Insight into the influences of pH value on Pb(II) removal by the biopolymer extracted from activated sludge

Abstract

The influences of pH value on Pb(II) removal by the biopolymer extracted from activated sludge were investigated from various perspectives, including removal rate, functional groups, binding site number between the biopolymer and Pb(II), and removal distribution. With the system pH value rising from 4.0 to 9.0, the total removal rate of Pb(II) without and with adding the biopolymer increased from 1.94% to 86.5% and from 32.2% to 95.4%, respectively. From the analyses of Fourier transform infrared (FTIR) spectroscopy and two-dimensional correlation spectroscopy (2D-COS), the continuous dissociation and deprotonation of functional groups in the biopolymer with rising system pH value promoted the biosorption removal of Pb(II) via complexation and ion exchange. According to the three-dimensional excitation emission matrix (3D-EEM) fluorescence spectrum, two protein-like fluorescence peaks of A (Ex/Em=280nm/326–338nm) and B (Ex/Em=220–230nm/324–338nm) were identified in the biopolymer. The binding site numbers, which were obtained via fluorescence quenching titration experiments, increased first and then decreased with rising system pH value for both peaks A and B. Biosorption removal of Pb(II) was dominant under acid conditions, but biosorption and Pb(OH)2 precipitation co-existed under neutral and alkaline conditions. The biosorption and the Pb(OH)2 precipitation were interestingly found to interfere with each other under alkaline conditions. The decreases in both binding site number and biosorption removal rate indicated that Pb(OH)2 precipitation excelled the biosorption in the competition of capturing Pb(II).