mineral treatment showed that the effect of exogenous Fe-bearing mineral on biochar stability was related to biochar category regulated by HTTs. This study indicated that endogenous minerals could increase biochar stability, and exogenous Fe-bearing minerals had dissimilar effects on different kinds of biochars, which critically regulates the potential of biochar for long-term carbon sequestration.

Sorption mechanisms of chlorinated hydrocarbons on biochar produced from different feedstocks: Conclusions from single- and bi-solute experiments

Biochar is increasingly deemed a potential sorbent for contaminants in soil and water remediation. We tested three biochars from different feedstocks (cattle manure, grain husk, and wood chips) produced at relatively low pyrolysis temperature (450 °C), for their sorption behavior towards trichloroethylene (TCE) and tetrachloroethylene (PCE) in single- and bi-solute systems. In single-solute experiments, all biochars show stronger sorption for TCE (about 50% based on solubility-normalized Freundlich coefficients). The lower sorption of PCE is attributed to steric effects, e.g. size exclusion in small micropores and specific interactions. Plant-derived, carbon-rich biochars with high specific surface area and microporosity predominantly sorb via pore-filling, as also observed in activated carbon. Biochar produced from manure, with higher ash content and polarity, and smaller total pore volume (PV_tot), shows significant contribution of partitioning. These findings also apply to bi-solute systems. TCE and PCE show different competition behavior depending on biochar properties. Plant-based biochars are pore-filling-dominated and show strong competition. However, competition behavior in microporous biochars depends on the concentration range. Manure biochar with high polarity and low PV_tot shows significant partitioning and therefore less competition. Compared to the plant-based chars competition in manure biochar is not concentration-dependent. These results indicate that biochars with a large fraction of non-carbonized phase facilitate non-competitive sorption and might be a valuable sorbent in mixed contaminant systems.

Effects of metal ions and pH on ofloxacin sorption to cassava residue-derived biochar

In this study, the impacts of various cations, cation strength and pH on ofloxacin (OFL) adsorption to cassava residue-derived biochars were determined. The associated adsorption mechanisms are discussed. The biochars were prepared at pyrolysis temperatures ranging from 350°C to 750°C, and labeled as CW350, CW450, CW550, CW650 and CW750. The Freundlich model provided the best fit to describe the adsorption capacity of OFL and the Freundlich coefficient (logK_f) increased with increasing pyrolysis temperature. The inclusion of Zn²⁺ or Al³⁺ increased OFL sorption capacities of five biochars, while Cu²⁺ reduced sorption to CW450 and CW550. No significant impacts on OFL sorption were observed in the presence of K⁺ and Ca²⁺. The concentration of Ca²⁺ affected the adsorption capacity of CW550, but had no significant impact on other biochars. The pH of OFL solution, ranging from 3 to 9, had no significant changes on OFL adsorption by all the tested biochars. Results of FTIR spectra and zeta potential indicated that electrostatic interactions, cationic exchange, metal bridging and micropore filling could be the main sorption mechanism between OFL and biochars. These studies indicated that cassava residue can be converted into biochars that are effective adsorbents for removing OFL from aqueous solution.

Comparative study for microcystin-LR sorption onto biochars produced from various plant- and animal-wastes at different pyrolysis temperatures: Influencing mechanisms of biochar properties

Diverse biochars produced from various feedstock sources (i.e., plant- and animal-wastes) at different pyrolysis temperatures (PTs) were characterized for their structural properties and sorption behaviors of aqueous microcystin-LR (MC-LR). Results indicated that MC-LR sorption capability of tested biochars varied as a function of biochar structural properties. Sorption mechanisms involved electrostatic attraction, pore-filling, hydrogen-bonding effect and π-π electron donor-acceptor interaction, but predominant mechanisms varied for different biochars. At the same PT (300 or 600°C), chicken manure-derived biochars (CMBs) exhibited stronger MC-LR sorption than others, with sorption coefficient (K_d) of 6.321-15.529Lg^-1 and 6.354-8.294Lg^-1 at aqueous equilibrium concentration (C_e) of 40 and 200μgL^-1, respectively. Higher mesoporosity, the point of zero charge and total surface groups concentration related to higher ash content of CMBs, which might be indispensable for enhancing MC-LR sorption. This study suggested that CMBs have great potential as low-cost sustainable sorbents to abate MC-LR contamination.

Challenges and recent advances in biochar as low-cost biosorbent: From batch assays to continuous-flow systems

Over the past few years, the increasing amount of pollutants and their diversity demand to develop versatile low-cost adsorption systems. The use of biomass feedstock such as agricultural residues, wood chips, manure or municipal solid wastes as source to produce low-cost biosorbent, and the new advances in their synthesis have encouraged remarkable efforts towards the development of biochar "on demand" in which their characteristics can be improved. This new trend opens the potential of biochar application in the removal of pollutants from wastewater, however, its use in environmental management requires the development of full-scale biosorption in engineered systems. Thus, this paper provides a brief review of recent progress in the research and practical application of biochar with a special emphasis on its potential to reduce the pollutants present in wastewater or to render them harmless. Furthermore, research gaps and uncertainties detected in their scale-up in continuous-flow systems are highlighted.

