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• BDOC promoted the dehydrochlorination of 1,1,2,2-tetrachloroethane in basic solution. • Synergies of dipole-dipole and conjugate bases was the dominant catalysis mechanism. • The promoting effect was enhanced at a higher pH by forming more conjugate bases. • >1 kDa fraction of BDOC presented greater promoting effect than <1 kDa fraction. • N-containing species and aromatic protein-/polyphenol-like matters played key roles. The environmental effects of biochar-derived organic carbon (BDOC) have attracted increasing attention. Nevertheless, it is unknown how BDOC might affect the natural attenuation of widely distributed chloroalkanes (e.g., 1,1,2,2-tetrachloroethane (TeCA)) in aqueous environments. We firstly observed that the kinetic constants (k e) of TeCA dehydrochlorination in the presence of BDOC samples or their different molecular size fractions (<1 kDa, 1∼10 kDa, and >10 kDa) ranged from 9.16×103 to 26.63×103 M−1h−1, which was significantly greater than the k e (3.53×103 M−1h−1) of TeCA dehydrochlorination in the aqueous solution at pH 8.0, indicating that BDOC samples and their different molecular size fractions all could promote TeCA dehydrochlorination. For a given BDOC sample, the kinetic constants (k e) of TeCA dehydrochlorination in the initial pH 9.0 solution was 2∼3 times greater than that in the initial pH 8.0 solution due to more formation of conjugate bases. Interestingly, their DOC concentration normalized kinetic constants (k e /[DOC]) were negatively correlated with SUVA 254 , and positively correlated with A 220 /A 254 and the abundance of aromatic protein-like/polyphenol-like matters. A novel mechanism was proposed that the C H dipole of BDOC aliphatic structure first bound with the C Cl dipole of TeCA to capture the TeCA molecule, then the conjugate bases (-NH-/-NH 2 and deprotonated phenol-OH of BDOC) could attack the H atom attached to the β-C atom of bound TeCA, causing a C Cl bond breaking and the trichloroethylene formation. Furthermore, a fraction of >1 kDa had significantly greater k e /[DOC] values of TeCA dehydrochlorination than the fraction of <1 kDa because >1 kDa fraction had higher aliphiticity (more dipole-dipole sites) as well as more N-containing species and aromatic protein-like/polyphenol-like matters (more conjugate bases). The results are helpful for profoundly understanding the BDOC-mediated natural attenuation and fate change of chloroalkanes in the environment. [Display omitted] [ABSTRACT FROM AUTHOR]

Biochar as an effective adsorbent can be used for the removal of triclocarban from wastewater. Biochar-derived dissolved organic carbon (BC-DOC) is an important carbonaceous component of biochar, nonetheless, its role in the interaction between biochar and triclocarban remains little known. Hence, in this study, sixteen biochars derived from pine sawdust and corn straw with different physico-chemical properties were produced in nitrogen-flow and air-limited atmospheres at 300–750 °C, and investigated the effect of BC-DOC on the interaction between biochar and triclocarban. Biochar of 600∼750 °C with low polarity, high aromaticity, and high porosity presented an adsorption effect on triclocarban owing to less BC-DOC release as well as the strong π-π, hydrophobic, and pore filling interactions between biochar and triclocarban. In contrast and intriguingly, biochar of 300∼450 °C with low aromaticity and high polarity exhibited a significant solubilization effect rather than adsorption effect on triclocarban in aqueous solution. The maximum solubilization content of triclocarban in biochar-added solution reached approximately 3 times its solubility in biochar-free solution. This is mainly because the solubilization effect of BC-DOC surpassed the adsorption effect of biochar though the BC-DOC only accounted for 0.01–1.5 % of bulk biochar mass. Furthermore, the high solubilization content of triclocarban induced by biochar was dependent on the properties of BC-DOC as well as the increasing BC-DOC content. BC-DOC with higher aromaticity, larger molecular size, higher polarity, and more humic-like matters had a greater promoting effect on the water-solubility of triclocarban. This study highlights that biochar may promote the solubility of some organic pollutants (e.g., triclocarban) in aqueous environment and enhance their potential risk. [Display omitted] • Biochar of 300∼450 °C improved the solubility of triclocarban, • Biochar of 600∼750 °C presented an adsorption effect on triclocarban, • Solubilization effect of biochar depended on the content and properties of BC-DOC, • Biochar might enhance the mobility and the potential risk of triclocarban. [ABSTRACT FROM AUTHOR]

