Biochar modification to enhance arsenic removal from water: a review.

Arsenic (As) contamination is a major threat to drinking water quality throughout the world, and the development of appropriate remediation methods is critical. Adsorption is considered the most effective method for remediation of As-contaminated water. Biochar is a promising adsorbent and widely discussed for As removal due to its potential low cost and environmental friendliness. However, pristine biochar generally exhibited relatively low adsorption capacity for As mainly due to the electrostatic repulsion between the negatively charged biochar and As. Biochar modification, especially metal modification, was developed to boost the adsorption capacity for As. A systematic analysis of As removal as affected by biochar properties and modification will be of great help for As removal. This paper presents a comprehensive review on As removal by biochars from different feedstock, preparation procedures, and modification methods, with a major focus on the possible mechanisms of interaction between As and biochar. Biochar derived from sewage sludge exhibited relatively high adsorption capacity for As. Considering energy conservation, biochars prepared at 401-500 °C were more favorable in adsorbing As. Fe-modified biochar was the most popular modified biochar for As remediation due to its low cost and high efficiency. In addition, the limitations of the current studies and future perspectives are presented. The aim of this review is to provide guidance for the preparation of low-cost, environmentally friendly, and high efficiency biochar for the remediation of As-contaminated water.

Embedding of biochar in soil mineral fractions: Evidence from benzene polycarboxylic acids molecular biomarkers.

Investigators are debating on the positive and negative priming effects of biochar on native soil organic carbon (SOC), which is largely attributed to the technical barrier of identifying biochar contribution to the apparently measured SOC or mineralized CO2. We combined benzene polycarboxylic acids (BPCAs) molecular biomarkers and soil particle density fractionation to identify biochar contributions to the carbon content in three representative allitic soils in Yunnan. The soil-biochar mixture was incubated for one-month to avoid significant biodegradation of biochar. The results showed that BPCAs were mainly distributed in free light fractions (fLF) up to 87 % of the total BPCAs contents after one month incubation. Recognition of BPCAs in occluded light fractions (oLF) and heavy fractions (HF) suggested a significant interaction between biochar and soil mineral particles. In addition, the percentage of B6CA is comparable or even higher in HF than in fLF or oLF. Thus, biochar-mineral interactions may be an additional stabilization mechanism besides the condensed aromatic structures in biochar. The apparently measured carbon contents increased after biochar application, and both positive and negative priming effects to native SOC were observed after deducting biochar contents based an accurate calculation from BPCAs. The most native SOC depletion (positive priming effects) was noted for the soil with the most favored biochar embedding in soil mineral compositions. This study emphasized that combining BPCAs molecular biomarkers and soil particle density fractionation could accurately quantify different carbon pools, and thus facilitate a comprehensive understanding on the stabilization and turnover of biochar in soils.

Comparative study on the relative significance of low-/high-condensation aromatic moieties in biochar to organic contaminant sorption.

Aromatic moieties of biochar are considered as key components for immobilizing hydrophobic organic contaminants in the environment. However, the relative importance of different aromatic moieties such as low-/high-condensation components in sorption has not been comprehensively investigated. In this study, biochar was produced from flue-cured tobacco straw (TB) and pine wood sawdust (WB) at various pyrolysis temperatures (200-600 °C). Aromatic moieties were characterized via elemental analysis, Fourier transform infrared spectroscopy, Raman spectroscopy, and benzene polycarboxylic acid molecular markers (BPCAs). The significance of different aromatic moieties in the sorption of phenanthrene (PHE) and bisphenol A (BPA) was assessed based on the individual BPCA patterns. The results indicated that aromaticity and aromatic moiety contents increased with increasing pyrolysis temperature. Biochar at 200 °C produced lower mellitic acid (B6CA) contents (18.7-27.9%) than the others. When the pyrolysis temperature was increased to 600 °C, the B6CA contents representing high-condensation aromatic moieties accounted for 55.4-60.9% of all the aromatic moieties. The unitary linear regressions between the individual BPCA distribution patterns and the n values and log Kd suggested that the high-condensation aromatic moieties played a more significant role than the low-condensation aromatic moieties (represented by B3CA-B5CA) in facilitating sorption nonlinearity (for PHE and BPA) and sorption capacity (for PHE). The elevated sorption of PHE can be attributed to the increased specific surface area and hydrophobicity of the newly formed aromatic moieties. Hydrogen bonds and π-π electron-donor-acceptor were the main mechanisms of BPA sorption. Because the WB biochar contained more aromatic moieties and more O-containing groups on the surface of the TB biochar, the WB exhibited a higher sorption for PHE; however, slightly elevated sorption was observed on the TB for BPA. This research may provide a new perspective in understanding the behavior of biochar aromatic moieties in sorption of organic contaminants.

