Arsenic(III) Remediation from Contaminated Water by Oxidation and Fe/Al Co-Precipitation

Wensheng Zhang, Pritam Singh, Touma B. Issa

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DOI: 10.4236/jwarp.2011.39075                

Abstract

Battery grade γ-MnO₂ powder was investigated as an oxidant and an adsorbent in combination with Fe/Al coagulants for removal of arsenic from contaminated water. Simultaneous oxidation of As(III) and removal by coprecipitation/adsorption (one step process) was compared with pre-oxidation and subsequent removal by coprecipitation/adsorption (two step process). The rate of As(III) oxidation with MnO₂ is completed in two stages: rapid initially followed by a first order reaction. As(III) is oxidised to As(V) by the MnO₂ with a release of approximately 1:1 molar Mn(II) into the solution. No significant pH effect on oxidation of As(III) was observed in the pH range 4 - 6. The rate showed a decreasing trend above pH 6. The removal of As(V) by adsorption on the MnO2 decreased significantly with increasing pH from 4 to 8. The adsorption capacity of the γ-MnO₂ with particle size 90% passing 10 µm was determined to be 1.5 mg/g at pH 7. MnO₂ was found to be more effective as an oxidant for As(III) in the two step process than in the one step process.

Keywords

Manganese Oxides, Iron Hydroxides, Arsenic Remediation, Fe/Al Coagulants, Contaminated Water

1. Introduction

Arsenic in contaminated groundwater occurs largely as arsenite (As(III)). [1] Effective and complete removal of arsenic by adsorption/coprecipitation methods requires pre-oxidation of As(III) to As(V). Oxygen or air is a cheap but kinetically slow oxidant for As(III). Various other oxidants for As(III) have been reported in the literature, including permanganate (), [2-4] ozone (O3), [5] hydrogen peroxide (H₂O₂), [6] chlorine (Cl₂), [7-10] or hypochlorite (ClO⁻), [11-13] catalyzed sulphite/O₂ (air) mixture, [14,15] and UV catalyzed systems. [16-19] These oxidants are effective but are either costly, or need rigid process controls for efficient oxidation. In recent years, manganese oxides, in both synthetic and natural forms, have been investigated for oxidation of As(III) [20-26].

Oscarson et al. [27] found that the oxidation of As(III) by birnessite, cryptomelane, and pyrolusite obeyed the first-order rate law with the rate constants at 298 K being 0.267, 0.189 and 0.44 × 10⁻³ h⁻¹, respectively. However, Chen and Fang [28] reported that the oxidation rate of As(III) by MnO₂ was rapid initially followed by a firstorder kinetics with respect to As(III) concentration. The activation energies for the oxidation reaction by the MnO₂ were measured to be in the range 26.0 - 32.3 kJ/mol [27]. The oxidation process was reported to be limited by diffusion of the reactant As(III) to or the reaction products away from the surface [27-29].

Scott and Morgan [29] proposed a surface mechanism that As(III) anion forms an inner-sphere complex followed by electron transfer between the surface metal ion and As(III) anion. The adsorption of As(III) on the surface was the slowest step. The surface mechanism was supported by the observation that the rate of As(III) oxidation directly depended on the concentration of surfacebound As(III) [30]. A mechanism of production of an intermediate reaction product, Mn(III) hydroxyl (MnOOH*), was proposed by Nesbitt et al. [31].

2MnO₂ + H₃AsO₃ = 2MnOOH* + H₃AsO₄(1)

2MnOOH* + H₃AsO₃ = 2MnO + H₃AsO₄ + H₂O (2)

Various forms of Mn oxides as adsorbent for arsenic removal have also been investigated, including pyrolusite and cryptomelane, [32] combination of pyrolusite with granular ferric hydroxide, [30] natural manganese oxides in a packed bed or column, [33] ferruginous manganese ore (FMO), [34] Mn dioxide-coated sand (MDCS), [35, 36] and Bi-enhanced Mn oxides [37]. Chiu and Hering [30] compared the adsorption capacities for different types of Mn oxides. They found that the surface saturation for pyrolusite and cryptomelane at pH 6.5 for As(V) species were 0.75 and 1.87 mg/g, respectively. The difference in the adsorption capacity was attributed to their crystallinity and specific surface areas. Poorly crystalline birnessite and cryptomelane possess higher specific surface areas than highly ordered pyrolusite [29].

This paper reports the investigation of MnO₂ as an oxidant for As(III) and as adsorbent in combination with Fe/Al coagulants for As(V) removal. The adsorption capacity of MnO₂ for As(V) was compared with commonly used iron and aluminium hydroxide adsorbents under similar experimental conditions. Simultaneous oxidation and coprecipitation/adsorption (one step process), and pre-oxidation followed by coprecipitation/adsorption (two step processes) were investigated for various combinations of MnO₂ with in-situ formed Fe/Al hydroxides. The objectives of this work were to determine the suitability of MnO₂ as an oxidant and adsorbent in combination with Fe/Al hydroxide for arsenic removal.

