Synthesis of MnO2 on Activated Carbon and its Potential Application in the Adsorption of As(V) and Pb(II) in Aqueous Solutions

The conditions for the synthesis of a material with MnO2 (OMD) on activated carbon (AC) were studied. These conditions were: reaction time, temperature, stirring speed, concentrations of AC, H2SO4, and O3 in solution, and particle size. Agglomerates on AC were observed by means of scanning electron microscopy (SEM) and microanalysis by energy dispersive spectroscopy (EDS) and revealed the presence of OMD deposited on the surface. The activation energy and the factor of frequency for the reaction were determined as Ea = 1.2 kcal/mol and A = 2.2. The value of Ea indicates that the precipitation of OMD on the AC was controlled by mass transfer in aqueous solution and the order of reaction was zero. The adsorption capacities of AC were q = 14 mg Pb(II)/g AC and q = 9.1 mg As(V)/g AC. Whereas, for the OMD/AC obtained in the following conditions: [AC] 1 or 2 g/L, particle size of AC of +0.59 mm, [H2SO4] 1 or 2 mol/L, 25 °C, stirring speed 600 rpm, and [O3] 1.35 mol/L, the adsorption capacities were q = 90.5 mg Pb(II)/g OMD/AC and 25.4 mg As(V)/g OMD/AC. Therefore, the fixing of OMD on the surface of the AC greatly improved the removal of both Pb(II) and As(V) from aqueous solutions.


Introduction
Studies on absorbent materials for natural and anthropogenic contaminant ions, in both surface water and groundwater, remain of interest.Arsenic and lead ions have notable deleterious effects on human health.In Mexico, their presence has been detected in aquifers, especially in the north of the country. 1 They cause various diseases including skin diseases, bone diseases and diseases in other vital organs.Moreover, they can lead to death when high-dose exposure is continuous.Thus, it is important to find efficient alternatives for the removal of these ions from water and to reduce the risks to human health.
Arsenic is present in the atmosphere, soil, rocks, natural water and organisms.Most environmental problems related to arsenic contamination are due to mobilization under natural conditions.For human beings, arsenic has an important impact in the form of mining, burning fossil fuels and use of arsenic-containing pesticides, herbicides and livestock feed. 2 The presence of arsenic in water usually indicates nearby mines or metallurgical industries in operation or agricultural areas where materials with arsenic are used. 3The average levels of arsenic in groundwater are between 0.001 and 0.002 mg/L; however, in areas with volcanic rocks and sulfide ore deposits, these levels are > 3 mg/L.The World Health Organization (WHO) established that the arsenic in water for human consumption should not exceed 0.01 mg/L. 4This is a provisional value given the uncertainty of the risks associated with a lower concentration of arsenic; in 1984, the WHO had set limit value of 0.05 mg/L.The U.S. Environmental Protection Agency (EPA) indicates a maximum permissible limit concentration of arsenic of 0.01 mg/L for drinking water. 5he Mexican official standard is a maximum permissible limit concentration of 0.025 mg/L. 6ead is present in the environment, including in the air, dust, soil and water.This element and its compounds are widely used for batteries, pigments, ammunitions, weldings, pipes, coated cables and bearings.Other sources of contamination are mining and smelting waste, petrol and paintings. 7The maximum permissible limit concentration of lead in drinking water established by the WHO for water is 0.01 mg/L.][9][10][11][12][13][14][15] In recent years, research has been conducted on low-cost and easily obtainable materials that may be useful for absorbing heavy metals.7][18][19] To increase the adsorption capacity of these natural adsorbents materials, changes to their surfaces have been proposed.][28][29][30] Oxidation of manganese ions by ozone in an aqueous solution has been widely studied, and it is a feasible alternative for the production of OMD. 31 Ozone is one of the strongest oxidants and offers the advantage of not introducing unwanted specimen into the system; therefore, its use guarantees the purity of the product obtained.OMD obtained via this method has better characteristics than if it was produced via other methods.3][34] Preferentially, OMD exchanges with other ions in the order Pb(II) > Zn(II) > Cd(II) > Tl(I). 35ctivated carbon (AC) is a low-cost adsorbent that, depending on the conditions of the adsorbate, can be used in a wide pH range.7][38] Manganese dioxide obtained by specific conditions of ozonation (OMD) has better adsorption capabilities than if it was obtained by electrolytic or chemical methods.The size of the OMD obtained by direct precipitation is relatively small.An interesting option is the use of a supporting material such as AC to obtain a larger material useful in traditional adsorption columns.The synthesis of OMD consumes electricity for the for-mation of ozone; however, with the use of alternate energy sources (i.e., solar), costs could be mitigated.Manganese sulfate can be obtained during the acid leaching of manganese ore, whose oxidation with ozone can be directly applied in the presence of AC.The cost of obtaining OMD from manganese ore is comparable to that of obtaining it by the electrolytic method.In addition, OMD/AC provides the features for adsorption of Pb(II) or As(V); furthermore, the reversibility of adsorption has been proved for cadmium(II) and zinc(II) with an acid solution at pH 1-2. 35he present investigation proposes the synthesis of an adsorbent material, which consists of OMD supported by AC to examine whether its physical and chemical characteristics are suitable for the adsorption columns used in water purification.The specific aims of this research are: 1) to establish the best conditions for the synthesis of OMD/AC, 2) to characterize the material, and 3) to determine the adsorption capacity of the material for Pb(II) and As(V) ions.