Biochar is composed of carbonaceous and inorganic (ash) fractions. The structural properties of carbonaceous fractions and the composition of ash in biochar are both variable with pyrolysis temperature. However, it is unknown whether ash may play different roles in sorption of organic compounds to the carbonaceous fraction of biochars produced at different temperatures. Hence, in this study, the pristine biochars produced at 300–900°C and their corresponding deashed biochars were investigated, and the combined roles of carbonaceous fraction and ash in sorption of triclosan were compared. The results showed that the biochars produced at 300–400°C had high content of uncarbonized organic structure with dominating partition effect. The combination of uncarbonized organic structure and ash had comparable or even higher sorption coefficient (KD) for triclosan at low concentration compared with a single uncarbonized organic structure. However, for the biochars produced at 600–900°C, which were mainly composed of carbonized or graphitized carbon structure, ash had significant effect on triclosan sorption by reducing surface adsorption and pore filling effect. The combination of carbonaceous fraction and ash decreased KD values for triclosan at any tested concentrations. In addition, the results of pH effect on sorption indicated that ash possibly decreased the electrostatic repulsion of deprotonated phenolic hydroxyl between biochars and triclosan. Accordingly, it will be more valuable to design biochars for pollutant sorption from the perspective of combined role of carbonaceous fraction and ash rather than a single role of carbonaceous fraction. [ABSTRACT FROM AUTHOR]

Abstract Currently, the role of amorphous carbon structure (ACS) in sorption of chlorinated hydrocarbons (CHs) to biochars remains little known. Therefore, three CHs (1,1,2,2-tetrachloroethane, 1,3,5-trichlorobenzene and γ-hexachlorocyclohexane) with different molecular properties were selected as model adsorbates to investigate the effect of ACS on sorption of CHs to biochars produced at seven different pyrolysis temperatures (300–900 °C). There were two main mechanisms for ACS controlling the sorption of CHs. First, the polar sites on ACS are hydrophilic, CHs with greater polarity could strongly compete with the water molecule for the hydrophilic sites. Second, ACS of low temperature (300–400 °C) produced biochars possessing the natural organic matter (NOM)-like structure occupied some hydrophobic sites on condensed graphitic structure (CGS) of biochars. CHs with great hydrophobicity possibly seized the hydrophobic sorption sites on CGS from the NOM-like structure. Therefore, ACS of biochar was more benefit for sorption of strong polar CHs (1,1,2,2-tetrachloroethane: π∗ = 0.95; Log K ow = 2.39) or strong hydrophobic CHs (1,3,5-trichlorobenzene: π∗ = 0.70; Log K ow = 4.19) than CHs (γ-hexachlorocyclohexane: π∗ = 0.68; Log K ow = 3.72) with relatively low polarity and hydrophobicity. The result reflects that the interaction between NOM and natural black carbon/biochars in soil and water environment possibly plays the similar role in controlling the environmental behavior of various polar or hydrophobic organic pollutants. Moreover, with increasing concentration of adsorbate (C e), the first mechanism enhanced, while the second mechanism weakened. This study gives a deep insight into the roles of ACS of biochars in controlling the fate and availability of CHs with different molecular properties in environment. Graphical abstract Image 1 Highlights • The role of ACS in sorption of biochars is closely related to the molecular properties of CHs. • CHs with greater polarity compete with water molecular for the hydrophilic sites on ACS. • CHs with stronger hydrophobic compete with NOM-like structure for the hydrophobic sites on CGs. • Sorption behavior of CHs on ACS depends on the concentration of CHs. [ABSTRACT FROM AUTHOR]

