Preparation of a novel amphoteric bio-adsorbent using wheat straw and mechanism for removing Pb2+ and As5+ from aqueous solutions

In recent years, methods in removal of heavy metal pollution become increasingly an interest in research. Numerous treatment technologies have been proposed for the removal of efficient heavy metal from waters, including chemical precipitation, ion exchange, adsorption, membrane separation, electrolysis, etc. Most of them are significantly costly and incapable of removing trace heavy metals. Among all these treatment processes, adsorption is one of the most widely used and promising techniques due to its high efficiency, low cost, easy operation, eco-friendly, and the availability of sorts of adsorbents. Lately, many studies have reported that straw modified by various methods as an adsorbent show an outstanding adsorption performance for pollutants owing to its structure features and functional groups such as amino, thiol, carboxyl. However, for some heavy metal ions (As, Cr, and Se) that exist in the form of anionic metals in water, adsorbent with only one type of functional group is usually incapable of eliminating those heavy metals. It is in general difficult for most adsorbents to remove both anionic metals and cationic metals in water at the same time. To our best knowledge, there are only a few quantitative data on the amphoteric resins based on agriculture adsorbent materials that have such capacity. Often, chloroacetic acid and simple quaternary ammonium salts as functional groups are grafted directly on those adsorbents. Therefore, in this work, a novel amphoteric adsorbent with two types of pincer functional groups (a cationic compound IA and an anionic reagent IM) based on wheat straw (WS) was prepared and characterized by scanning electron microscope (SEM), Fourier transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS), which were used to preliminarily analyze adsorption mechanism. The SEM showed that the surface of WS-IAIM was smooth, and the pore structure was compact and regular, indicating that the IA and IM groups were grafted successfully on WS after modification. FTIR showed that the active functional groups such as –COO- (1 589 cm-1 and 1 412 cm-1), C=N (1 536 cm-1) and C-N (1 358 cm-1) were found on the surfaces of WS-IAIM, indicating good adsorption potential for Pb2+ and As5+. The XPS suggested that WS-IAIM containing Pb2+ and As5+ were present in the amphoteric adsorbent after adsorption. Batch adsorption experiment results suggested that the optimized pH values for removal of Pb2+ and As5+ were 5.0 and 2.0, respectively. The data showed that adsorption isotherm fitted the Langmuir isotherm model with the maximum adsorption capacities of 180.12 mg/g for Pb2+ and 27.48 mg/g for As5+, respectively at 313 K, and adsorption kinetics were fitted well with the pseudo-second order model. Adsorption thermodynamic tests implied that the adsorption process was a spontaneous and endothermic reaction. Reusability studies confirmed that the WS-IAIM could be easily regenerated and repeatedly utilized in the wastewater treatment. Furthermore, the experimental results showed that the adsorption mechanism followed the monolayer chemical adsorption with an ion-exchange process and complexation. The adsorption capacities of adsorbent for Pb2+ and As5+ were still around 159.3 and 19.8 mg/g, respectively, after 5 times of recycle, which confirmed that the WS-IAIM could be easily regenerated and repeatedly utilized in the wastewater treatment. Therefore, we considered that WS-IAIM could be reused as a promising, effective and cheap adsorbent of Pb2+ and As5+ removal.