1. Synthesis of MnO 2 /AC
For the synthesis of the OMD, MnSO 4 (Karal S. A. de C. V., reagent grade) and H 2 SO 4 (Baker, reagent grade) were used.Granular AC type CAGR 8 × 30 (Clarimex), certified by the National Science Foundation and previously characterized was used. 39The AC is of lignite origin, is activated by vapor steam, has an average pore diameter of 3.5 nm and has a surface area of 664 m 2 /g.The determinations using the inductively coupled plasma (ICP-OES) spectrometry (Perkin Elmer, Optima 3300 DV) gave the following data: Al: 256 mg/kg, Si: 143 mg/kg, Na: 1085 mg/kg, K: 300 mg/kg, Ca: 872 mg/kg, Sr: 756 mg/kg and V: 14 mg/kg.
OMD synthesis was performed via the oxidation by ozonation of Mn(II) in aqueous solution.A solution of 1 g/L of MnSO 4 and 1 mol/L H 2 SO 4 was prepared with deionized water and was deposited in a glass reactor with 2 g of AC.The system was stirred to 600 rpm for 2 h, and the temperature was controlled at 25 °C.A gaseous mixture of O 2 /O 3 , produced in an ozone generator (PCI Ozone & Control Systems, mod.GL-1) supplied with oxygen was introduced into the reactor.Samples were taken at 0, 30, 60, 90 and 120 min of reaction.The samples were filtered and the solid was washed with 100 mL of deionized water.These samples were identified as OMD/AC.Finally, the samples were stored in a desiccator.Liquid samples were analyzed by ICP-OES.
The base conditions for the synthesis were: temperature, 25 °C; stirring, 600 rpm; concentration of AC, 2 g/L; [H 2 SO 4 ], 1 mol/L; [O 3 ] in solution, 1.32 mol/L; and particle size of AC, +0.59 mm.Each parameter was independently varied in order to find out the best adsorption capa-Contreras-Bustos et al.: Synthesis of MnO 2 on Activated Carbon ... city of the material for Pb(II) and As(V) ions.The temperature ranged from 25 to 70 °C; the concentration of AC from 0.5 to 5 g/L; the ozone concentration in solution from 0.35 to 1.65 mol/L; the concentration of H 2 SO 4 from 1 to 3 mol/L; the stirring speed from 200 to 800 rpm; and the particle size from AC of +0.59 to +1.6 mm.
The resulting solid material was characterized using scanning electron microscopy (SEM) and microanalysis by Energy Dispersive Spectroscopy (EDS) (Jeol, mod.JSM35CFLV) to corroborate the presence of manganese deposits on the surface of the AC and to identify other elements in the samples.

