Temperature effect on the mechanism of phosphate anions sorption by β-MnO2

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Abstract

Phosphate anions excess in stored water reservoirs are known to stimulate algal growth and scaling in vessels used for cooling purposes in industries. Several techniques such as ion exchange, adsorption on activated carbon, chemical precipitation and reverse osmosis have been used for its removal. The use of inorganic ion exchangers like oxide/hydroxides has been preferred recently due to their stability at high temperature, to ionizing radiations and high selectivities for specific anion and cations.

In the present investigation phosphate sorption on β-MnO2 is studied as function of concentration (1.07–11.85 × 10−1 mmol L−1) and temperature (293–313 K) at three different pH values of 3, 5 and 7. The sorption of phosphate is found to decrease with the increase in temperature in the pH range 3–7. Freundlich equation derived from the Law of mass action equation was found applicable to the sorption data. Effect of background electrolyte concentration and FTIR studies showed that mechanism of uptake process was the formation of outer-sphere complexes at all the temperatures under investigation. The thermodynamic parameters such as ΔH°, ΔS° and ΔG° values along with isosteric heats of adsorption were calculated, which showed that the solid preferred the multiply charged phosphate at low temperature and pH.

Introduction

Water is a prime need for human survival and industrial development. Effective management of water resources and control of pollution are becoming increasingly important for substantial development and human welfare. With the continuous growth in population, urbanization, industrialization and transportation, the number and amount of the environmentally damaging chemicals and toxic substances entering the biosphere have increased steadily, particularly during the last two decades.

Phosphate anions concentration exceeding about 2 μM in water reservoirs are known to stimulate algal growth (eutrophication), reducing the dissolved oxygen in aqueous environment, which not only kills the aquatic life but also disrupts the natural food chain [1], [2]. Besides, the phosphate pollution also causes scaling in vessels used for cooling purposes in industries [3]. It enters the natural waters from various fertilizers industries, domestic sewage, laundries, running water over phosphate rocks and extensive application of manure and fertilizers to soils. The run off from all these sources finds their way to water bodies [2]. It is therefore of the major concern to adopt an effective method for the removal of phosphate from natural aquatic system.

Oxide/hydroxides of aluminum, iron and manganese and other important inorganic ion exchangers are naturally occurring discrete minerals and are mostly used in various processes such as water treatment, nutrient recovery and various engineering processes [4], [5]. A number of these inorganic ion exchangers have been employed in the literature to study the sorption of various oxyanions anions like chromate, arsenate and selenate, etc. These researchers used the oxides of iron, aluminum and manganese to study the sorption of chromate, arsenate, selenate and selinite which are thoughts to be toxic in elevated levels both to plants and animals [6], [7], [8], [9], [10].

Phosphate in the soil environment is found to be fixed upon organic matter, oxides/hydroxides of aluminum, iron and manganese [11], [12]. While a number of studies have been reported in the literature about the phosphate anions sorption upon iron and aluminum oxides/hydroxides [12], [13], [14], [15], very little is reported about their interaction with oxides/hydroxides of manganese [16]. The present study therefore takes care of phosphate uptake by oxide of manganese (β-MnO2) as a function of temperature using three different initial pH values of 3, 5 and 7.

Section snippets

Reagents

All reagents used were analytical grade. The solutions were prepared in doubly distilled water. KH2PO4 supplied by MERCK was used without further purification. Similarly, nitric acid, potassium hydroxide solutions having concentrations 0.1, 0.5 and 1 M and standard buffers of pH 2 and 11 were also prepared in doubly distilled water.

The manganese dioxide purchased from MERCK was characterized as described in the following section.

Characterization of β-MnO2

The surface area of the solid β-MnO2 was determined by well-known

Characterization of solid β-MnO2

The surface area was found to be 214 m2 g−1 using the well-known nitrogen adsorption BET method. The X-ray diffraction pattern showed the solid to be crystalline β-MnO2 as reported by Bayliss et al. [19] and also by us elsewhere [18].

The point of zero charge (PZC) value (Fig. 1) for crystalline β-MnO2 is found to be equal to 7.5, which is reasonably close to the value of Stumm and Morgan [20].

Sorption studies

The adsorption of phosphate anions as a function of temperature at initial pH values of 3, 5 and 7 are

Conclusions

From the foregoing discussion, it can be concluded that the adsorption of phosphate is favoured by β-MnO2 both at low pH and temperature. The pH changes accompanying the sorption process, increase in phosphate sorption with the increase in back ground electrolyte concentration and the FTIR spectra show that the process responsible for the uptake of the phosphate anions is outer-sphere surface complexation at pH range 3–7 and temperature 293–313 K. Thermodynamic parameters derived from the

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