Ren-kou Xu

The forms of alkalis in the biochar produced from crop residues at different temperatures.

The forms of alkalis of the biochars produced from the straws of canola, corn, soybean and peanut at different temperatures (300, 500 and 700°C) were studied by means of oxygen-limited pyrolysis. The alkalinity and pH of the biochars increased with increased pyrolysis temperature. The X-ray diffraction spectra and the content of carbonates of the biochars suggested that carbonates were the major alkaline components in the biochars generated at the high temperature; they were also responsible for the strong buffer plateau-regions on the acid-base titration curves at 500 and 700°C. The data of FTIR-PAS and zeta potentials indicated that the functional groups such as -COO(-) (-COOH) and -O(-) (-OH) contained by the biochars contributed greatly to the alkalinity of the biochar samples tested, especially for those generated at the lower temperature. These functional groups were also responsible for the negative charges of the biochars.

Immobilization of Cu(II), Pb(II) and Cd(II) by the addition of rice straw derived biochar to a simulated polluted Ultisol.

To develop new remediation methods for acidic soils polluted by heavy metals, the chemical fractions of Cu(II), Pb(II) and Cd(II) in an Ultisol with and without rice straw biochar were compared and the effect of biochar incorporation on the mobility and bioavailability of these metals was investigated. In light of the decreasing zeta potential and increasing CEC, the incorporation of biochar made the negative soil surface charge more negative. Additionally, the soil pH increased markedly after the addition of biochar. These changes in soil properties were advantageous for heavy metal immobilization in the bulk soil. The acid soluble Cu(II) and Pb(II) decreased by 19.7-100.0% and 18.8-77.0%, respectively, as the amount of biochar added increased. The descending range of acid soluble Cd(II) was 5.6-14.1%, which was much lower than that of Cu(II) and Pb(II). When 5.0 mmol/kg of these heavy metals was added, the reducible Pb(II) for treatments containing 3% and 5% biochar was 2.0 and 3.0 times higher than that of samples without biochar, while the reducible Cu(II) increased by 61.6% and 132.6% for the corresponding treatments, respectively. When 3% and 5% biochar was added, the oxidizable portion of Pb(II) increased by 1.18 and 1.94 times, respectively, while the oxidizable portion of Cu(II) increased by 8.13 and 7.16 times, respectively, primarily due to the high adsorption affinity of functional groups of biochar to Cu(II). The residual heavy metal contents were low and changed little with the incorporation of biochar.

Adsorption of methyl violet from aqueous solutions by the biochars derived from crop residues

The adsorption of methyl violet by the biochars from crop residues was investigated with batch and leaching experiments--adsorption capacity varied with their feedstock in the following order: canola straw char>peanut straw char>soybean straw char>rice hull char. This order was generally consistent with the amount of negative charge of the biochars. Zeta potentials and Fourier transform infrared photoacoustic spectroscopy, combined with adsorption isotherms and effect of ionic strength, indicated that adsorption of methyl violet on biochars involved electrostatic attraction, specific interaction between the dye and carboxylate and phenolic hydroxyl groups on the biochars, and surface precipitation. Leaching experiments showed that 156 g of rice hull char almost completely removed methyl violet from 18.2 L of water containing 1.0 mmol/L of methyl violet. The biochars had high removal efficiency for methyl violet and could be effective adsorbents for removal of methyl violet from wastewater.

Adsorption of Pb(II) on variable charge soils amended with rice-straw derived biochar.

Two Ultisols and one Oxisol from tropical regions of southern China were incubated with rice straw biochar to investigate the effect of biochar on their surface charge and Pb(II) adsorption using batch methods. The incorporation of biochar induced a remarkable increase in soil cation exchange capacity after 30 d of incubation. The incorporation of biochar significantly increased the adsorption of Pb(II) by these variable charge soils; the enhancement of adsorption of Pb(II) by these soils increased with the addition level of biochar. Adsorption of Pb(II) involved both electrostatic and non-electrostatic mechanisms; however, biochar mainly increased Pb(II) adsorption through the non-electrostatic mechanism via the formation of surface complexes between Pb(2+) and functional groups on biochar. There was greater enhancement of biochar on the non-electrostatic adsorption of Pb(II) by the variable charge soils at relatively low pH. Therefore, the incorporation of biochar decreased the activity and availability of Pb(II) to plants through increased non-electrostatic adsorption of Pb(II) by acidic variable charge soils.

