Abstract
Rice husk and cotton straw were used to produce biochar under 300, 400, 500, 600, and 700 °C pyrolysis conditions, and the physicochemical properties of the obtained biochar samples were characterised. The effects of various adsorbent amounts, initial pH, and adsorption time on the Cd2+ adsorption performance were studied. The results showed that, at increasing pyrolysis temperatures, the biochar yield decreased, the ash content increased, the pH transitioned from acidic/neutral to basic/strongly basic, the biochar aromaticity gradually increased, and the biochar structure became more stable. In contrast, the hydrophilicity and polarity decreased, the specific surface area increased, and the number of oxygen-containing functional groups decreased. All these factors resulted in differences in the Cd2+ adsorption by the biochar samples. With increasing adsorbent content, the rate of Cd2+ adsorbed on the biochar gradually increased. The adsorption performance was optimal when the initial solution pH of the rice-husk and cotton-straw biochar samples was 5 and 6, respectively. The shortest time to achieve equilibrium was 30 min for rice-husk biochar, and 20 min for cotton-straw biochar. The Cd2+ adsorption data for both types of biochar were very well fitted with a pseudo-second-order kinetic model. Ion exchange and cation–π interactions may be the main factors influencing the Cd2+ adsorption by biochar. At the same time, the large specific surface area of biochar also plays a role in the Cd2+ adsorption.
Highlights
Featured Application: The results showed that there were differences in Cr2+ removal of biochar under different pyrolysis temperatures and initial pH conditions
It can be seen that the yield of both materials decreased with increasing pyrolysis temperature, while the ash content increased
This shows that higher temperatures improved the degree of pyrolysis, as the volatile components gradually escaped from the raw material, and the level of residual components decreased
Summary
Featured Application: The results showed that there were differences in Cr2+ removal of biochar under different pyrolysis temperatures and initial pH conditions. The hydrophilicity and polarity decreased, the specific surface area increased, and the number of oxygen-containing functional groups decreased All these factors resulted in differences in the Cd2+ adsorption by the biochar samples. Methods for the removal of heavy-metal ions mainly include redox coprecipitation, ion exchange and membrane separation, reverse osmosis and adsorption, etc. Current adsorbents include activated carbon, graphene, carbon nanotubes, etc., owing to their porous structure and unique surface characteristics, which are all advantageous for the removal of heavy metals. These materials are not cheap, and their large-scale use results in high costs [5]
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