Enzymatic grafting of carboxyl groups on to chitosan––to confer on chitosan the property of a cationic dye adsorbent

Chitosan (CTS) is a good adsorbent for dyes but lacks the ability to adsorb cationic dyes. In this study, chitosan was modified to posses the ability to adsorb cationic dyes from water. Four kinds of phenol derivatives: 4-hydroxybenzoic acid (BA), 3,4-dihydroxybenzoic acid (DBA), 3,4-dihydroxyphenyl-acetic acid (PA), hydrocaffeic acid (CA) were used individually as substrates of tyrosinase to graft onto chitosan. FTIR analysis provided supporting evidence of phenol derivatives being grafted. The grafting amounts of these phenol derivatives onto chitosan were examined by the adsorption of an anionic dye (amaranth) and reached a plateau value. The final contents of carboxyl groups in chitosan (mmol carboxyl groups per kg chitosan) were measured as 46.36 for BA, 70.32 for DBA, 106.44 for PA, and 113.15 for CA. These modified chitosans were used in experiments on uptake of the cationic dyes crystal violet (CV) and bismarck brown Y (BB) by a batch adsorption technique at pH 7 for CV and at pH 9 for BB and 30 °C. Langmuir type adsorption was found, and the maximum adsorption capacities for both dyes were increased with the following order CTS-CA > CTS-PA > CTS-DBA > CTS-BA.