Cross-linked-substituted (esterified/etherified) starch derivatives as aqueous heavy metal ion adsorbent: a review

Starch is a biopolymer with outstanding economic and environmentally friendly attributes which has driven technological innovations to enhance its applications in food and non-food industries. Starch is constituted by O-H groups with valency and electronic characteristics that can initiate adsorption of aqueous heavy metal ions (AHMIs). However, this can be enhanced using various modification sequences. A common procedure is the cross-linking and substitution of the O-H groups via esterification and/or etherification reactions to produce starch derivative adsorbents (SDAs) with improved structural and functional properties for adsorption of AHMIs. The efficiency of SDAs developed using these procedures depends on the botanical source of the native starch base, porosity and structural stability of the derivative (i.e. degree of cross-linking), substituted functional group(s), degree of substitution and the steric/conformation effects of the substituted groups. Many works have been done to optimize these factors, and this review highlighted some of the tailored procedures and the results obtained.