Adsorption of Lead and Arsenic
The solutions used for the experiment were one of lead and one of arsenic, each with a concentration of 1 g of the element per liter.They were prepared with Pb(NO 3 ) 2 (Baker, reactive grade) and Na 2 HAsO 4 • 7H 2 O (Mallinckrodt Chemical Works, reagent grade) in deionized water.These solutions were stored in polyethylene containers.The stock solutions were diluted up to a concentration of 100 mg/L for the adsorption experiments.0.1 g of OMD/AC prepared in specific conditions was put in contact with 100 mL of these solutions.For lead, the adsorption experiments were carried out at pH = 4.7 to avoid the precipitation of lead as hydroxide.According to the chemical species diagram at pH = 4.7 lead is present only as Pb(II). 40Arsenic experiments were done at pH 8.8, when As(V) can be found as the anionic species 30% (H 2 AsO 4 ) -. Contact was done in an isothermal bath with stirring at 300 rpm at 25 °C for 24 h.
After the contact the solutions were filtered and a nitric acid solution added to maintain the pH < 2, the samples were stored in polyethylene bottles.Lead was then present as Pb(II) and As(V) was ca.70% as H 3 AsO 4 and ca.30% H 2 AsO 2 4-all of which are water soluble. 40These samples were analyzed for Pb(II) or As(V) using ICP-OES.The OMD/AC was flushed with 100 mL of deionized water, dried at temperatures up to 35 °C for 12 h and stored in a desiccator with silica gel.
To determine the influence of each individual condition on the synthesis of OMD/AC, the adsorption capacity of the material for Pb(II) and As(V) was tested using the material obtained under different stirring speed, ozone concentration, acidity, AC concentration, temperatures, and AC particle sizes.
The equation used for the calculation of the adsorption capacity was: (1)   where q is the adsorption capacity (mg ion/g adsorbent), m is the mass of the adsorbent (g), C i is the initial concentration of ions in solution (mg/L), C f is the final concentration of ions in solution (mg/L) and V is the volume of the solution (L). 41The adsorption capacity of AC for Pb(II) and As(V) was measured as well.

1. 1. Stirring Speed
The values of Mn(II) when remaining in solution for the same amount of time are very similar (Table 1); this means that the parameter does not significantly alter the oxidation rate reaction.It can be deduced that the masstransfer phenomenon in that range of stirring speed is not important.A slight increase of Mn(II) deposition was observed, however, at a stirring speed of 800 rpm at 120 minutes of reaction.Mn(II) probably spreads in the AC by means of electrostatic attraction with the electrically charged sites of the surface, and then, when ions are already deposited, oxidation occurs to form OMD.

1. 2. Ozone Concentration
Table 1 shows the influence of the concentration of ozone on the oxidation rate reaction of Mn(II) in the presence of AC.The oxidation rate increases in relation to the concentration of ozone in solution.The best conditions for attaining the lowest concentration of Mn(II) in solution are 1.35 mol/L and 120 min of reaction.
With t > 90 min of ozonation and a concentration of O 3 of 1.65 mol/L, a re-dissolution of manganese ions was observed due to the oxidation of the ions Mn(IV) up to Mn(VII), revealed by the characteristic violet color.In accordance with the Pourbaix diagram at the experimental pH, Mn(II) can be oxidized to MnO 2 and subsequently to MnO 4 -. 42The calculation of the order of reaction was performed with the following equation: (2)   where r A expresses the variation of the concentration of a species (Mn(II)) with respect to time, C A is the concentration of ozone, k is the rate constant and n) is the order of reaction.
The graphics (not included) corresponding to 30, 60 and 90 min of stirring and ozone concentrations of 0.35 to 1.35 mol/L have slopes ≈ 0.5; this is considered a zero-order reaction.The value of log(-r A ) tended to be higher, indicating a change in slope, only in the case of 90 min and 1.35 mol/L ozone.For the lowest concentrations of ozone in solution the mass-transfer phenomena have more influence in the process than the chemical reaction itself during oxidation of Mn(II).However, as the concentration of dissociated ozone increases, the chemical reaction becomes more important. 43

1. Sulfuric-acid Concentration
The oxidation rate reaction of Mn(II) decreases with the increase in sulfuric-acid concentration (Table 2).For the range of acidity used, the best condition was 1 mol/L H 2 SO 4 .Moreover, the start-up time for reaction increases when the acid concentration increases.The low rate of oxidation with high concentrations of H 2 SO 4 may be due to a competition between protons and Mn(II) ions for the active sites of AC.Another explanation is that the low rate of oxidation is due to the high viscosity observed with a high concentration of acid.

