Adsorption of Pb 2+ by insolubilized humic acid extracted from sewage sludge

This study used excess sludge from a sewage treatment plant as raw material to extract humic acid (HA) and explore the ability of HA to adsorb Pb2+ from a solution. The effects of the adsorbent amount, solution pH, and co-existing cations on the adsorption process were investigated. The study showed that the humic acid derived from sludge (S-HA) surface had a loose, clustered texture. The S-HA surface contained many oxygen-containing functional groups, such as carboxyl groups, alcohol hydroxyl groups, and phenolic hydroxyl groups. The presence of co-existing cations, such as Na+, NH4+ and Ca2+, in the solution was not conducive to the adsorption of Pb2+ by S-HA. As the solution pH increased, the adsorption of Pb2+ by S-HA gradually increased. The process by which S-HA adsorbed Pb2+ conformed to a pseudo-second order kinetic model. Additionally, the overall adsorption rate was controlled by liquid membrane diffusion and intra-particle diffusion. The adsorption isotherm followed the Langmuir model, and the S-HA had a maximum adsorption capacity of 27.59 mg/g at 25 °C. The adsorption dynamics and thermodynamics results showed that the adsorption of Pb2+ by S-HA occurred via both physical adsorption and chemical adsorption processes.