Study of aluminium speciation in the coagulant composite of polyaluminium chloride-chitosan for the optimization of drinking water treatment

Abstract

The control of aluminium speciation in coagulant composites is the appropriate approach to remove turbidity and natural organic matter from raw water effectively. This suspended matter in water affects its organoleptic quality and triggers problems by causing interference with the water treatment process. This study examined the behaviour of composite coagulants polyaluminium chloride-chitosan (PAC-CTS) with different aluminium speciation and polymer ratio to remove oxidizable matter and turbidity residing in surface waters. The fraction of basicity ratio (Al/OH) in the preparation of polyaluminium chloride (PAC) and chitosan (CTS) were simultaneously evaluated and optimized according to aluminium speciation by experimental design. The interaction between PAC and CTS was examined via Al-Ferron timed spectrophotometric approach, theoretical study and fourier transform infrared (FTIR) analysis. Ferron analyses reveal that basicity ratio and CTS fraction affect the distribution of aluminium forms (mononuclear Ala, medium polymeric Alb, colloidal, and high polymeric Alc) in PAC-CTS. The theoretical study showed that Al(OH)2+, Al13, and Al30 species are more reactive than aluminium hydroxide Al(OH)3 at different magnitudes and sites with chitosan. The FTIR analysis confirmed the existence of an interaction between PAC and CTS by revealing a new peak for Al-NH2 stretches. The coagulation performance study of composite coagulant PAC-CTS with different compositions showed that the increase of chitosan and the preponderance of Alb and Alc species compared to Ala are suitable for removing colloidal suspensions. Further, incorporation of PAC with high basicity (74.1%) in 16.3% of chitosan (PAC-CTS1) removed 99.51% of turbidity and 66.66% of oxidizable matter at AlCl3 concentration of 10 mg l−1. However, increasing the percentage of chitosan to 34.1% at the same basicity (74.1%) in the PAC-CTS4 compound was not beneficial for oxidizable matter removal. It was speculated that the improvement in coagulation performance could be achieved by considering the aluminum speciation and polymer content in the composite flocculant PAC-CTS. The present work could be a useful model for synthesizing and studying organometallic interactions in developing new composite coagulants to improve coagulation performance.