Flocculation Performance of Modified Chitosan in an Aqueous Suspension Sk Akbar Ali, 1 Sagar Pal, 2 Ram Prakash Singh 3 1 Hindustan Gum and Chemicals Limited, Bhiwani 127021, Haryana, India 2 Indian School of Mines, Dhanbad 826 004, Jharkhand, India 3 Emeritus Scientist, Indian Institute of Science & Educational Research, Pune–411021, India Received 6 August 2009; accepted 7 April 2010 DOI 10.1002/app.32580 Published online 29 June 2010 in Wiley InterScience (www.interscience.wiley.com). ABSTRACT: A cationic moiety, N-(3-chloro-2-hydroxy- propyl)trimethyl ammonium chloride (CHPTAC), was incorporated onto chitosan in an aqueous alkaline solution. Thus, modified chitosan was prepared. A series of modified chitosans with different molecular weights and charge den- sities were synthesized through the alteration of the molar ratio of CHPTAC to chitosan in the reaction mixture. The synthesized modified chitosans were thereafter character- ized by a variety of physicochemical characterization techni- ques to confirm that modification did take place. Furthermore, the feasibility of applying modified chitosans as flocculants was assessed, and they were compared with the native chitosan in model colloidal suspensions of kaolin and iron-ore powder. The results indicated that the unmodi- fied chitosan itself was a good flocculating agent. The floc- culation performance of the chitosan could be altered by the incorporation of the CHPTAC moiety. Studies showed that not all the modified chitosans had superior flocculation per- formance versus the native chitosan. Among the different grades, the modified chitosan with a moderate molecular weight and a moderate charge density showed the best flocculation performance in both model suspensions. V C 2010 Wiley Periodicals, Inc. J Appl Polym Sci 118: 2592–2600, 2010 Key words: biopolymers; colloids; renewable resources; water-soluble polymers; cationic flocculant INTRODUCTION The mineral and paper-processing industries require very large volumes of fresh water every day. Because fresh water has become a valuable commod- ity, the recycling and reuse of wastewater have become indispensable for all processing industries to reduce processing costs and also to abide by envi- ronmental rules and regulations. 1 The wastewater emanating from these industries contains very fine suspended particles with sizes in the range of 0.001–10 lm. 2 Because of the very small size and the presence of surface charges, it is very difficult to bring these particles closer together to create a heavier mass for settling. 2 Sometimes, these particles remain suspended for years. 3 Hence, the re- moval of these colloidal particles from wastewater has become a serious challenge for industry. 4,5 Most of the inorganic particles carry charges in a neutral aqueous medium. 6 To destabilize the system, inorganic coagulants such as alum, poly(aluminum chloride), ferric chloride, ferrous sulfate, and magne- sium chloride have been used for decades. 7 How- ever, this process is strongly pH-dependent and decreases the alkalinity of water. 8 Moreover, large amounts of inorganic coagulants are required to achieve an effective settling rate. As a result, a large volume of metal hydroxide sludge is produced. This again creates a disposal problem. 9 Apart from the disposal problem, the use of inorganic salts such as alum and poly(aluminum chloride) is now very con- troversial because of the possible impact of residual aluminum in recycled water on Alzheimer’s disease. 10,11 To minimize the drawbacks of inorganic coagu- lants, synthetic cationic flocculants are used as alter- natives because of the rapid settling and lower pro- duction of sludge. 12 Cationic flocculants neutralize the surface charge of hydrated colloidal particles and reduce the electrical repulsion between approaching particles. Thus, the suspended particles come closer to one another. Cationic polymers also bridge the approaching particles with their long chains. 13 The bridging performance of cationic flocculants depends on the physiochemical properties of the polymer, such as the molecular weight, size, shape, charge den- sity, hydrophobicity, adsorption density, and confor- mation of the adsorbed polymer. 14 Water-soluble synthetic flocculants (mainly poly- acrylamide-based cationic flocculants with different molecular weights and charge densities) have been greatly used in industry because of their economic advantages and easy tailorability. 15 Correspondence to: Sk. A. Ali (akbarkgp@gmail.com). Journal of Applied Polymer Science, Vol. 118, 2592–2600 (2010) V C 2010 Wiley Periodicals, Inc.