Shaobo Liu

Biochar-based nano-composites for the decontamination of wastewater: A review.

Synthesizing biochar-based nano-composites can obtain new composites and combine the advantages of biochar with nano-materials. The resulting composites usually exhibit great improvement in functional groups, pore properties, surface active sites, catalytic degradation ability and easy to separation. These composites have excellent abilities to adsorb a range of contaminants from aqueous solutions. Particularly, catalytic material-coated biochar can exert simultaneous adsorption and catalytic degradation function for organic contaminants removal. Synthesizing biochar-based nano-composites has become an important practice for expanding the environmental applications of biochar and nanotechnology. This paper aims to review and summarize the various synthesis techniques for biochar-based nano-composites and their effects on the decontamination of wastewater. The characteristic and advantages of existing synthesis methods are summarized and discussed. Application of biochar-based nano-composites for different contaminants removal and the underlying mechanisms are reviewed. Furthermore, knowledge gaps that exist in the fabrication and application of biochar-based nano-composites are also identified.

Biochar as potential sustainable precursors for activated carbon production: Multiple applications in environmental protection and energy storage

There is a growing interest of the scientific community on production of activated carbon using biochar as potential sustainable precursors pyrolyzed from biomass wastes. Physical activation and chemical activation are the main methods applied in the activation process. These methods could have significantly beneficial effects on biochar chemical/physical properties, which make it suitable for multiple applications including water pollution treatment, CO2 capture, and energy storage. The feedstock with different compositions, pyrolysis conditions and activation parameters of biochar have significant influences on the properties of resultant activated carbon. Compared with traditional activated carbon, activated biochar appears to be a new potential cost-effective and environmentally-friendly carbon materials with great application prospect in many fields. This review not only summarizes information from the current analysis of activated biochar and their multiple applications for further optimization and understanding, but also offers new directions for development of activated biochar.

Investigation of the adsorption-reduction mechanisms of hexavalent chromium by ramie biochars of different pyrolytic temperatures

To investigate the relationship between Cr(VI) adsorption mechanisms and physio-chemical properties of biochar, ramie residues were oxygen-limited pyrolyzed under temperature varying from 300 to 600°C. Batch adsorption experiments indicated that higher pyrolysis temperature limits Cr(VI) sorption in terms of capacity and affinity due to a higher aromatic structure and fewer polar functional groups in biochar. Both electrostatic (physical) and ionic (chemical) interactions were involved in the Cr(VI) removal. For low-temperature biochar, the simple physical adsorption was limited and the significant improvement in Cr(VI) sorption was attributed to abundant carboxyl and hydroxyl groups. The adsorption-reduction mechanisms could be concluded that Cr(VI) ions were electrostatically attracted by the positively charged biochar surface and reduced to Cr(III), and then the converted Cr(III) was retained or discharged into the solution. The study demonstrates ramie residues can be converted into biochar as a low-cost and effective sorbent for Cr(VI) removal.

Effective removal of Cr(VI) using β-cyclodextrin–chitosan modified biochars with adsorption/reduction bifuctional roles

In this work, beta-cyclodextrin–chitosan modified walnut shell biochars (β-CCWB) were synthesized as a low-cost adsorbent for the removal of heavy metal Cr(VI) from aqueous solutions. Batch sorption experiments were carried out to investigate the adsorption characteristic of β-CCWB. The experimental data fitted a pseudo-second order equation and Freundlich isotherm model, and the optimum adsorption of the modified biochar was observed at pH 2.0 with an adsorption capacity of 206 mg g−1. Thermodynamic analysis showed that the adsorption process was spontaneous and endothermic. The removal efficiency of Cr(VI) by β-CCWB (about 93%) was higher than that by the pristine biochar (about 27%). Characteristic analysis indicated that amino and carboxyl groups were the major functional groups for Cr(VI) sorption, and implied that the electrostatic attraction of Cr(VI) to the positively charged biochar surface, reduction of Cr(VI) to Cr(III) ions and the complexation between Cr(III) ions and β-CCWB functional groups were responsible for Cr(VI) removal mechanism in this research. Furthermore, the environmentally friendly and low-cost β-CCWB could be applied as a potential effective adsorbent to remediate Cr(VI) contamination from aqueous solution.

Biochar to improve soil fertility. A review

Soil mineral depletion is a major issue due mainly to soil erosion and nutrient leaching. The addition of biochar is a solution because biochar has been shown to improve soil fertility, to promote plant growth, to increase crop yield, and to reduce contaminations. We review here biochar potential to improve soil fertility. The main properties of biochar are the following: high surface area with many functional groups, high nutrient content, and slow-release fertilizer. We discuss the influence of feedstock, pyrolysis temperature, pH, application rates, and soil types. We review the mechanisms ruling the adsorption of nutrients by biochar.

Competitive removal of Cd(II) and Pb(II) by biochars produced from water hyacinths: performance and mechanism

Three biochars converted from water hyacinth biomass at 300, 450, and 600 °C were used to investigate the adsorption properties of Cd2+ and Pb2+. In addition, the competitive adsorption mechanisms between Cd2+ and Pb2+ were also conducted. Adsorption kinetics and isotherms indicated that the maximum adsorption capacity of Pb2+ was larger than that of Cd2+, and the adsorption process in the mixed solutions of two heavy metals (Cd2+ and Pb2+) was more favorable for Pb2+. Further investigation about the characterization of biochars demonstrated that cation exchange, surface complexation, cation–π interaction and precipitation were the main mechanisms responsible for the heavy metal removal. In this study, competitive adsorption may also be explained by these mechanisms. These results are useful for the application of biochars in selective adsorption and in practical wastewater treatment.

