**6.1 Super adsorbent**

Colored effluents of industries can be highly toxic and carcinogenic, causing a great danger to living organisms. As organic dye effluents have complex chemical structure, high hydrophilicity and stability, resistant to their removal poses difficulties for wastewater treatment [21]. Technologies such as coagulation and flocculation, oxidation, adsorption, membrane separation, and electro-coagulation methods [22, 23] have been used to treat colored wastewater. Among these, the adsorption method is regarded as one of the most attractive processes because of its easy operation and high removal efficiency. The xylan-based hydrogels showed outstanding adsorption capacity for organic dyes removal instead of its excellent properties of biodegradability and renewability [24–26]. Sun and co-workers were prepared a novel adsorbent composite hydrogel of acylated xylan and silanized graphene oxide via free radical polymerization reactions. The composite hydrogel based-adsorbent showed excellent removal capacity of Cu2+ ions from an aqueous solution. Study of adsorption thermodynamics showed the adsorption of Cu2+ ions was endothermal and spontaneous, and the adsorption amount rose with an increase in temperature. In addition, higher desorption percentages of Cu2+ ions from the used hydrogel were also achieved up to 77.3% after recycling for six times. Thus, all obtained results were indicated that the prepared hydrogel is promising for water treatment and collection of metal ions [27].