The molecular markers provide complementary information for biochar characterization before and after HNO3/H2SO4 oxidation.

Biochar inevitably goes through long-term aging under biotic and abiotic processes in the environment, which results in various changes in its physicochemical properties. However, the traditional characterization methods based on particle separation cannot effectively monitor biochar in complex matrixes. Molecular markers, especially benzene polycarboxylic acids (BPCAs), can be used to directly identify the source and properties of biochar. In this study, biochars were prepared using corn straw (CS) and pinewood (PW) and were oxidized with HNO3/H2SO4 to simulate the aging processes. Molecular markers of lignin-derived phenols showed that PW has more vanillyl unit and thus more stable than CS. The overall BPCAs content and the relative content of mellitic acid (B6CA) both increased with pyrolysis temperature, indicating increased aromatic condensation/aromaticity. The pristine CS biochar has a higher BPCAs content compared to PW biochar. HNO3/H2SO4 treatment greatly decreased the lignin components and more vanillyl and cinnamyl units were removed from CS biochar than PW biochar. In addition, BPCAs contents decreased by 41-60 mg/g for CS biochar, while increased by 86-133 mg/g for PW biochar after HNO3/H2SO4 oxidation. This is owing to the release of the condensed aromatic structures in CS biochars, but the concentration of the condensed aromatic structures in PW biochars after oxidation. These results showed that PW biochars are more stable than CS biochars. The application of the molecular markers can help understanding the dynamic change of biochar in the environment.

Organo-mineral complexes protect condensed organic matter as revealed by benzene-polycarboxylic acids.

Condensed organic matters (COM) with black carbon-like structures are considered as long-term carbon sinks because of their high stability. It is difficult to distinguish COM from general organic matter by conventional chemical analysis, thus the contribution by and interaction mechanisms of organo-mineral complexes in COM stabilization are unclear and generally neglected. Molecular markers related to black carbon-like structures, such as benzene polycarboxylic acids (BPCAs), are promising tools for the qualitative and quantitative analysis of COM. In this study, one natural soil and two cultivated soils with 25 y- or 55 y-tillage activities were collected and the distribution characteristics of BPCAs were detected. All the investigated soils showed similar BPCA distribution pattern, and over 60% of BPCAs were detected in clay fraction. The extractable BPCA contents were substantially increased after mineral removal. The ratios of BPCA contents before and after mineral removal indicate the extent of COM-mineral particle interactions, and our results suggested that up to 73% COM were protected by mineral particles, and more stronger interactions were noted on clay than on silt. The initial cultivation dramatically decreased COM-clay interactions, and this interaction was recovered only slowly after 55-y cultivation. Kaolinite and muscovite are important for COM protection. But a possible negative correlation between BPCAs and reactive iron oxides of the cultivated soils suggested that iron may promote COM degradation when disturbed by tillage activities. This study provided a new angle to study the stabilization of COM and emphasized the importance of organo-mineral complexes for COM stabilization.

Combining bulk characterization and benzene polycarboxylic acid molecular markers to describe biochar properties.