Biochar for volatile organic compound (VOC) removal: Sorption performance and governing mechanisms

Sorption is one of the most efficient and low cost strategies for volatile organic compound (VOC) removal, but VOC sorption by biochar has seen limited research. In this work, gas phase sorption experiments were conducted to determine the sorption potential and mechanisms of VOCs onto biochar. A total of 15 biochars produced from 5 common feedstocks at 300, 450, and 600°C were evaluated as sorbents. Three common VOCs (acetone, cyclohexane, and toluene) were chosen as sorbates. The results showed that all the tested biochars had VOC sorption capacity in the range of 5.58-91.2mgg^-1. The sorption capacities were mainly influenced by both the surface area of biochar and its noncarbonized organic matter content. The vapor sorption process was exothermic, and the removal of VOCs by the biochars decreased with increasing feedstock temperature. Both the physical adsorption and partition mechanisms played important roles in controlling the VOC removal by the biochars. Biochar maintained its VOC removal ability after five consecutive sorption-desorption cycles, which indicated good reusability. Findings of this work suggest that biochar is a promising alternative sorbent for gaseous VOC removal.

Roles of polar groups and aromatic structures of biochar in 1-methyl-3-octylimidazolium chloride ionic liquid adsorption: pH effect and thermodynamics study

Adsorption mechanisms of 1-methyl-3-octylimidazolium chloride ([OMIM]Cl) on rice straw-derived biochars produced at 400, 500, and 700 °C (referred as RB400, RB500, and RB700, respectively) were evaluated. Adsorption affinity followed the order of RB700 > RB400 > RB500. Electrostatic attraction and hydrogen bond controlled adsorption of [OMIM]Cl on RB400, while π-π EDA interaction between [OMIM]Cl and the aromatic rings of biochar dominated adsorption of RB500 and RB700. With increasing solution pH, -COOH and -OH on biochar became deprotonated. Consequently, [OMIM]Cl binding to these sites changed from hydrogen bond to electrostatic attraction. Adsorption capacity of [OMIM]Cl increased with increasing pH during the adsorption process. Solid concentration induced by -OH of [OMIM]Cl was higher than that of -COOH. Thermodynamics study indicated that adsorption process was spontaneous and endothermic. ∆H ⁰ values indicated that [OMIM]Cl adsorption on biochars was a physisorption.

The importance of nano-porosity in the stalk-derived biochar to the sorption of 17β-estradiol and retention of it in the greenhouse soil

Natural estrogens in greenhouse soils with long-term manure application are becoming a potential threat to adjacent aquatic environment. Porous stalk biochar as a cost-effective adsorbent of estrogen has a strong potential to reduce their transportation from soil to waters. But the dominant adsorption mechanism of estrogen to stalk biochars and retention of estrogen by greenhouse soils amended with biochar are less well known. Element, function groups, total surface area (SA_total), nano-pores of stalk biochars, and chemical structure of 17β-estradiol (E2, length 1.20 nm, width 0.56 nm, thickness 0.48 nm) are integrated in research on E2 sorption behavior in three stalk-derived biochars produced from wheat straw (WS), rice straw (RS), and corn straw (CS), and greenhouse soils amended with optimal biochar. The three biochars had comparable H/C and (O + N)/C, while their aromatic carbon contents and total surface areas (SA_total) both varied as CS > WS > RS. However, WS had the highest sorption capacity (logK _oc), sorption affinity (K _f ), and strongest nonlinearity (n). Additionally, the variation of Langmuir maximum adsorption capacity (Q ⁰) was consistent with the trend for SA_1.2-20 (WS > RS > CS) but contrary to the trend for SA_total and SA_<1.2 (CS > WS > RS). These results indicate that pore-filling dominates the sorption of E2 by biochars and exhibits "sieving effect" and length-directionality-specific via H-bonding between -OH groups on the both ends of E2 in the length direction and polar groups on the inner surface of pores. After the addition of wheat straw biochar, the extent of increase in the sorption affinity for E2 in the soil with low OC content was higher than those in the soil with high OC content. Therefore, the effectiveness for the wheat straw biochar mitigating the risk of E2 in greenhouse soil depended on the compositions of soil, especially organic matter.

Properties of biochar-amended soils and their sorption of imidacloprid, isoproturon, and atrazine

Biochars produced from rice straw, wheat straw and swine manure at 300, 450 and 600°C were added to soil at 1, 5, 10, or 20% levels to determine whether they would predictably reduce the pore water concentration of imidacloprid, isoproturon, and atrazine. The sorption capacity of the mixtures increased with increasing biochar amounts. The enhanced sorption capacity could be attributed to the increased organic carbon (OC) content and surface area (SA) as well as the decreased hydrophobicity. Biochar dominated the overall sorption when its content was above 5%. The OC contents of the mixtures with 10% and 20% biochar were generally lower than the predicted values. This implies possible interaction between soil components and biochar and/or the effect of biochar oxidation. For soils amended with biochars produced at 300°C, the N2 SA (N2-SA) values were underestimated. The predicted CO2 SA (CO2-SA) values of the mixtures at the biochar content of 10% and 20% were generally higher than the experimental values. Sorption of imidacloprid to the soils amended with biochar at 10% and 20% levels, excluding the soils amended with rice (SR300) and wheat (SW300) straw-derived biochar produced at 300°C, was lower than the predicted value. For SR300 and SW300, the intrinsic sorption capacity of biochar was enhanced by 1.3-5.6 times, depending on the biochar, solute concentration, and biochar dose. This study indicates that biochars would be helpful to stabilize the soil contaminated with imidacloprid, isoproturon, and atrazine, but the sorption capacity of the mixtures could exceed or fall short of predicted values without assuming a cross-effect between soil and biochar.