Effects of pH and metal ions on oxytetracycline sorption to maize-straw-derived biochar

Biochars produced from biomass residues have been recognized as effective sorbents to hydrophobic compounds, but knowledge on sorption of antibiotics to biochar and its mechanisms are still inadequate. Sorption of oxytetracycline (OTC) in aqueous solution to maize-straw-derived biochar, and the effect of pH and metal ions, was investigated in batch experiments, and the main sorption mechanisms were elucidated using FTIR and zeta potential measurements. The results showed that sorption of OTC on biochar was highly pH-dependant. The amount of sorbed OTC first increased and then decreased with increasing pH, and maximum sorption was achieved at pH 5.5. Cu(2+) enhanced the sorption of OTC, while Pb(2+) slightly reduced the sorption under acidic conditions. Other metal ions had no significant effect on the sorption of OTC to biochar. Surface complexation, through π-π interaction and metal bridging, was the most important sorption mechanism although cation exchange might have played a role.

Adsorption and desorption of Cu(II) and Pb(II) in paddy soils cultivated for various years in the subtropical China

The adsorption and desorption of Cu(II) and Pb(II) on upland red soil, and paddy soils which were originated from the upland soil and cultivated for 8, 15, 35 and 85 years, were investigated using the batch method. The study showed that the organic matter content and cation exchange capacity (CEC) of the soils are important factors controlling the adsorption and desorption of Cu(II) and Pb(II). The 15-Year paddy soil had the highest adsorption capacity for Pb(II), followed by the 35-Year paddy soil. Both the 35-Year paddy soil and 15-Year paddy soil adsorbed more Cu(II) than the upland soil and other paddy soils. The 15-Year paddy soils exhibited the highest desorption percentage for both Cu(II) and Pb(II). These results are consistent with the trend for the CEC of the soils tested. The high soil CEC contributes not only to the adsorption of Cu(II) and Pb(II) but also to the electrostatic adsorption of the two heavy metals by the soils. Lower desorption percentages for Cu(II) (36.7% to 42.2%) and Pb(II) (50.4% to 57.9%) ,ere observed for the 85-Year paddy soil. The highest content of organic matter in the soil was responsible for the low desorption percentages for the two metals because the formation of the complexes between the organic matter and the metals could increase the stability of the heavy metals in the soils.

Application of crop straw derived biochars to Cu(II) contaminated Ultisol: Evaluating role of alkali and organic functional groups in Cu(II) immobilization

When Cu(II) contaminated Ultisol was mixed with biochar derived from straw and incubated for 120 d, acid-soluble Cu(II) decreased by 0.08-0.33 mmol/kg due to the liming effect of biochar; 1.00-1.93 mmol/kg due to organic functional groups of biochar when it was added to the soil at 30 g/kg, and by 1.40-2.43 mmol/kg at 50 g/kg. The total functional groups and volatile matter (VM) were significantly related to Cu(II) immobilization (P<0.01), suggesting that it is functional groups in VM that are essential to Cu(II) immobilization in soil. The percentage of acid soluble Cu(II) decreased from 43.07% for the control, to 18.83-27.45% and 11.03-20.97% for the treatments with 30 and 50 g/kg of crop straw biochars added, respectively. The immobilized Cu(II) was primarily transformed to reducible and oxidizable forms. Biochar could retain Cu(II) for at least 120 d, indicating the long-term stability of biochar in Cu(II) immobilization.

