Removal of lead from aqueous solutions with a treated spent bleaching earth

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Abstract

A spent bleaching earth from an edible oil refinery has been treated by impregnation with a normal sodium hydroxide solution followed by mild thermal treatment (100 °C). The obtained material (TSBE) was washed, dried and characterized by X-ray diffraction, FTIR, SEM, BET and thermal analysis. The clay structure was not apparently affected by the treatment and the impregnated organic matter was quantitatively removed. We have investigated the sorption of lead on this material, the spent bleaching earth (SBE) and the virgin bleaching earth (VBE). The kinetic results fit the pseudo second-order kinetic model and the Weber & Morris, intraparticle diffusion model. The pH had effect on the sorption efficiency. The sorption isotherms followed the Langmuir model for various sorbent concentrations with good values of determination coefficient. A comparison between the results obtained with this material and those of the literature highlighted a good removal capacity of the treated spent bleaching earth at low cost.

Introduction

The increasingly various industrial activities produce liquid effluents containing toxic compounds for humans and the environment. Among these compounds, heavy metals are most dangerous due to their toxicity at very low concentration in water. Many works were devoted to their elimination, the objective being to develop an effective and economic process. Heavy metals removal methods cited in the literature generally involve adsorption processes on activated carbon [1], [2], ions exchange mechanism [3], [4] or complexation by natural and synthetic reagents [5], [6], [7]. The cost of these processes led to numerous studies on alternative removal methods by use of less expensive natural materials and waste by-products such as chitosan, zeolites, clays, peat moss, fly ash, rice husk and sawdust [8]. The waste by-product chosen in this work is a spent bleaching earth obtained from an edible oil refinery.

The refining process of crude edible oil includes four operations: degumming, neutralization, bleaching and deodorization. The bleaching step is performed by use of acid-activated clay, called bleaching earth. This bleaching operation produces great quantities of solid wastes which usually are disposed of in a landfill near the factory. In Algeria, edible oil refineries produce about 8000 tonnes of spent bleaching earth per year. This disposal poses an acute problem of management and storage.

The spent bleaching earth (SBE) contains about up to 30% (w/w) of residual oil that rapidly oxidizes to the spontaneous auto-ignition point, and also produces unpleasant odors [9]. The constant changes in environmental legislation and the growing importance of safety in disposal techniques has led to many restrictions in solid waste management. In order to minimize the risk of pollution, many studies are devoted to the conversion and the reuse of these wastes in various applications [9], [10], [11], [12], [13], [14], [15], [16]. Three methods were reported for the treatment of spent bleaching earth: (1) recovery of the entrapped oil by washing with non polar solvents followed by steam treatment or washing with a solution of anionic detergent and a solution of sodium triphenylphosphate, (2) regeneration by burning and (3) regeneration by solvent extraction [17].

Recently, the regeneration of spent bleaching earth was conducted by preliminary acid impregnation followed by thermal processing [9], [10], [11], [13], [14], [15], [16]. In a previous work we have studied the decolorizing properties of a spent bleaching earth regenerated by thermal treatment followed by acid leaching [14] and by impregnation with a sodium hydroxide solution followed by a mild thermal treatment [18]. The aim of this work is to apply our recently published spent bleaching earth regeneration method to heavy metals removal from aqueous solution. In our previous publication we have observed that the regenerated material removed only cationic dyes leading us to suppose that the process was an ion exchange one. The regeneration method reported was based on a saponification reaction of the edible oil, leading to the formation of fatty acids sodium salts which could be removed by washing with water and afforded the original bentonite with its well known cation exchange properties. The sorption properties of the treated material were studied for lead removal from aqueous solutions. Lead is one of the most toxic elements and is present in industrial wastewaters from battery manufacturing, metallurgy, metal finishing, and chemical industry.

In this study, we have investigated the kinetics, sorption isotherms and pH effect of the lead sorption on the treated spent bleaching earth and we have compared the sorption capacities with those of the virgin and the untreated spent bleaching earth.

Section snippets

Materials

The spent bleaching earth (SBE) was obtained from an edible oil-refining factory (ENCG at Oran, Algeria). The virgin bleaching earth (VBE) was provided by Bental company (Algeria). The chemical composition of the virgin bleaching earth is shown in Table 1. All chemicals used were of analytical grade.

SBE treatment

The spent bleaching earth sample was impregnated with 1N NaOH solution (solid/solution ratio: 1/5, w/w) and heated at 100 °C overnight. The obtained solid was washed with distilled water, dried at 80 

Characterization of materials

The X-ray powder diffraction patterns (Fig. 1) of VBE, SBE and TSBE show the same diffraction peaks of the montmorillonite (M) and the presence of quartz impurities in the samples (at 2θ = 20.89° and 26.65°). From this observation, we deduce that the edible oil bleaching process and the treatment of the SBE with NaOH did not affect the main structure of the used bentonite. Fig. 1 also shows that the (0 0 1) reflection of SBE and TSBE did not correspond to any organic intercalation product after the

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