Removal of lead from aqueous solution by activated carbon prepared from Enteromorpha prolifera by zinc chloride activation

Abstract

Activated carbon was prepared from Enteromorpha prolifera (EP) by zinc chloride activation. The physico-chemical properties of EP-activated carbon (EPAC) were characterized by thermal stability, zeta potential and Boehm titration methods. The examination showed that EPAC has a porous structure with a high surface area of 1688 m²/g. Batch adsorption experiments were carried out to study the effect of various parameters such as initial pH, adsorbent dosage, contact time and temperature on Pb(II) ions adsorption properties by EPAC. The kinetic studies showed that the adsorption data followed a pseudo second-order kinetic model. The isotherm analysis indicated that the adsorption data can be represented by Freundlich isotherm model. Thermodynamic studies indicated that the adsorption reaction was a spontaneous and endothermic process.

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.