Removal of copper on composite sewage sludge/industrial sludge-based adsorbents: The role of surface chemistry

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Abstract

Sewage sludge and industrial waste oil sludge were pyrolyzed in an inert atmosphere at 650 or 950 °C, either as single components or as 50:50 mixtures. Composite materials were used as adsorbents of copper ions from aqueous solution. The capacity for copper removal was comparable to that of commercial activated carbon. To relate the performance of materials to their properties, the surface features were characterized using adsorption of nitrogen, thermal analysis, XRF, potentiometric titration, and elemental analysis. The results indicated that a high copper removal capacity could be linked to basic surface pH and specific compounds present on the surface. The high removal ability of materials obtained at 650 °C is attributed to cation exchange reactions between calcium and magnesium in aluminosilicates, formed on their surface during heat treatment, and copper. On the other hand, the high degree of mineralization of the surface of the materials obtained at 950 °C promotes copper complexation and its surface precipitation as hydroxides or hydroxylcarbonate entities.

Introduction

Removal of heavy or transition metals from water is an important environmental problem and the search for economically feasible technologies continues. As adsorbents, mainly activated carbons [1], [2], [3], [4], clays [5], [6], [7], [8], iron oxides/minerals [9], [10], [11], and other natural or synthetic fibrous adsorbents have been investigated [12]. Usually the adsorption ability of the above-mentioned materials is attributed to the presence of functional groups on their surfaces (activated carbons, fibers) or to ion-exchangeable sites where surface complexation or ion-exchange reactions can occur (clays, oxides).

Recently, sewage-sludge-based adsorbents were indicated as promising media for removal of such metals as copper [13] and mercury [14], [15]. Their adsorption capacity was around 80 mg g−1 and it was linked to cation-exchange reactions with calcium present in sewage sludge. While the toxic effects of mercury on the environment are well known [16], the presence of elevated levels of copper, besides damage to plants and decrease in the production of farmlands, results in health effects such as liver and kidney failure or Wilson's disease [17]. The main sources of copper environmental pollution are smelters, foundries, power stations, various combustion sources, and copper mining. The last results in the presence of copper in tailings and acid mine waters. So far, cation exchange has been the main mechanism used to remove significant quantities of copper from the environment [18].

Since sewage sludge and other industrial sludges are produced in abundant quantities by industrialized nations, the problems of feasible disposal/utilization of sludge arises. The efficient method of their industrial “recycling” and thus waste minimization is via pyrolysis, in which adsorbents are produced. It was recently demonstrated that such adsorbents can effectively remove dyes [13], [19], [20], heavy metals [13], [14], [15], phenols [13], or hydrogen sulfide [21], [22], [23], [24], [25], [26], [27]. Their high removal capacity is a result of complexity of surface chemistry, specific porosity, and high volume of mesopores. That surface chemistry includes basic pH and presence of catalytic centers based on iron and other transition metals. These entities are formed as a result of solid state reactions during pyrolysis of complex inorganic/organic matrices [24], [25], [27]. Moreover, there is an indication that the pyrolysis conditions can be adjusted toward obtaining the most effective adsorbents for a desired application [28], [29].

The objective of this paper is to investigate the performance of complex adsorbents derived from industrial sludges (sewage sludge/waste oil sludge) in the process of copper removal from aqueous solution. The materials are obtained at two temperatures, 650 and 950 °C, to investigate the effect of surface chemistry on the copper removal capacity. Based on the surface features of the materials studied (porosity, specific surface chemistry), the mechanism of copper adsorption is proposed. This kind of investigation can open a new window of opportunity for a wide range of waste materials ranging from microbial biomass to byproducts derived from industrial, agricultural, and fishery wastes.

Section snippets

Materials

Industrial oil sludge (WO) from Newport News Shipyard was mixed with dewatered sewage sludge from Wards Island Water Pollution Control Plant (SS) in a 50:50 ratio based on the wet mass, homogenized, dried at 120 °C for 48 h, and then carbonized at 650 or 950 °C in nitrogen in a fixed-bed horizontal furnace. The heating rate was 10 °C min−1 with holding time half an hour. The same treatment was applied separately to both single components of the mixture, sewage sludge and waste oil sludge. The

Results and discussion

The adsorption capacity of sludge-based composite adsorbents for the removal of copper from aqueous solution was compared to that of commercial carbon, WVA. Adsorption kinetics experiments were carried out to determine the time needed to reach equilibrium (Fig. 1). Although after about 50 h the equilibrium value is reached for the majority of samples, the isotherms were measured after 3 days (72 h) exposure to copper solution. The kinetics of Cu2+ adsorption were analyzed using the

Conclusions

The results presented in this paper show the applicability of industrial sewage sludge composite materials as adsorbents of copper from aqueous solution. On these materials, the adsorption capacity is higher than that reported on commercial activated carbons and modified carbons. The performance of our materials depends on the pyrolysis temperature. Low pyrolysis temperature results in a high capacity, which is linked to cation-exchange reactions. As a result of this, magnesium, calcium, and

Acknowledgements

The experimental help of Anna Kleyman is appreciated. The authors are grateful to Professor Daniel Akins, Dionne Miller, and Shiunchin (Chris) Wang for their help in receiving FTIR spectra.

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