Removal of lead from aqueous solution by activated carbon prepared from Enteromorpha prolifera by zinc chloride activation
Introduction
The presence of lead ions in the aquatic environment has been of great concern to scientists due to their increased discharge, non-biodegradable, toxic, and other adverse effects on human being as well as the fauna and flora [1]. Many methods such as chemical precipitation [2], electrochemical reduction [3], ion exchange [4], reverse osmosis, and membrane separation [5] have been developed to remove lead from wastewater. However, these technologies are either expensive for the treatment and disposal of the secondary toxic metal sludge or ineffective when lead is present in the wastewater at low concentrations [6]. Currently, activated carbon adsorption is a widely used technology because it is simple, low cost and effective for removing low lead concentration waste streams. The properties such as surface charge, type of surface functional groups, specific surface area, and pore-size distribution affect the adsorption capabilities of metal ions on activated carbon. While all above mentioned physical and chemical properties of activated carbon depend on the precursor materials and activation methods used. So, research interest in the production of activated carbon with high qualities has been focused on two aspects.
One way is to search for suitable precursors. Recently, biomass have attracted increasing attention due to their high efficiency, low operating cost, minimization of chemical sludge, regeneration of biosorbent, and the possibility of metal recovery [7], [8]. Activated carbon produced from a variety of biomass of Tamarind wood [9], Polygonum orientale Linn. [10], hazelnut husks [11], coconut shell [12], palm shell [13], and coffee residue [14] have shown attracting experimental results for lead removal.
Another approach is to explore appropriate activation methods. Generally, two different processes of physical and chemical activations were used to prepare activated carbon [15], [16]. Physical activation is to carbonize raw materials at the high temperatures in an inert atmosphere followed by oxidation treatment with steam, air, or CO2 [17]. Chemical activation involves impregnation of the raw materials with dehydrating chemical agents including phosphoric acid [18], sulfuric acid [14], KOH [19], NaOH [20], and ZnCl2 [21], [22], [23]. The advantages of lower temperature and high carbonization yield of the chemical activation make it be widely used to produce activated carbon. ZnCl2 is an effective dehydrating chemical agent. The impregnation with ZnCl2 plays an important role in increasing carbonization ability of the precursors and acquiring a desired pore structure of activated carbon.
EP, one of the widely available marine biomaterials in china [24], [25], have proved to be economic, eco-friendly, and effective heavy metal adsorbents due to their special cell wall structures containing functional groups such as amino, hydroxyl, carboxyl and sulphate, which can act as binding sites for metals via both electrostatic attraction and complexation [26], [27], [28]. However, to our knowledge, no investigations have been carried on the use of activated carbon from EP as an adsorbent for heavy metal removal.
The purpose of this study aims to develop an effective adsorbent from EP through ZnCl2 activation and investigate its lead removal capability. TGA/DTA analysis was used to characterize the pyrolysis properties of EP. The SEM, Zeta potential, and BET were applied to analyze the physico-chemical properties of EPAC. The effect of various experimental parameters on the lead adsorption process, such as pH, adsorbent dosage, contact time, and temperature were studied.
Section snippets
Preparation of EPAC
The precursor used for the preparation of the activated carbon was EP, collected from the Yellow Sea coast in Qingdao, China. It was washed several times using tap water to remove impurities and dried in an oven at 100 °C for 24 h until all the moisture evaporated. The dried EP was ground by ball milling (XQM-0.4L) to 120 meshes. The pyrolysis property of EP was carried out at a heating rate of 10 °C/min from 30 to 800 °C in an atmosphere of N2 at a flow rate of 50 mL/min using a Mettler TGA/STDA 851
Pyrolysis characterization of EP
The TGA/DTA plots (Fig. 1) indicate that the pyrolysis process of EP exhibits three obvious degradation stages. The first stage is a process of dehydration and desiccation to lose the water in the cells and the external water bounded by surface tension [25]. The release of absorbed water in EP leads to a weight loss of 12.5% at temperature range of 30–175 °C and an endothermic peak of 69.5 °C is found in the DTA curve. The significant weight loss appears at the second stage and it reaches 47.6%
Conclusions
Activated carbon with high Pb(II) ion adsorption capacity was prepared from EP by zinc chloride activation at 500 °C. The physico-chemical characterization of EPAC shows that EPAC has a large specific surface area of 1688 m2/g and a IEP of 2.71. Boehm titration study shows that there are large quantity of the functional groups such as carboxylic, hydroxyl, and phenolic groups existed on the surfaces of EPAC. Batch adsorption experiments show that Pb(II) ion adsorption properties of EPAC are great
Acknowledgements
This work was supported by the National Natural Science Foundation of China (50802045 and 20975056/B050902) SRF for ROCS, SEM, the Middle-aged and Youth Scientist Incentive Foundation of Shandong Province (BS09018) and the Taishan Scholar Program of Shandong Province, China.
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