2. Materials and Methods

First, Battery grade g-MnO₂ powder with particle size 90% passing 10 µm, supplied by Aldrich Australia, was used for all the experiments. All the other chemicals used were of AR grade without further treatment. As(III) stock solution was prepared from As₂O₃ in accordance with the procedure provided by Vogel [38]. As(V) stock solution was prepared by dissolving Na₂HAsO₄ in demonized water. Fe(III) and Al(III) stock solutions were prepared from their chloride salts. Solution pH was adjusted with dilute HCl and NaOH solutions.

All the experiments were conducted at 25˚C in 250 ml conical flasks equipped with magnetic stirring units for liquid-solid mixing. In the one step process, a dose of MnO₂ and a desired volume of equal molar Fe(III)/Al(III) solution were simultaneously added to water containing known amount of As(III) or As(V). Solution pH was adjusted and maintained at the desired value throughout the experiment. Samples were taken and filtered through a 0.2 µm membrane filter. The filtrate was analysed for As(III) and As(V) by hydride generation followed by inductively coupled plasma and atomic emission spectroscopy (ICP-AES), and for total soluble Mn by ICPAES at the Marine and Freshwater Research Institute, Environmental Science, Murdoch University, Western Australia. For the two step process, the arsenic bearing solution was first treated with MnO₂ followed by adsorption/precipitation with Fe(III)/Al(III) coagulants at pH 7. All the other procedures were the same as the one step process.

3. Results and Discussion

3.1. Oxidation of Arsenic(III) by MnO2

3.1.1. Stoichiometry of Oxidation of As(III)

The stoichiometry of oxidation of As(III) by MnO₂ was determined by measuring residual reactants and reaction products after 2 hours contact of one gram of the MnO₂ powder with initial 1 ppm As(III) solution at pH 7 in the absence of oxygen maintained by bubbling nitrogen gas through the solution. The analysis results for residual concentrations of As(III), As(V) and Mn(II) in the final solution are given in Table 1. The important observations are:

No As(III) remained in the solution, indicating that all the As(III) was oxidized to As(V).

The residual arsenic in the solution accounted for only 80% of the amount initially present in the reaction mixture.

The solid phase contained the remaining 20% of the arsenic which could be assumed to be As(V).

The solution contained Mn(II) as much as would be expected if all the reacted MnO₂ were converted to Mn(II) during its reaction with As(III). Thus, the oxidation of As(III) was accompanied by a reduction of MnO₂ yielding Mn(II) into solution at an approximately equal molar stoichiometry with respect to the total oxidized As(III):

Mn(IV) + As(III) = Mn(II) + As(V) (3)

3.1.2. Rate of Oxidation of As(III)

The rate of As(III) oxidation are plotted in Figure 1 and analyzed with respect to the first order rate law:

Ln[As(III)]/[As(III)] = kt(4)

where [As(III)] is the initial As(III) concentration (mg/L), [As(III)] the concentration at time t (min), k the rate constant (min^–1) which is a function of MnO₂ dose and tem-

Table 1. Concentrations of reaction products in the final solutions after 2 hour contact time at pH 7 and 25˚C. Initial 1 ppm As(III), 1 g/L MnO₂.

perature. As can be seen in Figure 1, the oxidation of As(III) by MnO₂ could be characterized by very fast kinetics within the first 30 minutes, followed by a first order rate which is indicated by the fact that Ln[As(III)]/ [As(III)] vs reaction time t is graphically linear. The rate increases with MnO₂ dose, suggesting that the reaction depends on surface area or available reaction sites on the surface of MnO₂. This two stage kinetic feature was also observed by Chen and Fang [28]. The slow-down in the rate at later stage of the reaction is indicative of competition for active adsorption sites between As(III) and As(V).

3.1.3. Effect of pH on Oxidation of As(III)

The pH effect was investigated by varying solution pH in the range 4 - 8 and measuring the residual As(III) in solution after 2 hour contact time for each fixed pH. The initial As(III) concentration was 6 mg/L. It was observed that about 80% of the As(III) ions was oxidized for each pH in two hours in the pH range 4 - 6. Above pH 6, a decreased trend occurred up to pH 8. This is likely to be caused by formation of manganese hydroxide on the surface which blocks some sites for reaction with As(III) on the surface.

3.2. MnO₂ as Adsorbent

3.2.1. Effect of pH on As(V) Adsorption on MnO₂

Figure 2 shows that the %As(V) adsorption decreases linearly when solution pH increases from pH 4 to pH 8. This effect can be explained by the surface charge characteristics of the MnO₂ phase. The point of zero charge (PZC) of chemically or electrochemically prepared MnO₂ materials such as α-MnO₂, γ-MnO₂ and δ-MnO₂ lies in the pH range 1.5 - 4.15 [39,40]. Therefore, it is not surprising to observe the decreasing effect because the adsorption of As(V) species must overcome the increased

Conflicts of Interest

The authors declare no conflicts of interest.

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