Effect of pyrolysis temperature of biochar on Cd, Pb and As bioavailability and bacterial community composition in contaminated paddy soil.

To further investigate the effect of pyrolysis temperature on bioavailable Cd, Pb and As, as well as the bacterial community structure in multi-metal(loid) contaminated paddy soil, six types of biochar derived from wood sawdust and peanut shell at 300 °C, 500 °C and 700 °C were prepared and incubated with Cd, Pb and As contaminated paddy soil for 45 days. The results showed that adding biochar decreased bioavailable Cd by 31.3%- 42.9%, Pb by 0.61-56.1%, while bioavailable As changed from 9.68 mg kg-1 to 9.55-10.84 mg kg-1. We found that pyrolysis temperature of biochar had no significant effect on Cd bioavailability while Pb bioavailability decreased obviously with pyrolysis temperature raising. Biochar reduced the proportion of soluble and exchangeable Cd from 45.0% to 11.2-15.4% in comparison with the control, while no significant effect on the speciation of Pb and As. Wood sawdust biochar (WSBs) had more potential in decreasing bioavailable Cd and Pb than peanut shell biochar (PSBs). Although high-temperature biochar resulted a larger increase in bacterial species than low-and mid- temperature biochar, feedstock played a more important role in altering soil bacterial diversity and community composition than pyrolysis temperature. PSBs increased the diversity of soil bacteria through elevating soil dissolved carbon (DOC). Biochar altered soil bacterial community structure mainly by altering the level of soil electricity conductivity, DOC and bioavailable Cd. In addition, applying high-temperature PSBs increased the genus of bacteria that relevant to nitrogen cycling, such as Nitrospira, Nitrosotaleaceae and Candidatus_Nitrosotalea.

Characteristics of dissolved organic matter composition in biochar: Effects of feedstocks and pyrolysis temperatures.

Biochar has widely used in soil pollution remediation due to its advantages of high efficiency and environmental sustainability. Dissolved organic matter (DOM) released by biochar plays a non-negligible role in the migration and transformation of pollutants in environment, and its composition was regarded as main impact factor. In this study, 28 biochar were investigated to detect the effect of pyrolysis temperature and feedstock on DOM content and components. Results showed that the content of DOM released from biochar at low pyrolysis temperatures (300-400 ℃) was higher than that from high pyrolysis temperatures (500-600 ℃). In addition, the specific UV-Visible absorbance at 254 nm (SUVA254) results expressed that DOM from peanut shell biochar (PSBC), rice husk biochar (RHBC) and bamboo biochar (BBC) had higher humification at high temperatures. Moreover, one fulvic acid-like (C2) and two humic acid-like (C1, C3) substances were main fluorescent components of biochar-derived DOM identified by parallel factor analysis based on excitation emission matrices fluorescence spectroscopies (EEM-PARAFAC). With the increase of pyrolysis temperature, humic acid substances content gradually decreased. The correlation analysis results revealed that pyrolysis temperatures and O/C, H/C, DOM content, the biological index (BIX), humification index (HIX), C1% and C3% was negatively correlated (p < 0.001). Thus, the pyrolysis temperatures take important roles in composition of DOM released from biochar, and this research would provide a reference for the application of biochar in the environment.

Characteristics and chlorine reactivity of biochar-derived dissolved organic matter: Effects of feedstock type and pyrolysis temperature.

Increasing biochar application worldwide may release more biochar-derived dissolved organic matter (BDOM) to the source water for drinking water supply. However, it is unclear how feedstock types and pyrolysis temperatures for biochar production would affect the characteristics and chlorine reactivity of BDOM. Here, we studied the spectroscopic characteristics of BDOM pyrolyzed from pine needle, wheat straw, walnut shells, alfalfa, pig manure, and sludge derived biochars at 300, 500, and 700 °C, as well as the formation potential of disinfection byproducts (DBPs) and their bulk toxicity after BDOM chlorination. The N/C ratio of biochar was higher for N-rich than C-rich feedstocks. Optical analyses indicated that BDOM aromaticity was highest at 700 °C, while the impact of pyrolysis temperature on the molecular weight of BDOM varied greatly among feedstocks. Increasing pyrolysis temperature caused consistently decreased BDOM reactivity toward haloketone formation but did not show consistent impacts on the other DBPs. Among feedstocks, the N-rich sludge showed the highest specific haloacetonitrile formation potential of BDOM at any given pyrolysis temperature. The DBP formation potential from biochar was consistently highest at 300 °C and was higher for N-rich than C-rich feedstocks. The microtoxicity of DBP mixture was highest for the BDOM derived from sludge produced at 300 °C. This study highlights the high variations in characteristics and chlorination reactivity of BDOM with varying feedstocks and pyrolysis temperatures, which implies that more attention should be paid to the environmental impacts of the intensive application of low-temperature biochar from N-rich feedstock such as sludge.

