| Straw, as a kind of crop waste, is the stem or leaf part of mature crops. Since straw is rich in nitrogen, phosphorus, potassium, and organic substances such as cellulose and lignin, it is a sort of versatile and renewable biological resource. However, the general treatment for excess straw in China is still incineration, which is not only destruction of the atmospheric environment, but waste of resources. So it is very important to improve the comprehensive utilization of straw materials, and realize reclamation and industrialization of straw for promotion of environmental protection and farmers’ income. Since straw material is rich in cellulose and lignin, which are both environmentally friendly natural polymers and contain a large number of hydroxyl, carboxyl and other active groups, it can be applied as a kind of green water treatment agents to eliminate pollutants from the waste water through flocculation or adsorption effects. Furthermore, through proper modification by destroying the original ordered structures and introducing new functional groups, the reactivity of straw can be improved and its application performance can be further strengthened.In this work, the wheat straw materials have been simply modified by etherification to prepare three kinds of adsorbents, including anionic, cationic and amphoteric straw based ones. Chloroacetic acid and3-chloro-2-hydroxypropyl trimethylammonium chloride was used as ethification reagents. Methylene blue (MB), a kind of cationic dye, together with methyl orange (MO) and acid green25(AG25), two sorts of anionic dyes, have been employed as synthetic wastewater, respectively. The adsorption behaviors of the aforementioned dye matters by the three kinds of modified straw materials as mentioned above have been studied systematically. The adsorption mechanisms of those modified straw materials for removal of the ionic dyes from aqueous solutions have been discussed in detail. Several interesting results have been drawn: (1) Anionic straw was effective in eliminating cationic MB but useless in adsorption of anionic dyes. Similarly, cationic straw was useful in removal of anionic MO and AG25but ineffective in adsorption of cationic dyes. However, the amphoteric straw was proven effective in eliminating both cationic (MB) and anionic (AG25) dyes, especially at corresponding favored pH conditions, as shown in Fig.l. There was no doubt that the amphoteric straw expanded the limited application range of anionic and cationic adsorbents largely. Fig.l The effects of pH on MB (a) and AG25(b) adsorption by straw (?) and amphoteric straw (o), respectively.(2) The anionic, cationic and amphoteric straw based adsorbents all showed obvious pH dependence on adsorption of ionic dyes. This was attributed to the change of electrostatic interactions between adsorbents and dye matters brought by the change of surface charge as solution pH varied. For the three kinds of aforementioned adsorbents, the adsorption isotherms were all well fitted by the Langmuir model while the adsorption kinetices were all well described by both pseudo-second order equation and the Elovich equation. Besides, the Thomas model was suitable to describe the adsorption behaviors in column study. In addition, the adsorption behaviors were all temperature independence. It indicated that the adsorption behavior of the various straw based adsorbents for ionic dyes was a monolayer chemical adsorption with an ion-exchange process.(3) In addition, recycling is an important factor influencing the practical application of the adsorbents. Normally, disused adsorbents are recovered from dilute acidic or alkaline solutions in order to recycle and collect some valuable contaminants. In this work, the amphoteric straw after adsorption and desorption for six cycles was still able to maintain high adsorption capacity, which made it suitable for real applications. For anionic and cationic straw based adsorbents, a new reuse method was applied in this work. On the one hand, the adsorbents after saturated adsorption of dyes become useless; On the other hand, the dye-loaded adsorbents are stable enough in certain pH range, so they can be regarded as a kind of new adsorbent materials for altered surface structures and applied in a secondary adsorption for removal of other pollutants. The final experimental results showed that, the MB-loaded anionic straw had good adsorption capacity for MO, while the MO-or AG25-loaded cationic straw materials had good adsorption capacity for MB, as shown in Fig.2. It was indicated that the secondary adsorption was an appropriate way for reuse of these disused adsorbents.(a) Fig.2The reuse process in a secondary adsorption for anionic (a) and cationic (b) straw-based adsordentsAbove all, it was believed that the aforementioned straw based adsorbents could be considered as a sort of promising adsorbents for the removal of various dye matters. |
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