Preparation of Amino-Functionalized Carbon Microspheres and Their Adsorption Characteristics for Pb2+ in Aqueous Solutions

Carbon microspheres (CMSs) were synthesized from soluble starch via a straightforward and efficient hydrothermal carbonization process. Soluble starch was chosen as a cost-effective, non-toxic, and sustainable precursor, offering a green approach to the production of carbon microspheres. Two types of amino-functionalized carbon microspheres (NH₂-CMSs) were subsequently prepared via surface amino-modification using aqueous ammonia or ammonium persulfate. Quantitative analysis revealed that NH₂-CMSs modified with ammonia exhibited a surface amino content 2.5 times higher than those modified with ammonium persulfate. Structural and morphological characterizations confirmed the successful synthesis of uniformly sized spherical NH₂-CMSs, with the amino-functionalization maintaining the integrity of the carbon framework. Adsorption studies demonstrated that NH₂-CMSs achieved a significantly enhanced theoretical maximum adsorption capacity of 130.96 mg·g⁻¹ for Pb²⁺, surpassing that of unmodified CMSs (80.87 mg·g⁻¹) by 1.62 times. The primary adsorption mechanism involved the formation of covalent bonds between amino groups and Pb²⁺, resulting in stable metal complexes. The adsorption kinetics indicated a single-molecule adsorption behavior dominated by chemical adsorption as the rate-determining step. These findings underscore the potential of NH₂-CMSs as highly effective adsorbents for the removal of Pb²⁺ from aqueous solutions.