Removal of phosphorus by a composite metal oxide adsorbent derived from manganese ore tailings

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Abstract

The selective adsorption of phosphate (P) from wastewater is a promising method for controlling eutrophication in water bodies. In this study, an adsorbent of composite metal oxides (CMOMO) was synthesized from manganese ore tailings by the process of digestion–oxidation–coprecipitation. CMOMO was characterized using several methods, and its adsorption behaviors for phosphate were investigated. Based on the results from SEM and BET analysis, CMOMO exhibited a rough surface and a large surface area (307.21 m2/g). According to the results of EDAX, XRD and XPS, its main constituents were determined to be amorphous FeOOH, MnO2 and AlOOH. The kinetic data were best fit using the Elovich model due to its complicate composites. The maximal adsorption capacity of P would increase with elevated temperatures. Additionally, it was found that the P removal efficiency decreased with an increase of pH (4–10) or a decrease of ion strength (1–0.01 M). The coexisting anions had little effects on phosphate removal, implying the specific adsorption of P by CMOMO. Furthermore, the desorption and reuse results indicated that this adsorbent could be regenerated using alkali solutions.

Highlights

► A new adsorbent of composite metal oxides (CMOMO) was synthesized from manganese ore tailings and used for phosphorus (P) removal. ► The kinetics data were best fitted by the Elovich model due to the high heterogeneity of the adsorbent. ► P adsorption by CMOMO was an endothermic process, with a satisfactory maximum adsorption capacity. ► This adsorbent showed specific adsorption capacity towards P, and P is adsorbed mainly by formation of inner-sphere surface complexes. ► P adsorption by CMOMO was reversible using alkali solution as an eluent.

Introduction

The enhanced removal of phosphate (P) from wastewater may become required urgently when it is discharged to freshwater bodies, which are at a risk of eutrophication [1]. Several methods have been developed for P removal, such as chemical precipitation [2], biological processes [3], [4], adsorption [5], ion exchange [6], and membrane technologies [7]. Among these available approaches, chemical precipitation and biological processes are generally not able to meet the stringent effluent standards, while ion exchange and membrane technologies need high investment and operation cost. Comparing with these methods, adsorption process showed advantages of easily-handle operation, high efficiency and lower cost. Various adsorbents have been used for P removal, such as zeolite [8], Ca-based materials [9], iron oxides [10], aluminum oxides [11], etc. In recent years, based on economical and environmental concerns, considerable attention has been paid to the utilization of low-cost adsorbents, including agro-industrial and municipal waste materials [12]. Some of these waste materials were used directly for P removal from wastewater. Razali et al. [13] studied the effectiveness of drinking water treatment sludge in removing different phosphorus species from aqueous solution, and found the Al-based sludge had the potential to be used as a raw material for a wide range of P species removal in simulated P-enriched wastewater. Wei et al. [14] used acid drainage sludge for the P removal from secondary effluents of municipal wastewater treatment plants. It was found that the metal oxides (such as iron (Fe), aluminum (Al), manganese (Mn), etc.) contained in these low-cost adsorbents played an important role for their capacity for phosphate removal [14]. However, the adsorption capacities of these waste materials were generally limited due to the less active components [13], [14]. Moreover, second pollution may be caused due to the complicate components of these materials. Thus, some researchers have made efforts to effectively utilize these waste materials for the preparation of adsorbents with low cost and high adsorption capacity. Gong et al. [15] prepared a new adsorbent with hydrated lime and blast furnace slag for phosphorus removal from aqueous solution, and optimized the preparation conditions for the premium P removal efficiency. Sibrell et al. [16] prepared an adsorption media from acid mine drainage sludge for the P removal from agricultural wastewater, which was more safe and had a greater P adsorption capacity than that of acid mine drainage sludge.

Manganese ore tailings were produced hundreds of thousands of tons every year in China, due to the exploitation of manganese ores. It has been reported that the metals contained in the wastes of abandoned mines may cause serious environmental problems in nearby groundwater, stream and cultivated lands [17], [18]. So these mine wastes should be disposed of properly to minimize the threat for the ecology environment. A mineralogical study showed that this mine waste is composed primarily of Mn, Fe and silicon (Si) oxides (pyrolusite, hematite, quartz, …) [19]. It may be beneficial to synthesize a material of composite metal oxides from manganese ore tailings and use it as an adsorbent for removing phosphate, which provides not only an effective residual management option for manganese ore tailings, but also an alternative approach for P removal.

Therefore, the objective of this study was to explore the feasibility of the composite mental oxides derived from manganese ore tailings (CMOMO) for P removal. The surface characteristics research of CMOMO was conducted. The kinetics and isotherm behaviors of phosphate adsorption by CMOMO were investigated. The parameters influencing P removal were also studied, including solution pH and coexisting anions. Additionally, the P desorption from CMOMO was performed to assess the regeneration feasibility.

Section snippets

Materials

The mine tailings used in this study were gotten from a mine of manganese ores at the city of Hanzhong in Shaanxi Province, China. The metal compositions were shown in Table 1.

All chemicals were of analytical grade and purchased from Beijing Chemical Co., and all solutions were prepared with deionized water. Sodium hypochlorite (NaClO) was used for the preparation the adsorbent. The phosphorus stock solutions were prepared with sodium dihydrogen phosphate (NaH2PO4). Additionally, sodium nitrate

Characterizations

Fig. 1 presents the morphology and surface elements distribution of the CMOMO examined by SEM/EDAX. It can be seen that the adsorbent particles were aggregated with many small particles (Fig. 1a), resulting in a rough surface and a porous structure. EDAX results showed that the adsorbent mainly contained the elements of O, Fe, Mn, Al and Ca, indicating the composition of mixed metal oxides. Chemical composition analysis revealed that Fe and Mn oxides were the dominant components of the

Conclusions

A new adsorbent was synthesized from manganese ore tailings for P removal using the process of digestion–oxidation–coprecipitation. It had an amorphous structure and a large BET area of 307.21 m2/g, with the pHIEP value of 5.4. The kinetics data for P adsorption were best fitted by Elovich model due to the complicate composites of CMOMO. The maximum adsorption capacity for phosphate increased with an increase of temperature. Additionally, phosphate removal efficiency decreased with an increase

Acknowledgements

This work was supported by the financial supports from Chinese Universities Scientific Fund (Grant No. QN2009037) and the Natural Science Foundation of China (Grant No. 51108377).

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