[Display omitted] • Biochars were produced in three atmospheres at the temperatures of 300–750 °C; • Air-limitation made biochar have less C and more inorganic elements than CO 2 and N 2 ; • Biochars produced in CO 2 and N 2 had greater energy densification ratios; • Biomass pyrolysis in CO 2 and N 2 had greater C stability and C sequestration potential; • Temperature-variability of biochar followed the order: air-limitation > CO 2 > N 2. This study systemically investigated the characteristics of biochars derived from thermo-conversion of pine sawdust and wheat straw in air-limitation, CO 2 , and N 2 atmospheres at the temperatures of 300–750 °C. Meanwhile, their energy and C stability parameters were also evaluated here. The results showed that biochar produced in air-limitation had less yield, fixed C and bulk C, as well as more volatile matter and inorganic elements than that produced in CO 2 and N 2. Biochars derived from thermo-conversion of pine sawdust in CO 2 and N 2 at 450–750 °C had the greatest energy densification ratios (EDR) (range: 1.40–1.61), because pine sawdust contained more lignin than wheat straw, and the thermo-conversion of lignin in N 2 and CO 2 at 450–750 °C benefited for the formation of fixed C. Recalcitrance potential (R 50) results showed that the biochars produced in CO 2 and N 2 at 600–750 °C had the highest carbon stability (R 50 : 0.54–0.64) for given biomass, owing to the thermo-conversion of biomass in CO 2 and N 2 at 600–750 °C preferring to form the organic C with high aromaticity and low polarity. Nonetheless, thermo-conversion of biomass in CO 2 and N 2 at 300 °C presented the greatest C sequestration potential, owing to high biochar yields under these conditions. Generally, the temperature-variability for the composition, EDR, and C sequestration potential followed the order: air-limitation > CO 2 > N 2 , whereas carbon stability presented an opposite order. Our results contributed to selecting the appropriate atmosphere to optimize the properties and performances of biochars. [ABSTRACT FROM AUTHOR]

Biochar colloids are likely to control the environmental fate of heavy metals because of the widespread application of this carbon-based material in environmental remediation. Low-molecular-weight aromatic acids (LWMAAs), a class of common organic substances in the environment, are critical in affecting biochar colloid-mediated transport of heavy metals in aquifer media. Nevertheless, thus far, little is known about the effects of LWMAAs on their co-mobility behaviors. Herein, the influences of two aromatic acids, including cinnamic acid and caffeic acid, on the biochar colloid-assisted mobility of Cd2+ in quartz sand were examined. Generally, LMWAAs promoted biochar colloid mobility because of the enhanced electrostatic repulsion, competitive retention between LMWAAs and colloids, and steric hindrance. More importantly, biochar colloids could act as carriers for contaminants to considerably facilitate Cd2+ transport, mainly due to the strong binding affinities of colloids toward Cd2+ and the high mobility of colloids. Intriguingly, although the presence of LMWAAs inhibited Cd2+ adsorption onto biochar colloids, the two aromatic acids enhanced the contaminant-mobilizing transport capacity of colloids, owing to the increased colloid mobility and the formation of stable Cd-LMWAA complexes. Furthermore, the promotion effects of caffeic acid on colloid mobility or colloid-mediated transport of Cd2+ were greater than those of cinnamic acid. This pattern was related to the different chemical properties (e.g., molecular size and/or hydrophobicity) of the two LMWAAs. The observations of this paper provide useful information for understating the effects of coexisting LMWAAs on the co-mobility behaviors of carbonaceous materials and heavy metals in the groundwater environment. [Display omitted] • LMWAAs considerably enhance the transport of biochar colloids. • Steric repulsion and deposition site competition are critical mechanisms. • Highly mobile biochar colloids serve as carriers to enhance Cd2+ mobility markedly. • LMWAAs promote the colloid-facilitated transport of Cd2+ via multi-mechanisms. • The improved effect on colloid-assisted mobility was LMWAA-type dependent. [ABSTRACT FROM AUTHOR]