The mechanism of chromate sorption by three variable charge soils

Adsorption of chromate and desorption of the pre-adsorbed chromate were studied using three representative variable charge soils from the south of China. The mechanisms of the adsorption were discussed based on the hydroxyl release and the change of zeta potential during the chromate adsorption. The adsorption and desorption of chromate followed the same order: the Hyper-Rhodic Ferralsol>the Rhodic Ferralsol>the Haplic Acrisol. The adsorption and the desorption both increased with elevation of the equilibrium chromate concentration and decreased with increasing of the soil solution pH. The percentage of the specific adsorption of chromate was 54.0-59.4%, 54.3-60.3%, and 43.9-46.2% for the Hyper-Rhodic Ferralsol, the Rhodic Ferralsol, and the Haplic Acrisol, respectively; the percentage of the electrostatic adsorption was 40.0-46.6%, 39.7-45.8%, and 50.8-56.5% for the three soils, respectively. These findings suggest that both the specific adsorption and the electrostatic adsorption contributed to the chromate adsorption by the variable charge soils. The hydroxyl release during the chromate adsorption shared the same trend with the adsorption envelopes, and decreased with increasing of pH. This is attributed to the exchange of chromate with the hydroxyl on the soil particle surfaces and the formation of a chemical bond between chromate and the surface. Our results indicate that the adsorption of chromate resulted in a shift of zeta potential-pH curves of the soil colloids to negative values, which suggests that the adsorption increased the negative surface charge and decreased the surface potential of the soil colloids.

Effects of biochars generated from crop residues on chemical properties of acid soils from tropical and subtropical China

The chemical compositions of biochars from ten crop residues generated at 350°C and their effects on chemical properties of acid soils from tropical and subtropical China were investigated. There was greater alkalinity and contents of base cations in the biochars from legume residues than from non-legume residues. Carbonates and organic anions of carboxyl and phenolic groups were the main forms of alkalis in the biochars, and their relative contributions to biochar alkalinity varied with crop residues. Incubation experiments indicated that biochar incorporation increased soil pH and soil exchangeable base cations and decreased soil exchangeable acidity. There were greater increases in soil pH and soil exchangeable base cations, and a greater decrease in soil exchangeable acidity, for biochars from legume than from non-legume residues. The biochars did not increase the cation exchange capacity (CEC) of soils with relatively high initial CEC but did increase the CEC of soils with relatively low initial CEC at an addition level of 1%. The incorporation of biochars from crop residues not only corrected soil acidity but also increased contents of potassium, magnesium, and calcium in these acid soils from tropical and subtropical regions and thus improved soil fertility.

Removal of Cr(VI) from aqueous solutions by Na2SO3/FeSO4 combined with peanut straw biochar

Discharge of Cr(VI)-containing industrial effluents leads to the pollution of surface waters and ground waters. In this study, Cr(VI) was first reduced by Na2SO3 or FeSO4 and then biochar generated from peanut straw at 500 °C was used to remove the Cr(III). Results indicated that the reduction of Cr(VI) by Na2SO3 must be conducted under strongly acidic conditions within a narrow pH range of 2.0-2.4, whereas the reduction of Cr(VI) by FeSO4 can be conducted under acidic, neutral and weak alkaline conditions because protons are generated from the hydrolysis of Fe(3+) via Fe(2+) oxidation. When the initial concentration of Cr(VI) was no more than 1.5mM, and after Cr(VI) had been reduced by Na2SO3 at pH 2.0 or FeSO4 at pH 7.6, 4 g L(-1), peanut straw biochar was able to neutralize solution acidity and remove Cr from the aqueous solution. The optimal reaction time for biochar in the Cr-containing solutions was 6h. The precipitation of Cr(OH)3 and the formation of Cr(3+) surface complexes with the functional groups on the biochar were the main mechanisms for Cr(III) removal by biochar. These results suggested that the combination of reductants (Na2SO3/FeSO4) and biochar generated from peanut straw can be used to efficiently remove Cr(VI) from aqueous solutions.