Removal of Cr (VI) with wheat-residue derived black carbon: Reaction mechanism and adsorption performance

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Abstract

The removal of Cr (VI) from aqueous solutions using black carbon (BC) isolated from the burning residues of wheat straw was investigated as a function of pH, contact time, reaction temperature, supporting electrolyte concentration and analytical initial Cr (VI) concentration in batch studies. The effect of surface properties on the adsorption behavior of Cr (VI) was investigated with scanning electron microscope (SEM) equipped with the energy dispersive X-ray spectroscope (EDS) and Fourier transform-infrared (FTIR) spectroscopy. The removal mechanism of Cr (VI) onto the BC was investigated and the result showed that the adsorption reaction consumed a large amount of protons along the reduction of Cr (VI) to Cr (III). The oxidation of the BC took place concurrently to the chromium reduction and led to the formation of hydroxyl and carboxyl functions. An initial solution pH of 1.0 was most favorable for Cr (VI) removal. The adsorption process followed the pseudo-second order equation and Freundlich isotherm very well. The Cr (VI) adsorption was temperature-dependent and almost independent on the sodium chloride concentrations. The maximum adsorption capacity for Cr (VI) was found at 21.34 mg/g in an acidic medium, which is comparable to other low-cost adsorbents.

Introduction

Increasing industrialization and urbanization worldwide had substantially ravaged our aquatic environment through the discharge of industrial and domestic wastes. These wastewaters are frequently laden with toxic heavy metals in which significant amounts are deposited into the natural aquatic and terrestrial ecosystems [1]. Among these heavy metals, chromium (Cr) is one of priority pollutants in surface water and groundwater resulting from numerous industrial activities such as the preservation of wood, textile dyeing, leather tanning, electroplating and metal finishing [2], [3]. Both of the common Cr (VI) anions, chromate (CrO42−) and dichromate (Cr2O72−), are strong oxidants, and chromate is a known carcinogen and a suspected mutagen and teratogen in biological systems [4], [5]. The US EPA requires Cr (VI) in drinking water and inland surface waters is 0.05 and 0.1 mg/L, respectively [6]. Accordingly, chromium containing wastewaters must be treated to lower the Cr (VI) to allowable limits before discharging into the environment.

Among available conventional processes used to remove Cr (VI), the most commonly applied are reduction and precipitation as chromium hydroxide. However, these methods suffer from some disadvantages due to their relatively high operational costs [5]. There is therefore a need for the developments of cheaper alternative technologies that can be applied as complement to the usual methods mentioned above.

Adsorption process was studied and emerged as one of the promising technique [5] due to its low initial cost, simplicity of design, ease of operation and insensitivity to toxic substances. Activated carbon is the most widely used adsorbent with great success due to its large surface area, micro-porous structure, high adsorption capacity, etc. However, its use is limited due to its high cost and low selectivity, which have led many workers to search for more economic substitutes [3], [5], [6], [7], [8], [9], [10], [11], [12].

Black carbon (BC), a significant extrinsic carbonaceous component arising from the incomplete combustion of fossil and biomass, is regarded as a potential contaminant adsorbent in soils and sediments [13], [14]. In China, about 600 billion kg of straws are produced annually as a by-product of crop production, more than 50% of which are burned for the purposes of quick waste disposal and immediate land clearing [15]. The smoke caused by open-field burning wheat straw frequently results in serious air pollution and traffic trouble, hence new economical technologies for crop straw disposal and utilization must be developed [16].

Numerous studies have demonstrated that black carbon in soils is a highly effective adsorbent for organic contaminants (including pesticides) [14], [17], [18], [19]. However, little effort has been made to address the adsorptive role of BC for heavy metals and the associated underlying mechanisms. Qiu et al. [15] evaluated the adsorptive ability of BC isolated from the burning residues of rice straw and wheat straw for lead ions and suggested the electrostatic interactions between positively Pb (II) and negatively charged functional groups as the primary adsorptive forces. However, the interactions of Cr (VI), mainly as oxyanions (i.e. CrO42− and Cr2O72−), with BC are poorly understood, which is of importance to predict the environmental behavior of Cr (VI) in soils. In this study, the removal of Cr (VI) from aqueous solution by a BC derived from wheat-residue was examined in order to understand the mechanisms that govern Cr (VI) removal. The influences of the solution pH, contact time, reaction temperature, ionic strength and initial Cr (VI) concentration on the Cr (VI) uptake were investigated. Moreover, in addition to the macroscopic Cr (VI) adsorption studies, the surface properties change before and after Cr (VI) adsorption was characterized by SEM-EDS and FTIR spectroscopy.

Section snippets

Materials

Potassium dichromate, hydrochloric acid, hydrofluoric acid, nitric acid and sodium hydroxide were purchased from Shanghai Chemical Reagent Company (Shanghai, China) and are of A.R. grade. The wheat straw (Triticum aestivum L.) was collected from the croplands of Yuntai District (Lianyungang, China). The black carbon samples were prepared according to reference [15]. Briefly, after air-drying for month, 5 kg of wheat residues were burned in an open field under local weather conditions. The

Surface characterization

The black carbon surface, before and after reacting with Cr (VI) in aqueous solution, was characterized using SEM-EDS (Fig. 1, Fig. 2). The surface of black carbon was smooth before the reaction with Cr (VI), but became coarse with a tiny particle on it. Fig. 2 shows the EDS spectra. The un-reacted surfaces contained elements of C, O, Si, K, and Ca. After reaction, the surfaces only contained elements C, O, Si, and Cr.

pH dependence of Cr (VI) removal

The aqueous solution pH is an important controlling parameter in the heavy

Conclusions

A black carbon (BC) sample isolated from the burning residues of wheat straw was applied to remove Cr (VI) from aqueous solution. The following conclusions can be drawn:

  • (1)

    The Cr (VI) adsorption was strongly pH-dependent. The higher the proton concentration, the higher the efficiency of the Cr (VI) removal. The Cr (VI) adsorption was temperature-dependent and almost independent on NaCl concentrations (supporting electrolyte), indicating that Cr (VI) removal was not principally by anionic

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