1. 4. AC Concentration
As can be seen in Table 2, the values corresponding to the change of Mn(II) for the oxidation reaction without AC have tendency very similar to that of the tests carried out with the presence of AC.Rodríguez-Santillan et al. mention that the AC favors the decomposition of O 3 in solution even at low pH; 44 however, in the conditions of the present work, this was not observed.
According to the calculations done in accordance with Equation 2, the oxidation reaction is of zero-order with respect to this variable.This means that, in the interval of the AC used, this parameter does not have a significant influence on the rate of oxidation of Mn(II) by ozone.

1. 5. AC Particle Size
The rate reaction is quite similar for the three particle sizes, namely 0.59, 1.19 and 1.6 mm (Table 3).Thus, it can be deduced that this variable does not influence the process of oxidation of Mn(II).

1. 6. Temperature
Increasing temperature to 70 °C and reaction times to greater than 60 min favors oxidation; for lower temperatures, there was no noticeable difference (Table 3).Similar observations have been reported previously. 35he k values for temperatures of 25, 40, 60, and 70 °C were calculated considering the slopes of the graphs of Figure 1A.These values were used to determine the activation energy by the Arrhenius linear equation: (3)   where k is the rate constant, A is the factor of frequency, E a is the activation energy (kcal/mol), R is the universal constant of the gases, and T is temperature (K).
Figure 1B shows a graph in which the activation energy and the factor of frequency are the slope and the intercept of the line, respectively.These values are E a = 1.2 kcal/mol and A = 2.2.According to Peters and Bolton, E a values under 4.7 kcal/mol and occasionally under 2.3 kcal/mol correspond to mechanisms controlled by mass-transfer in an aqueous solution; 45 therefore, deposition of MnO 2 on AC can be considered as such.Thus, temperature does not influence the process of MnO 2 precipitation.

2. Characterization by SEM
Figure 2 shows typical images obtained by SEM.In the micrograph taken to the scale of 150× and 100 μm (a), fine particles were observed on the AC.Yet, in the micrograph taken to the scale of 2000× and 10 μm (b), agglomerates of OMD were deposited on the surface of the AC.The EDS (c) revealed the presence of manganese and silicon, which were located on the AC surface.
When low ozone concentrations were used for synthesis, the images showed less deposits of manganese on the AC than on samples synthesized with high concen-trations of ozone; thus, the increase of ozone can be said to favors precipitation.

