Comparison of coagulation and magnetic chitosan nanoparticle adsorption on the removals of organic compound and coexisting humic acid: A case study with salicylic acid

Removals of salicylic acid (SA) and humic acid (HA) in individual and bi-component systems by coagulation employing aluminum sulfate (alum) and adsorption using magnetic chitosan (MCS), were comparatively investigated in this study. In single-component system, alum at sufficient dose (12.0 mg/L), although efficient in removing HA, was not desirable in removal of SA. Conversely, higher SA removal rate in the adsorption process was achieved due to its superior adsorption tendencies and size-exclusion effects than that of HA. Moreover, individual SA/HA by MCS adsorption fit with Freundlich isotherm model, indicating a multilayer adsorption process. In the bi-component system, the SA and HA removal rates were observed to decrease to varying extents. Dynamic floc sizes during coagulation processes were studied using a laser diffraction particle sizing device. The results demonstrated high molecular weight (MW) HA-Al(III) complex could grow into enough large size and consequently tended to separate in water; while more coagulants (>14.0 mg/L) were required in the bi-component system compared with individual HA water when the system emerged obvious flocs, which was associated with the discrepancy of charge neutralization and competition of adsorption sites. For sole adsorption, the reaction between HA and MCS was faster than that of SA, and parts of adsorption sites were preoccupied by HA and thereby result in the low SA removal efficiency. Furthermore, combination of alum and MCS could significantly enhance organic matter removal, and alum with low dose of 8.0 mg/L coupled with MCS could evidently improve flocs settlement performance and increase the floc sizes.