Biochar aging alters the bioavailability of cadmium and microbial activity in acid contaminated soils.

The effects of biochar aging on heavy-metal bioavailability and microbial activity are not fully understood. This study determined the effect over 270 days of poultry-litter biochar (PBC) and sugar-gum-wood biochar (SBC) on the bioavailability of Cd and microbial activity in acidic soils differing in organic matter content. Soil basal and substrate-induced respirations, microbial properties, Cd bioavailability and plant Cd bioaccumulation were evaluated at 1, 30, 90 and 270 days. The addition of PBC decreased Cd bioaccumulation by 81% and 85% while SBC decreased bioaccumulation by 47% and 56% in high (Chromosol) and low (Sodosol) organic matter soils, respectively, at Day 1. By Day 270, Cd bioaccumulation significantly (P < 0.05) increased in SBC-amended soils but decreased in PBC-amended soils. The addition of PBC increased both basal and substrate-induced microbial respirations compared to the other treatments over 270-day aging. However, SBC increased microbial biomass C compared to the PBC after Day 30. Aging decreased microbial respiration and biomass C in biochar-amended soils. It is concluded that Cd bioaccumulation increased in SBC-amended soils during aging whereas the PBC decreased Cd bioaccumulation and that the selection of biochar is important to enhance remediation efficiency in the long term.

Biochar surface complexation and Ni(II), Cu(II), and Cd(II) adsorption in aqueous solutions depend on feedstock type.

Biochar is a promising material for efficient removal of toxic metals from wastewater to meet standards for discharge into surface water. We characterized adsorption behaviour of willow (Salix alba) wood (WW) and cattle manure (CM) and their biochars, willow wood biochar (WWB) and cattle manure biochar (CMB), and elucidated the mechanisms for the removal of Ni(II), Cu(II) and Cd(II) from aqueous solutions. The kinetic adsorption suggests that the adsorption of Ni(II), Cu(II) and Cd(II) by feedstock and their biochars was controlled by mass transport, and chemisorption also played a role in the adsorption process. The Elovich model also well described the adsorption kinetics for WW and CM (R2 > 0.92), indicating that heterogeneous diffusion was the mechanism. The Sips isotherm model fitted best (R2 > 0.98) for Ni(II), Cu(II) and Cd(II) adsorption by the feedstocks and their biochars, indicating that both monolayer and multilayer adsorption played roles on the heterogeneous surfaces of the four adsorbents. The WWB had a higher while the CMB had a lower adsorption capacity than their respective feedstock due to the presence of abundant -COOH functional group on WWB surface to interact with Ni(II), Cu(II) and Cd(II) to form surface complexes. The higher specific surface area and lower pH of point of zero charge (PZC) of WWB were other contributing factors for its greater removal capacity. Therefore, we conclude that proper feedstocks need to be selected to produce biochars that are efficient for the removal of toxic metals from wastewater.

Phosphorus sorption capacity of biochars varies with biochar type and salinity level.

Biochar is recognized as an effective material for recovering excess nutrients, including phosphorus (P), from aqueous solutions. Practically, that benefits the environment through reducing P losses from biochar-amended soils; however, how salinity influences P sorption by biochar is poorly understood and there has been no direct comparison on P sorption capacity between biochars derived from different feedstock types under non-saline and saline conditions. In this study, biochars derived from wheat straw, hardwood, and willow wood were used to compare P sorption at three levels of electrical conductivity (EC) (0, 4, and 8 dS m-1) to represent a wide range of salinity conditions. Phosphorus sorption by wheat straw and hardwood biochars increased as aqueous solution P concentration increased, with willow wood biochar exhibiting an opposite trend for P sorption. However, the pattern for P sorption became the same as the other biochars after the willow wood biochar was de-ashed with 1 M HCl and 0.05 M HF. Willow wood biochar had the highest P sorption (1.93 mg g-1) followed by hardwood (1.20 mg g-1) and wheat straw biochars (1.06 mg g-1) in a 25 mg L-1 P solution. Although the pH in the equilibrium solution was higher with willow wood biochar (~ 9.5) than with the other two biochars (~ 6.5), solution pH had no or minor effects on P sorption by willow wood biochar. The high sorption rate of P by willow wood biochar could be attributed to the higher concentrations of salt and other elements (i.e., Ca and Mg) in the biochar in comparison to that in wheat straw and hardwood biochars; the EC values were 2.27, 0.53, and 0.27 dS m-1 for willow wood, wheat straw, and hardwood biochars, respectively. A portion of P desorbed from the willow wood biochar; and that desorption increased with the decreasing P concentration in the aqueous solution. Salinity in the aqueous solution influenced P sorption by hardwood and willow wood but not by wheat straw biochar. We conclude that the P sorption capacity of the studied biochars is dependent on the concentration of the soluble element in the biochar, which is dependent on the biochar type, as well as the salinity level in the aqueous solution.