Characterization of red mud granular adsorbent (RMGA) and its performance on phosphate removal from aqueous solution

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Abstract

Red mud granular adsorbent (RMGA), which was applied to remove phosphate from aqueous solution in this research, was manufactured with red mud (a solid waste from alumina industry) as the main raw material. Based on the combined adsorption–regeneration experiment, RMGA sintered at 1000 °C was selected for further characterization. Through the process of sintering, both the alkalinity and the surface of RMGA were ameliorated. X-ray diffraction analysis showed that the components with single bondOH and single bondSO4 were the functional groups for phosphate adsorption. The removal of phosphate by RMGA was weakly affected by the common coexisting ions in solution (such as Cl, NO3- and SO42-), but strongly depended on pH and contact time. RMGA performed relatively better at pHi (the initial pH in solution) of 3.0–6.0, and equilibrium could be reached after 7 h. The kinetics studies presented that phosphate removal by RMGA mainly followed the pseudo second-order model. The initial phosphate removal rate was faster at lower pHi, since the electrostatic repulsion between RMGA and phosphate was enhanced as pH increased. The pH in solution rose during phosphate removal process, and the mechanism for phosphate removal was divided into two stages: the removal of phosphate within 1 h was mainly attributed to phosphate adsorption on RMGA; while that after 1 h was the combined effect of adsorption and precipitation, which resulted in the appearance of the maximum phosphate removal capacity (6.64 mg g−1) at pHi 5.00.

Highlights

► Red mud is applied to prepare granular adsorbent (RMGA) for phosphate removal. ► Components with single bondOH and single bondSO4 are effective for phosphate adsorption on RMGA. ► Pseudo second-order model can describe the phosphate removal by RMGA well. ► The largest P removal capacity was obtained at pHi 5.00 after equilibrium. ► P removal includes two stages: adsorption and combined adsorption–precipitation.

Introduction

Phosphate is a limiting nutritional factor for plant growth, but it will induce eutrophication to rivers and lakes if it is excessively supplied. According to a report in 2004, more than 40% of the receiving surface waters around the world are facing eutrophication problems [1]. Moreover, as P is considered as a nonrenewable and irreplaceable resource that will be scarce in the future [2], it is of great importance to remove and collect phosphates from the drainage basin before being dispersed into environment. An efficient method of reducing phosphates during wastewater treatment is the fundamental factor to prevent eutrophication in surface water. Various physical, chemical and biological techniques have been applied to remove dissolved phosphate from wastewater, such as adsorption or ion exchange [3], coagulation [4], precipitation–microfiltration [5] and aerobic–anaerobic–anoxic systems [6]. Among them, thanks to the comparatively simple and economical process as well as the less sludge producing and easy disposing operation, adsorption methods are considered promising [7].

In recent decades, various adsorbents have been studied for their application in phosphate removal, which include modified natural minerals like bentonite [8] and montmorillonite [9], agricultural residues like shells [10] and wheat-straw [11], industrial wastes like fly ash [12] and red mud [13], etc. Among these materials, red mud is a highly alkaline waste residue of bauxite via the alumina producing process, and one to two tons of red mud is generated for per ton alumina production [14]. Only in China, approximately 28.9 million tons of alumina was created during 2010 [15], and red mud is produced even more. Because of the alkaline in it as well as the lack of proper disposal methods, large amount of red mud is cumulated continuously, and this results in serious environmental problems. Since red mud is a potential adsorbent, the application of it in water treatment for phosphate removal is a good way, by which both the objects of disposing and reusing red mud can be obtained.

Recently, powdered red mud has been studied by many researchers, which performed well for phosphate removal due to its relatively larger surface area and higher calcium, aluminum, and iron content [16]. However, the characteristics such as easy-blocking and hard-recovering prevent the application of powdered red mud in practical column techniques [17], so the project of making red mud into a granular adsorbent has been put forward. Because the cohesiveness of red mud is not strong enough for granulation, some researchers have created granular red mud employing additives (such as Na2SiO3 [18]), which plays the role of adhesive, but this method is not very satisfactory in economic concern. Based on these attempts, we intend using clay, which is a natural cementing agent [19], instead of those expensive chemical adhesives to reduce the high cost in granular red mud producing.

In this research, red mud granular adsorbent (RMGA) was developed from red mud as the main raw material. Bentonite (a kind of clay) and starch were employed as natural additives instead of expensive chemical agent. For the purpose of accumulating basic operating data, the removal of phosphate from aqueous solution using RMGA was investigated according to adsorption experiments conducted in capped conical flasks. Through the regeneration experiment, RMGA sintered at 1000 °C (RMGA-1000 °C) was selected for further study. RMGA was characterized using X-ray diffraction (XRD) and scanning electron microscopy (SEM) microanalysis. The influences of some operation parameters on phosphate removal were investigated, including initial pH, coexisting ion and contact time as well as the kinetics studies.

Section snippets

Materials

In this research, raw materials for RMGA producing included dewatered red mud, bentonite and starch, all of which were under 100 mesh particle sized (less than 0.149 mm). Red mud, the main raw materials, was obtained from Shandong Aluminium Industry Corporation in Zibo, Shandong Province of China. Bentonite and starch played the role of cementing agent and aperture producer, respectively, taking the place of expensive chemical additives. The powdered materials were evenly blended at a mass ratio

Components analysis of raw materials

The chemical composition of two raw materials determined by EDAX is shown in Table 1: calcium oxide was the dominant component in red mud with 31.12% in mass, and the other principal components were the oxides of Si, Fe, Al and Na; while for bentonite, SiO2 was dominant with mass percentage of 66.18%, and the results were similar with our previous research [20]. The mass ratio of chemical compositions in mixed raw materials was calculated according to their corresponding proportion in red mud

Conclusions

RMGA prepared in this research was characterized and investigated for removal of phosphate from aqueous solution. During the sintering process, the alkalinity of RMGA was reduced and a rougher outer surface was obtained due to the flux component melting and the gas producing reaction, respectively. RMGA sintered at1000 °C was selected for further study based on its reasonable recycling performance. According to the XRD analysis, chemical components with single bondOH and single bondSO4 were considered as the

Acknowledgements

The authors are grateful to the support of National Major Special Technological Programmes Concerning Water Pollution Control and Management in the Twelfth Five-year Plan Period (No. 2012ZX07203004) Research Fund for the Doctoral Program of Higher Education of China (20100131110005), the National Natural Science Foundation of China (21007034), Natural Science Foundation of Shandong Province (ZR2010EQ031), Foundation for Young Excellent Scientists of Shandong Province (BS2009NY005) and Shandong

References (33)

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