[Display omitted] • Biochars are produced in air-limitation (AL), CO 2 , and N 2 to adsorb tetracycline. • Biochars from AL and CO 2 pyrolysis present better sorption performance. • The better sorption performance results from high SSA, pore volume, and ash content. • The sorption behavior of biochar from N 2 pyrolysis depends on its graphitic degree. • The sorption ability of biochars from CO 2 and AL pyrolysis is more pH-dependent. Presently, as the prevalent pyrolysis atmospheres, N 2 is widely used, while air-limitation and CO 2 are rarely considered, to produce biochar to adsorb tetracycline. This study thus used N 2 , CO 2 , and air-limitation to produce various biochars at 300 ∼ 750 °C, and explored their structure-performance relationship for tetracycline sorption. The maximum sorption capacities of biochars produced in CO 2 and air-limitation were 55.36 mg/g and 71.11 mg/g (at 750 °C), respectively, being 2.34 and 3.01 times that of biochars produced in N 2 (23.60 mg/g at 750 °C). Interestingly, except for high pore volume and specific surface area supported pore filling and sites providing effect, ash (containing metal cations, P-O, and S=O) induced complexing effect was the primary mechanism for tetracycline sorption, rather than hydrophobic effect, π-π interaction, and hydrogen bond caused by C composition. This study provides important information about adjusting the pyrolysis atmosphere to improve the sorption performance of biochar toward tetracycline. [ABSTRACT FROM AUTHOR]

• N 2 and CO 2 pyrolysis retained more organic carbon than air-limitation (AL) pyrolysis. • AL, CO 2 , and N 2 pyrolysis made chemical groups have different temperature trends. • CO 2 and N 2 pyrolysis made biochar have more C C/C C groups than AL pyrolysis. • AL and CO 2 pyrolysis supported C O to retain in biochar relative to N 2 pyrolysis. • With elevated temperature, AL pyrolysis decreased the graphitic carbon content. Currently, the effects of various pyrolysis atmosphere types (air-limitation, carbon dioxide, and nitrogen) on the chemical compositions and carbon structures of biochar remain little known. Elemental analysis, X-ray photoelectron spectroscopy (XPS), two-dimensional perturbation correlation infrared spectroscopy (2D-PCIS), and Raman spectroscopy were applied to investigate the chemical compositions and carbon structures of pine sawdust- and wheat straw-derived biochars produced in air-limitation, carbon dioxide, and nitrogen atmospheres, as well as their variation with the charring temperature. The results showed that, of the three atmospheres, air-limitation pyrolysis made biochar contain more inorganic elements, while carbon dioxide and nitrogen preferred to retain organic carbon in biochars. The 2D-PCIS spectra indicated air-limitation pyrolysis supported to retain methylene of glucopyranose ring, while carbon dioxide and nitrogen pyrolysis promoted its breaking. Carbon dioxide and nitrogen pyrolysis made an earlier decline in H-bond of alcohol and phenolic groups than methylene, and air-limitation pyrolysis presented an opposite trend. The XPS spectra showed that, at 450–750 °C, biochars produced in carbon dioxide and nitrogen had more surface C C/C C groups than biochars produced in air-limitation. Furthermore, the content of C O followed the order of nitrogen > or ≈ carbon dioxide > air-limitation at 300 °C, and was approximately equivalent at 450–750 °C for the three atmospheres. Air-limitation and carbon dioxide pyrolysis supported more C O to retain in biochars than nitrogen pyrolysis. With elevated charring temperature, the carbon content of graphitic structure decreased for biochars produced in air-limitation and increased for biochars produced in carbon dioxide and nitrogen. The carbon content of graphitic structure followed the order of air-limitation > carbon dioxide ≈ nitrogen at a relatively low charring temperature and carbon dioxide > nitrogen > air-limitation at a relatively high charring temperature. The results provide an important direction for selecting suitable pyrolysis atmospheres to optimize the properties of biochars. [ABSTRACT FROM AUTHOR]

