Synthesis of a chitosan-based flocculant CS-g-P(AM-IA-AATPAC) and evaluation of its performance on Ni2+ removal: Role of chelating-coordination and flocculation

Ni2+ rich wastewater has caused critical water contamination, a growing issue of environmental pollution as well as serious threat to aquatic organisms and human health. It is of great significance and urgent to develop a novel and efficient wastewater treatment technology for Ni2+ removal. The efficient removal of Ni2+ by flocculation has always become a hot and focus topic. In this study, a novel chitosan (CS) graft modified flocculants CS-g-P(AM-IA-AATPAC), namely CS-g-PAAI, is prepared by microwave initiation and polymerization, and is applied in the removal of Ni2+. The effects of microwave power, microwave irradiation time, initiator concentration and pH on the CS graft polymerization efficiency and flocculant molecular weight were investigated and optimized, so as to further understand the rules of microwave initiation and CS-g-PAAI graft and polymerization. Various characterization methods indicated the successful grafting and synthesis of CS-g-PAAI. Meanwhile, the structural characteristic results manifested that CS-g-PAAI possessed a good thermal stability, a coarse and porous apparent morphology, and its chemistry and crystal structure was evidently changed and affected after graft polymerization. Compared with CS, PAA and CS-g-PAA, the removal efficiency of Ni2+ by CS-g-PAAI is more exceedingly favorable and desirable. The flocculation effect of Ni2+ by CS-g-PAAI is as high as 86.3% at the optimal condition of pH = 9, flocculant dosage = 4 mg/L, the stirring speed = 400 rpm, and flocculation time = 30 min. Meanwhile, the generated Ni2+ floc size (d50) and fractal dimension (Df) can reach a high value (d50 = 339.3 µm, Df = 1.98), which accelerates the sedimentation and separation of the formed Ni2+ flocs. CS-g-PAAI shows excellent flocculation and coordination ability by its powerful coordination groups to strengthen the adsorption and chelating of Ni2+ ions and generate colloidal particles for subsequent flocculation. Then, efficient flocculation was carried out and proceeded through the bridging effect, the concomitant sweeping and netting action.