The physicochemical properties of biochar determined its sorption of organic contaminations, and the environmental aging process changed the biochar properties. However, the correlation between biochar heterogeneous properties and their sorption characteristics is unclear. In this study, peanut shell biochars were produced at 200-700 °C, and HNO3/H2SO4 was used to oxidize 400 °C biochar for 2-10 h to simulate the enhanced aging process of biochar in the environment. Benzene polycarboxylic acid (BPCA) molecular markers, and bulk characterization were analyzed to describe biochar physicochemical properties and to further predict the sorption characteristics to bisphenol A (BPA). For pristine biochars, the mellitic acid/BPCAs (B6CA/BPCAs) increased with the raise of pyrolysis temperature and the H/C atomic ratio was positively correlated with benzenepentacarboxylic acid/B6CA (B5CA/B6CA) (P < 0.01), which indicated the increased aromatic condensation. After HNO3/H2SO4 treatment, the aromaticity (H/C ratio) decreased while the highly condensed components in biochars were enriched (increased B6CA/BPCAs values). Multiple regression models were adopted to establish a quantitative relationship between biochar heterogeneous properties and their sorption of BPA. Both nonlinearity coefficient N values (N = 0.08 + 0.103 B5CA/B6CA + 0.721 (O + N)/C, R2 = 0.985) and single-point sorption coefficients log Kd (log Kd = 1.236 + 0.006 BPCAs + 1.449 (O + N)/C, R2 = 0.936) could be estimated combining molecular markers and polarity parameters for biochars.

Benzene polycarboxylic acid - A useful marker for condensed organic matter, but not for only pyrogenic black carbon.

Benzene polycarboxylic acid (BPCA) molecules are a widely used marker method for the qualitative and quantitative analysis of pyrogenic black carbons (BC). Based on an overview of the development and chemical reaction mechanism of the BPCA method, we propose that the commonly used BPCA markers may not be solely indicative of BC but more generally of condensed organic matter in soils and aquatic systems. First, we sequentially removed the soil fractions and observed that the BPCA contents were abundant in humic acids (HAs). After sequential treatment, the residual particles were supposed to contain BC and minerals; however, the BPCAs in the residue accounted for only 2.4-10.1% of that detected in the entire soil. In addition, substantial quantities of BPCAs were detected in both thermally treated samples and composted biomass. Furthermore, humic acids extracted from all the samples showed that obvious BPCA contents in the samples accounted for 0.1-121.7 mg/g. Therefore, soil fractionation may also partly extract BCs as suggested by BPCAs in the HAs of the biochars. However, organic matter without any thermal treatment may contain BPCAs. A series of standard substances without any BC showed high BPCA content in the samples from 5.9-124.5 mg/g. These observations create a serious concern for the proper application of BPCAs as a marker for BCs. Combining a systematic literature review of BPCA that deviates from BC content, we suggest that the BC-like structure of organic matter may be referred to as BPCA-probed organic matter content, which could be a more useful term for studies on the multimedia environmental behaviors of contaminants.

Molecular markers of benzene polycarboxylic acids in describing biochar physiochemical properties and sorption characteristics.

Biochar function in soil is based on properties such as sorption characteristics, and these are expected to change throughout the life cycle of the biochar. Because biochar particles cannot easily be separated from soil particles, this change is seldom investigated. Biochar-related molecular markers, such as benzene polycarboxylic acids (BPCAs) are promising tools for studying the properties of biochars in complex environmental matrices. In this study, biochars were derived from corn straw and pine wood sawdust at 200-500 °C, and their aging was simulated with NaClO. Biochar properties were characterized by elemental analysis, BET surface characterization and BPCA molecular marker analysis. Chemical oxidation decreased the surface area (SA) but increased the O content of biochars. The oxidation decreased the amount of biochars, with a mass loss in the range of 10-55%. A similar mass loss was also observed for BPCAs and was negatively related to both the pyrolysis temperature and the extent of the condensed structure (higher aromaticity). The biochar amounts were calculated quantitatively using the sum of BPCA contents, with a conversion factor (the ratio of biochar amount to BPCA content) in the range of 3.3-5.5, and were negatively related to the B5CA content. Three model pollutants, namely, bisphenol A (BPA), sulfamethoxazole (SMX), and phenanthrene (PHE), were chosen to study the sorption characteristics of biochar before and after oxidation. Chemical oxidation generally increased SMX sorption but decreased PHE sorption. The nonlinear factor n, based on Freundlich equation modeling, was negatively related to B6CA for all three chemicals. The BPCA molecular markers, especially B5CA and B6CA, were correlated to the biochar properties before and after oxidation and are thus a potentially useful technique for describing the characteristics of biochar in the environment.