Effect of Cu(II) ions on the enhancement of tetracycline adsorption by Fe3O4@SiO2-Chitosan/graphene oxide nanocomposite.

Fe3O4@SiO2-Chitosan/GO (MSCG) nanocomposite was investigated by various techniques (SEM, TEM, XRD, VSM, FT-IR, XPS) for the removal of tetracycline (TC). Effects of pH, zeta potential and initial contaminant concentration were studied in detail. Four background cations (Na+, K+, Ca2+ and Mg2+) with a concentration of 0.01M showed little influence on the TC adsorption at the studied pH range while the divalent heavy metal cation Cu(II) could significantly enhance the adsorption. The results indicated that the highest adsorption capacity of TC were 183.47mmol/kg and 67.57mmol/kg on MSCG with and without Cu(II), respectively. According to mechanism investigation for the adsorption of TC by pH impact study and XPS analysis, besides electrostatic interaction and π-π interactions, the Cu(II) also acts as a bridge between TC and MSCG, which significantly improve the adsorption of TC. This study provided valuable guidance and effective method for the removal of TC from aquatic environments.

Competitive adsorption of Pb(II), Cd(II) and Cu(II) onto chitosan-pyromellitic dianhydride modified biochar

In this work, a novel engineered biochar prepared through modification with chitosan and pyromellitic dianhydride (PMDA) was investigated as an adsorbent for the removal of heavy metal ions from single metal and mixed-metal solutions (Cd, Cu and Pb). Characterization experiments with FTIR and XPS suggested that the novel modified biochar had more surface functional groups compare to the pristine biochar. Adsorption experiments indicated that the initial pH of the solution influenced the ability of biochars to adsorb heavy metals in single- and multi-metal systems. Moreover, the chitosan-PMDA modified biochar had strong selective adsorption of Cu(II). Mechanism studies showed that chemisorption was the major mechanism for heavy metal removal by the chitosan-PMDA modified biochar. Furthermore, the types of effective functional group for these heavy metal removal were different. The NCO group played a dominant role in the process of Pb(II) removal, while several N-containing functional groups and CC groups participated in the adsorption of Cd(II). The novel engineered biochar had selective adsorption capacity for copper due to the N-containing functional groups, meanwhile abundant carbonyl groups also participated in the removal of copper, and may reduce Cu(II) to Cu(I).

Adsorption of Estrogen Contaminants by Graphene Nanomaterials under Natural Organic Matter Preloading: Comparison to Carbon Nanotube, Biochar, and Activated Carbon

Adsorption of two estrogen contaminants (17β-estradiol and 17α-ethynyl estradiol) by graphene nanomaterials was investigated and compared to those of a multi-walled carbon nanotube (MWCNT), a single-walled carbon nanotube (SWCNT), two biochars, a powdered activated carbon (PAC), and a granular activate carbon (GAC) in ultrapure water and in the competition of natural organic matter (NOM). Graphene nanomaterials showed comparable or better adsorption ability than carbon nanotubes (CNTs), biochars (BCs), and activated carbon (ACs) under NOM preloading. The competition of NOM decreased the estrogen adsorption by all adsorbents. However, the impact of NOM on the estrogen adsorption was smaller on graphenes than CNTs, BCs, and ACs. Moreover, the hydrophobicity of estrogens also affected the uptake of estrogens. These results suggested that graphene nanomaterials could be used to removal estrogen contaminants from water as an alternative adsorbent. Nevertheless, if transferred to the environment, they would also adsorb estrogen contaminants, leading to great environmental hazards.

Tetracycline absorbed onto nitrilotriacetic acid-functionalized magnetic graphene oxide: Influencing factors and uptake mechanism

A novel magnetic nanomaterial was synthesized by grafting nitrilotriacetic acid to magnetic graphene oxide (NDMGO), which was applied as an adsorbent for removing tetracycline (TC) from aqueous solutions. The nanomaterial was characterized using TG-DTA, SEM, TEM, XRD, VSM, XPS, Raman, BET surface area and zeta potential measurements. Several experimental conditions (solution pH, adsorption time, temperature, ionic strength and foreign ions) affecting the adsorption process were investigated. The results showed that the TC adsorption capacity could be affected by solution pH. The adsorption capacity of TC increased rapidly in the initial 20min and finally reached equilibrium was about 600min. The pseudo-second-order kinetics provided the better correlation for the experiment data. Various thermodynamic parameters indicated that the adsorption was a spontaneous and endothermic process. The presence of NaCl and background electrolytes in the solution had a slight influence on TC adsorption. Hydrogen bonds, amidation reaction, π-π and cation-π interaction between NDMGO and TC could be used to explain the adsorption mechanism. The regeneration experiment demonstrated that this nanomaterial possessed an excellent regeneration performance. Based on the experimental results and comparative analysis with other adsorbents, the NDMGO was a high-efficiency and reusable adsorbent for TC pollution control.