Adsorption of Pb(II) in OMD/AC
Table 4 shows the influence of different conditions on the synthesis of OMD/AC in terms of the composite's adsorption capacity for lead.This adsorption capacity increases with the stirring speed, probably due stirring speed's creation of more homogeneous deposits of OMD.However, the OMD/AC formed at 800 rpm does not present the best features for removing Pb(II), because the dissemination processes do not favor the solid-liquid interface or because the contact time between the particles of OMD and AC was reduced.Moreover, this condition of synthesis could also create fractures of AC particles; hence, the OMD deposited on them may become detached.
The adsorption capacity of lead in the OMD/AC increased when the concentration of ozone used in the synthesis of the material was changed from 0.35 to 1.32 mol/L (Table 4); however, with higher concentrations, this adsorption capacity decreased.This can be explained by a partial re-dissolution of manganese as MnO 4 -during synthesis, and resulting decrease of manganese as an oxide on the surface of the AC, which is the chemical species that allows for efficient adsorption.
Table 4 also shows the influence of sulfuric-acid concentration.When the concentration of the acid reaches 2 M, the formation of γ-MnO 2 is promoted, thereby increasing adsorption capacity.A higher concentration of sulfuric acid (3 mol/L) favors the formation of Mn(III), an intermediate species between Mn(II) and MnO 2 . 35In addition, in this case, the decrease of manganese as an oxide on the surface of the AC reduces the lead adsorption capacity of the material.The adsorption capacity of Pb(II) by OMD/AC decreases when the concentration of AC used in the synthesis of the material increases (Table 4), for the amount of OMD present in the adsorbent material is proportionally smaller.
The best adsorption capacity with respect to the temperature of the formation of the OMD/AC was 82.7 mg Pb(II)/g in the material prepared at 40 °C.The adsorption capacity of the material is reduced at higher temperatures during synthesis.Umetsu et al. reported a similar behavior; they observed that OMD does not present adsorption for a synthesis at 70 °C. 46Although there are significant deposits of OMD in the synthesized material at that temperature, the results indicate that the main factor responsible for the removal of Pb(II) is AC.
Regarding particle size, the best adsorption capacity (85.8 mg Pb(II)/g OMD/AC) was obtained using the synthesized material with the largest AC particle, because it presents more deposits of OMD on the surface.In the intermediate-sized synthesized material with AC, the removal of Pb(II) was reduced because of small amount OMD deposits, which was observed in the micrographs for particles sizes of 0.59 and 1.6 mm.Finally, when using the smallest AC particles, the Pb(II) removal was also high (71.7 mg Pb(II)/g OMD/AC); the presence of few OMD deposits, the adsorption capacity increased if the size of the AC was reduced.
The conditions of synthesis of OMD/AC with the maximum adsorption capacity of Pb(II) are: 2 mol/L of sulfuric acid, temperature at 25 °C, stirring at 600 rpm, AC par- The adsorption-capacity value in these conditions is 90.5 mg Pb(II)/g OMD/AC; for AC, however, it is 14 mg Pb(II)/g AC.Therefore, the fixing of MnO 2 on the surface of the AC greatly improves the removal of ions Pb(II) of aqueous solutions.

4. Adsorption of As(V) in OMD/AC
The results of the experiments carried out to determine the adsorption capacity of As(V) on the material in each of the conditions of synthesis was calculated in the same way as described for lead, following Equation 1 and determining the influence of each individual parameter.The values obtained are shown in Table 4.
At the experimental pH (8.8), As(V) can be found as an anionic species H 2 AsO 4 2-. 40][49] Table 4 shows that the adsorption capacity was better with a slow stirring speed during synthesis (200 and 400 rpm); the highest value obtained was for 400 rpm (18.5 mg As(V)/g OMD/AC).The increased stirring speed resulted in a drastic decrease in the adsorption capacity because of fewer deposits of manganese oxide, which is responsible for adsorption.
The ozone concentration used for the synthesis of OMD/AC does not have a significant influence on the adsorption capacity for arsenic ions.The maximum adsorption capacity is less than 6 mg As(V)/g OMD/AC for a concentration of O 3 = 1.65 mol/L.This behavior is different from that observed with the adsorption of Pb(II) for the same parameter because the mechanism of ion exchange of the OMD is different for cations and anions.
The adsorption capacity of As(V) in OMD/AC depends on H 2 SO 4 concentration; it increased when that concentration increased from 1 to 3 M.The best adsorption capacity was 19.5 mg As(V)/g OMD/AC for 3 mol/L of H 2 SO 4 .The increase of the anion exchange of the OMD is given by the presence of sulfate ions on the surface, which allowed for a greater exchange of anions.However, the variation of the adsorption capacity between 2 and 3 mol/L did not produce noticeable changes.The amount of OMD deposited on the AC decreased when the concentration of the acid increased; therefore, the adsorption of As(V) also decreased.
An adsorption capacity of 25.1 mg As(V)/g OMD/AC was obtained when the concentration of AC was 1 g/L; with higher concentrations, smaller values were obtained.This behavior is similar to that observed for Pb(II), and similarly related to the amount of OMD pre-Table 4. Adsorption capacities of OMD/AC for Pb(II) and As(V) with respect to different synthesis parameters for OMD/AC.Initial concentrations = 100 mg/L.Without OMD, q 0 = 14.0 mg Pb(II)/g AC and q 0 = 9.1 mg As(V)/g AC.

