Arsenic(iii) Remediation from Contaminated Water by Oxidation and Fe/al Co-precipitation

Battery grade γ-MnO 2 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 MnO 2 is completed in two stages: rapid initially followed by a first order reaction. As(III) is oxidised to As(V) by the MnO 2 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 MnO 2 decreased significantly with increasing pH from 4 to 8. The adsorption capacity of the γ-MnO 2 with particle size 90% passing 10 µm was determined to be 1.5 mg/g at pH 7. MnO 2 was found to be more effective as an oxidant for As(III) in the two step process than in the one step process.

MnO    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 MnO 2 was rapid initially followed by a first-order kinetics with respect to As(III) concentration.The activation energies for the oxidation reaction by the MnO 2 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][28][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].
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 MnO 2 as an oxidant for As(III) and as adsorbent in combination with Fe/Al coagulants for As(V) removal.The adsorption capacity of MnO 2 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 MnO 2 with in-situ formed Fe/Al hydroxides.The objectives of this work were to determine the suitability of MnO 2 as an oxidant and adsorbent in combination with Fe/Al hydroxide for arsenic removal.

Materials and Methods
First, Battery grade -MnO 2 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 Na 2 HAsO 4 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 MnO 2 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 ICP-AES 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 MnO 2 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.

Stoichiometry of Oxidation of As(III)
The stoichiometry of oxidation of As(III) by MnO 2 was determined by measuring residual reactants and reaction products after 2 hours contact of one gram of the MnO 2 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 MnO 2 were converted to Mn(II) during its reaction with As(III).Thus, the oxidation of As(III) was accompanied by a reduction of MnO 2 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)

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: 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 MnO 2 dose and tem- .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).

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.

Effect of pH on As(V) adsorption on MnO 2
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 MnO 2 phase.The point of zero charge (PZC) of chemically or electrochemically prepared MnO 2 materials such as α-MnO 2 , γ-MnO 2 and δ-MnO 2 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  repulsion force as pH rose above the PZC.Additionally, the proportion of the more negatively charged As(V) species increases as the solution pH increases, e.g. 2 4 H AsO  inate in pH range 2 -6.5.The H AsO ions dom  ions er hand dominate in pH range 6. 5 -11.8.This should also contribute to the observed pH effect.on the oth .

3
The effect of As(V) concentration on was investigated by varying initial As(V) concentration at a fixed dose of 1 g/L MnO 2 at pH 7. As shown in Figure 3, the results reasonably fit the Langmuir isotherm model: l/L) is the equilibrium concentration in the solution, Qe (mmol/g) is the amount adsorbed on the adsorbent at equilibrium, Q˚ and b are the Langmuir constants related to adsorption capacity and binding energy of adsorption respectively.From the slope of best fit, the adsorption capacity of the γ-MnO 2 is determined to be 1.5 mg/g.
For compar only used iron and aluminum hydroxide adsorbents were also similarly studied and the reagents summarized in Table 2.The published data from the literature are also included in the table.As seen, the adsorption capacity of MnO 2 increased in the order -MnO 2 < -MnO 2 < -MnO 2 << δ-MnO 2 .Clearly, the capacity depended on the form of MnO 2 and its preparation method which determine the crystalline properties and surface area.For example, the amorphous δ-MnO 2 possesses highest surface area and thus highest adsorption capacity compared with other well crystalline forms of MnO 2 .However, the capacity of the amorphous δ-MnO 2 is found to be significantly lower than the amorphous iron hydroxide (ferrihydrite).The capacity of As(V) removal also depends on the method used.The removal of As(V) by adsorp-apacity of Mn oxides with iron oxides.

Fe(III)/Al(III) Coagulants
T hydroxides was investigated in two ways: One step process: this involved simultan MnO 2 oxidant and the Fe or Fe/Al coagulant followed by pH adjustment.Two step proce s(III) by MnO 2 and subsequent removal of As(V) by coprecipitation/adsorption on in-situ formed Fe Fe/Al hydroxides.

 One step
As shown in Figur creased exponentially from 0.2 mg/L to 0.02 mg/L as the MnO 2 dose increased from 0.1 g/L to 0.5 g/L.A dose of 1 g/L of MnO 2 was needed to lower the residual arsenic to 0.01 mg/L (standard drinking water limit by US Environmental Protection Agency).This reflects the relative inefficiency of the one step process for removal of very low level of arsenic.


The results can be summar Fe/Al alone is an effe g/L, Expt.Soluble Mn(II) was effectively removed from the system in The arsenic removal is not affected when MnO 2 , Fe/Al, and As(V) are co-present in the one step pro de 3).
r compared with the two step process.T fa The As(III) removal by the one step process is relatively poo wo ctors are likely to affect the removal process.First, the in-situ formed amorphous Fe/Al 'hydroxide' may partially cover the surface of MnO 2 , which slows down the diffusion process of As(III) to its surface.Second, the As(III) initially adsorbed on the surface of Fe/Al "hydroxide" is desorbed by As(V) ions, establishing a new equilibrium.As(III), which is known to occur as unionised H 3 AsO 3 in the pH range 2 -9 is weakly bonded on the surface of ferrihydrite compared with As(V) which occur as H AsO  .The As(V) species is strongly bonded on the surface specific inner sphere adsorption more, the As(III) adsorbed on the surface of ferrihydrite may need further treatment to oxidize the As(III) to As(V) for safe disposal.The cost for this treatment may not justify using the one step process in real applications where safe disposal is the prime objective.As(III) is oxidized ost lu-e tion.The rate of the oxidation is characterised by an initial fast kinetics followed by a first order rate reaction.No significant variation in the oxidation rate occurs in the pH range 4 -8, except for a decreased trend at pH above 6, is observed.The adsorption of As(V) with the γ-MnO 2 is favored in low pH 4 and decreases rapidly where the pH rises to 8. The adsorption capacity of MnO 2 (90% passing 10 µm) is 1.5 mg (As(V)/g at pH 7. The MnO 2 is more efficient when used as an oxidant in the two step process than in the one step process.All the soluble Mn(II) ions are removed in the solution in both the processes.

Figure 2 .
Figure 2. Effect of solution pH on adsorption of As(V) on the MnO 2 .

Figure 4 .
Figure 4. Effect of MnO 2 dose on removal of arsenic by coprecipitation with Fe/Al coagulant.Initial 1 mg/L As(III), mechanism.Further to As(V) by the γ-MnO 2 with alm qual molar stoichiometric release of Mn(II) into so dgment DEST, Australia for fina pport under INDO-AUS Strategic Research Fund ences .T. Emett, M. Zaw and P. Prasad, " om Tubewell Water in Bangladesh, Reol.82, 1993, Arsenic with KMnO 4 .Oxidation and Manal of Arsenic in Waste Water from ution by , Vol.

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 MnO 2 .
Reaction product ratio As:Mn  1 perature.As can be seen in Figure1, the oxidation of As(III) by MnO 2 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 MnO 2 dose, suggesting that the reaction depends on surface area or available reaction sites on the surface of MnO 2