Effects of Pyrolysis Temperatures and Modified Methods on Rice Husk-Derived Biochar Characteristics and Heavy Metal Adsorption

Biochars were prepared from rice husk at different pyrolysis temperatures (300, 400, and 500 °C) and then modified by nitric acid (HNO3) and potassium hydroxide (KOH). The chemical and physical properties were characterized, and the adsorption ability of biochars for the removal of Cd (II) and Pb (II) was investigated. The results showed that with increasing pyrolysis temperature, the aromaticity of rice husk biochar increased while its polarity decreased and both specific surface area and total pore volume significantly increased. Both HNO3 and KOH modification significantly changed the oxygen-containing functional groups in biochar, especially biochars prepared at lower pyrolysis temperatures. HNO3 modification introduced nitro and carboxyl groups on the surface of HNO3-BC300, increasing the ether bond functional groups, while KOH modification increased the content of hydroxyl groups on KOH-BC300 and reduced the ether bond groups. At the same time, the modification of rice husk-derived biochar greatly enhanced the ability to absorb Cd (II) and Pb (II) from aqueous solution. Notably, KOH-BC300 exhibited the highest adsorption capacities, reaching 72.14 mg·g−1 for Cd (II) and 170.84 mg·g−1 for Pb (II). These results demonstrate that KOH modification was more effective than HNO3 modification at enhancing the adsorption of Cd (II) and Pb (II) onto rice husk-derived biochar. In addition, the specific surface area and total pore volume of biochar increased significantly after HNO3 and KOH modification. It was concluded that biochar’s adsorption performance might be greatly improved by increasing its oxygen-containing functional groups and specific surface area, but the effect of oxygen-containing functional groups was greater than that of specific surface area. Thus, KOH-modified biochar (KOH-BC300) can be used as an effective sorbent for heavy metal removal from wastewater.