Synthesis parameters
q (mg Pb(II)/g q(mg As(V)/g for OMD With respect to the temperature of the synthesis of OMD/AC, the tendency is the same for the As(V) as for the Pb(II).At low synthesis temperatures the adsorption capacity was higher showing the best results at 40 °C, although it remained virtually constant up to 60 °C, with 19.0 and 18.5 mg As(V)/g OMD/AC, respectively.At 70 °C a significant decrease in the adsorption capacity was observed.The reduction of the ion-exchange capacity of OMD/AC meant that the capacity for retention of the material decreased when more deposits of manganese were present in the adsorbent material.
The adsorption of As(V) with respect to particle size displayed the opposite behavior as the adsorption of Pb(II).In this case, with the smallest particle size, the adsorption capacity was the lowest.The sample used for this experiment probably had few OMD deposits, so the main adsorbent was AC.For other particle size, OMD deposits were confirmed; however, no high adsorption capacity was observed.The highest adsorption capacity was 19.7 mg As(V)/g, for a particle size of AC = +1.19mm.The removal of As(V) does not present a clear tendency with regard to the particle size used for the synthesis of OMD/AC in the present work.
The best adsorption capacity obtained was 25.4 mg As(V)/g OMD/AC with a material prepared as follows: AC concentration 1 g/L, temperature at 25 °C, stirring speed at 600 rpm, AC particle size at +0.59 mm, [O 3 ] 1.35 mol/L, and [H 2 SO 4 ] 1 mol/L.The adsorption capacity of As(V) in AC is 9.1 mg As(V) /g AC; therefore, it can be seen that the presence of OMD greatly improves the absorption of As(V).
The mechanism of removing the As(V) in anionic form from water by the OMD/AC is complicated.The first stage is probably the formation of complexes on the surface of the AC, which is followed by ion exchange.The adsorption of anions has been related to reactions of complex formations on protonated sites or the presence of electrophilic sites.The As(V) is quite basic, suggesting that its adsorption in AC has a close relationship with the concentration of functional strong acid groups.

5. Comparison of the Adsorption Data for Pb(II) and As(V)
One published review focused on adsorbents for the removal of arsenic, cadmium, and lead from contaminated waters. 50Table 5 shows a comparison of the present work's data and some data from the literature for the same kind of adsorbent material.The adsorption capacity of OMD/CA is similar to that of Al 2 O 3 -pillared layered MnO 2 and Alpillared montmorillonite (A, B) and better than that of other materials.Regarding the adsorption capacity of OMD/CA for As(V), the result obtained is not too low; though there are other materials with higher values of q.

Conclusions
The conditions for the synthesis of a material with MnO 2 (OMD) on activated carbon (AC) were studied.It can be deduced that mass-transfer phenomena in the range of the stirring speed (200-800 rpm) are not important.The oxidation rate increases according to the concentration of ozone in solution and the best conditions for attaining the lowest concentration of Mn(II) in solution which results in more deposits of manganese dioxide are 1.35 mol/L and 120 min of reaction.The rate of reaction of Mn(II) oxidation decreases with increased in sulfuric-acid concentration.Neither the concentration nor the particle size of AC influences the process of oxidation of Mn(II).Increasing temperature to 70 °C and a reaction time to greater than 60 min favors oxidation; for lower temperatures, oxidation is not significantly different.
For all experimental conditions agglomerates on AC were observed by means of SEM and EDS and the presence of manganese dioxide deposited on the surface of the AC was identified.The activation energy and the factor of frequency for the reaction were determined as E a = 1.2 kcal/mol and A = 2.2, respectively.The value of E a indicates that the precipitation of manganese dioxide in the AC is controlled by mass transfer in aqueous solution.The order of this reaction is zero.
The best conditions for the synthesis of OMD/AC to ensure maximum adsorption capacity for Pb(II) and

Table 1 .
Manganese concentration (mg/L, in italics) remaining in solution, with respect to stirring speed (rpm), ozone concentration (mol/L) and reaction time (min).

Table 2 .
Manganese concentration (mg/L, in italics) remaining in solution, with respect to initial sulfuric-acid concentration (mol/L), quantity of activated carbon (g) and reaction time (min).

Table 3 .
Manganese concentration (mg/L, in italics) remaining in solution, with respect to the particle size of activated carbon (mm), temperature (°C) and reaction time (min).