The textile finishing industry has a high specific water consumption (approx. 100 L per kg of product), part of which is due to dyeing and subsequent rinsing processes. Dyes need to be removed from wastewater for water recycling in the industry but also to prevent those persistent substances from being released into the environment.

In this study the suitability of various sorbents for sorptive removal of reactive dyes was investigated. The macrocyclic ligand cucurbituril, a new type of sorbent, as well as several commercial sorbents were used.

The characteristics and behavior of cucurbituril were studied in depth: Cucurbituril is partly dissolved in pure water (approx. 15 mg/L). It dissolves more readily in salt solutions (e.g. 200 mg/L are dissolved in 1 M Ca-chloride solution). Cucurbituril solubility in pure salt solutions (alkaline and alkaline earth salts) is reported and a stoichiometric model for cation complexation is suggested. Complexes with divalent cations are more stable than with monovalent ions. Also, larger ions form stronger complexes than smaller ones. However, due to its solubility cucurbituril is not suitable for practical purposes, even though high removal rates for reactive dyes (above 1 mol dye per mol cucurbituril) can be obtained under optimized conditions. Permanent (covalent) fixation on a support material would remedy this problem, but no such fixation has been described in literature.

Zeolithes and microporous resins were found to be ineffective for reactive dye sorption. Moderate to high affinity and capacity were observed with granulated ferric hydroxide (GEH), macroporous polymeric resins and anion exchangers. GEH has the lowest capacity and is ineffective at high pH or high sulfate concentration. Regeneration through oxidative destruction of sorbed dyes with hydrogen peroxide is possible. The polymeric resins had insufficient capacity for large dyes (above 1000 g/mol). The anion exchangers showed high affinity and high capacities for all dyes (maximum loading 0.2-1 mmol/g). The strongly basic type has to be regenerated with HCl or a HCl-methanol mixture. For some dyes, which are precipitated at very low pH, regeneration is unsatisfactory. The weakly basic type can be regenerated with alkaline methanol for all dyes. Methanol recovery through distillation would be feasible. The only drawbacks are the necessity of HCl-conditioning after regeneration and the fact that the sorbent cannot be used above pH 8.