N 2 and air-limitation environments are two common pyrolysis atmospheres to produce biochars. However, their influence on the sorption mechanism and performance of biochar toward heavy metal ions (HMs) remains unknown. Hence, biochars produced in N 2 and air-limitation environments at 300–750 °C were applied to sorb Cd(II) and Ni(II) in this study. The results show that the maximum adsorption quantities (Q m) of Cd(II) and Ni(II) on biochars of air-limitation pyrolysis were 1.08–1.84 folds and 1.66–3.11 folds, respectively, of those on biochars of N 2 pyrolysis. The Q m of both studied HMs on biochars produced in the two studied atmospheres both generally improved with increasing pyrolysis temperature due to the increased contents of inorganic elements such as P, S, and Si at higher temperatures. Besides, the Q m of both studied HMs on biochars of N 2 pyrolysis improved with the increasing graphitic C content. The results indicate the sorption mechanism of HMs on biochars of air-limitation pyrolysis was dominated by the mineral anions complexation, while the sorption mechanism of HMs on biochars of N 2 pyrolysis was controlled by both the mineral anions complexation and cation-π interaction. Additionally, the protonated oxygen-containing groups (e.g., hydroxyl groups) linked to C fraction possibly formed hydrogen-bonds with mineral anions and thus suppressed the sorption of HMs on biochars. A pyrolysis condition of the maximum adsorption efficiency for HMs is the N 2 pyrolysis at 750 °C. The results of this study are beneficial for selecting suitable pyrolysis methods to produce biochars and raising their efficiency in HMs sorption. [Display omitted] • Higher temperature generally supported greater sorption quantity of HMs on biochars. • Air-limitation (AL) made greater sorption quantity of HMs on biochar than N 2. • Mineral anions complexing controlled the sorption of HMs on biochar of AL pyrolysis. • Cation-π effect was an important sorption mechanism for biochars of N 2 pyrolysis. • The method of the N 2 pyrolysis at 750 °C presents the maximum adsorption efficiency. [ABSTRACT FROM AUTHOR]

Biochar, a cost-effective carbonaceous material, has shown great promise in many applications such as soil remediation and wastewater treatment. Citric acid in soil and water environments may affect interactions between biochar and organic contaminants (e.g., tetracycline). To evaluate the effect of citric acid on the adsorption of tetracycline onto biochars, this study investigated the adsorption behavior of tetracycline onto biochars of two different pyrolysis temperatures (300 and 450 °C, referred to as BC_300 and BC_450) in the present of citric acid, accompanying with the effects of solution pH and Cu2+. The results indicated that citric acid significantly inhibited the adsorption of tetracycline onto both biochars, that was mainly due to the multi-mechanisms including pore blocking effect, surface adsorption sites competition, and steric hindrance induced by citric acid. Moreover, citric acid more significantly suppressed the adsorption of tetracycline onto the two biochars at pH 9.0 than at pH 5.0 and 7.0. Because increasing pH could promote electrostatic repulsion between the negative tetracycline and the negatively charged surface of biochars. Cu2+ had greater enhancing effect on the sorption of tetracycline onto BC_300 than BC_450 via cation-bridging effect. Meanwhile, citric acid inhibited the adsorption of tetracycline onto BC_300 to a larger extent than the adsorption onto BC_450 in the presence of Cu2+. That is possibly due to BC_300 has more surface oxygen-containing functional groups than BC_450. Thus, the effect of coexisting citric acid needs to be taken into account when biochar is increasingly used as an amendment in soil and water systems. [ABSTRACT FROM AUTHOR]