Phosphate removal from water using freshly formed Fe–Mn binary oxide: Adsorption behaviors and mechanisms

https://doi.org/10.1016/j.colsurfa.2014.04.034Get rights and content

Highlights

  • A freshly formed Fe–Mn binary oxide (FMBO) suspension was synthesized.

  • FMBO had different physicochemical properties from the mixture of Fe and Mn oxides.

  • Phosphate adsorption followed trend FMBO > mixed oxide > ferric oxide at pH 3–10.

  • The mechanism of phosphate adsorption on FMBO was clarified.

Abstract

Freshly formed metal oxide suspensions usually have high reactivity and are easy to achieve in-situ preparation and dosing in water treatment. In this study, a freshly formed Fe–Mn binary oxide (FMBO) suspension was prepared for phosphate removal using simultaneous oxidation and coprecipitation method. FMBO was characterized by SEM, TEM, XRD and zeta potential and compared with the mixture of ferric oxide and manganese oxides (FMMO), hydrous ferric oxide (HFO) and hydrous manganese oxide (HMO). Results indicated that ferric oxide was relatively enriched on the surface of nanostructured binary oxide, resulting in its pHZPC and XRD pattern close to HFO. At pH 3–10, phosphate removals followed trend: FMBO > FMMO > HFO > HMO. The pseudo-second-order kinetics model provided a better fit for phosphate adsorption onto FMBO, indicating that chemisorption may play a dominant role. The maximum adsorption capacity estimated from the Langmuir equation was 0.223 mol-P/mol-Fe at pH 5.0. The competition effects of coexisting substances such as sulfate, bicarbonate, silicate and DOC on phosphate adsorption using FMBO were not significant. Phosphate adsorption onto FMBO could be attributed to electrostatic interaction and replacement of surface hydroxyl groups via formation of monodentate and bidentate complexes.

Introduction

Phosphate is an essential nutrient element for living organisms. However, in aquatic environments, its excessively high concentration may deteriorate ecosystems and bring about subsequent water quality problems [1]. Therefore, it is crucial for researchers to develop effective technologies for phosphate removal in water and wastewater treatment systems. Among various available technologies for decontamination of phosphate, the adsorption method is relatively simple, economical, and highly efficient [2], [3].

In adsorption process, metal (hydr) oxides adsorbents such as iron, aluminum and manganese (hydr) oxides are widely used in water and wastewater treatments [4], [5]. In natural waters, these metal oxides are omnipresent and they are also found in combination with each other to form composite oxides [6], [7], which may display physicochemical properties different from those of their single component oxides. Thus the development of composite oxides adsorbents containing two (or more) different metal oxides has drawn great attention in recent years [7], [8], [9], [10], [11], [12]. In these studies, researchers only concentrate on adsorptive characteristics of relatively well-defined solid oxides for pollutants. However, in aquatic environments when these oxides are freshly formed, they often take on hydrous and amorphous forms with bulk surface properties distinctly different from aged solid oxides [3], [13]. Further, adsorptive capability of aged solid oxides might decrease due to drying preparations. In particular, these solid oxides are mostly powdery, making their direct engineering applications difficult. By contrast, freshly formed oxide suspensions can maintain high surface reactivity and adsorptive capability [14]. When utilized as adsorbents in water treatment, freshly formed oxide suspensions are easy to prepare in situ, dose in the water to be treated and subsequently rapid adsorptive reaction occurs. This is very favorable for its engineering applications in the treatment of water or wastewater, especially the remediation of phosphate-contaminated water body. Among various oxides, iron and manganese (hydr) oxides are well known due to their affinity towards phosphate [4], [15]. However, until now, very little is known about the adsorption behaviors of phosphate on freshly formed Fe–Mn binary oxide.

Thus, the objectives of the current study are to: (1) evaluate physicochemical properties of different freshly formed oxide suspensions (FMBO, FMMO, HFO and HMO) via determination of particle size distribution, zeta potential, X-ray diffraction (XRD) and other analytic techniques; (2) illustrate the characteristics of phosphate adsorption on freshly formed FMBO such as pH effects, adsorption kinetics, isotherms and effect of coexisting substances through batch experiments; and (3) propose the dominant mechanisms involved in phosphate removal by freshly formed FMBO using Fourier transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS) techniques.

Section snippets

Materials

In this study, with the exception of humic acid purchased from Sigma–Aldrich Corporation (USA), all other chemicals were purchased from Sinopharm Chemical Reagent Company (China) and were of analytical reagent grade. The phosphate standard stock solution was made from anhydrous potassium dihydrogen orthophosphate (KH2PO4). All solutions were prepared using deionized water.

Preparation of freshly formed oxides

Four kinds of freshly formed oxide suspensions were prepared in the current study, which included FMBO, FMMO, HFO and HMO.

Physicochemical properties of freshly formed oxides

Phosphate adsorption on metal oxides was significantly affected by their morphologies. In this study, the particle size distribution curves of FMBO, HFO and HMO suspensions are shown in Fig. 1. It can be found that the main particle size of FMBO ranged from 2 to 15 μm, which was different from that of HFO and HMO. The main particle size of HFO and HMO was approximately in the range of 5–35 μm and 0–23 μm, respectively. The SEM image (Fig. 2) of FMBO revealed that there were many micropores on the

Conclusions

Freshly formed FMBO suspension synthesized by the oxidation and coprecipitation method exhibited the highest selective adsorption capability towards phosphate as compared to FMMO and their simple oxide (i.e., HFO and HMO). The surfaces of FMBO were found to be rough and nanostructured. Zeta potential, XRD, EDAX and XPS analyses showed that ferric oxide was enriched on the surface of binary oxide, resulting in the surface mineralogy (XRD) and charge properties of FMBO microparticles similar to

Acknowledgments

This study was funded by the National Science Foundation for Distinguished Young Scholars of China (No. 51225805), the National Natural Science Foundation of China (Nos. 51108298 and 51138009) and the Natural Science Foundation of Tianjin (No. 12JCYBJC14800). The authors also acknowledge the financial support from the Research Fund of Tianjin Key Laboratory of Aquatic Science and Technology (No. TJKLAST-2011-13).

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