Survey of the magnetic field effect on arsenic removal from drinking water with and without iron filings

Introduction
Arsenic is a toxic metalloid and exists in nature in the two organic and mineral forms. Arsenate is the oxidized form and is predominant in the surface waters, while arsenite is a reduced form and is often found in the groundwater. Besides, toxicity and solubility of arsenite is more than that of arsenate. Excessive and prolonged human intake of inorganic arsenic, through drinking water and food, causes arsenicosis, which includes skin disorders, skin cancer, internal organ cancer, arm and leg vascular diseases and diabetes. The World Health Organization (WHO) guideline value for arsenic in drinking water is set as 10 µg/l. It should be noted that this standard is 50 µg/l in Asia and also in Iran. There are three main methods for arsenic removal from drinking water, including membrane filtration, coagulation-precipitation and adsorption. Water treatment by means of the magnetic field has been recently considered. Ma Wei et al. investigated the arsenic removal via sulfide ions in the magnetic field. Stephen and Coey carried out a study on water treatment using the magnetic field. Lipus et al. surveyed the influence of magnetic field on the aragonite precipitation. Also, Ambashta et al. investigated water purification using magnetic assistance. Besides, Ochkov et al. conducted a study on magnetic treatment of water. Gholizadeh et al. investigated the effect of magnetic field on scale prevention in the industrial boilers. Amiri et al. carried out a study on the reduction of the surface tension of water due to magnetic treatment. In addition, Banejad et al. conducted an experiment on the effect of magnetic field on water hardness reducing. In this study, the efficiency of iron filings in arsenite removal from polluted water was primarily surveyed, and then the effect of magnetic field on the process was investigated. The innovation of this research will be to understand the importance of physical forces such as magnetic field in the field of water purification and its application in the water industry.

Materials and Methods
Preparation of Adsorbent
The required iron filings were prepared in the turning workshops of Tehran, Iran. The iron filings were passed through a sieve with pores of 2 mm and were made wet using deionized water, and ferric hydroxide precipitation was allowed to be formed on the surface of the filings.

Making the Magnetic Column
A circular magnet with the magnetic field intensity of 0.01 T was primarily placed around a glass column and a iron spiral was located in front of the magnet inside the column, so that the sediments on it could be reacted. (Discharge velocity was 2 mm/s).

Preparation of the Samples
The synthetically arsenic-polluted water samples were prepared by diluting 0.1N sodium arsenite solution (Merck) with de-ionized water. In addition, to prevent the oxidation of arsenite to arsenate the required solutions were prepared on a daily basis at 0.5 and 2 mg/l concentrations.

The Tests
The prepared arsenite solutions at 0.5 and 2 mg/l concentrations were reacted in contact with the iron filings adsorbent at 0, 2.5 and 5 g/l dosages over contact times of 5, 10 and 15 minutes within a beaker with the volume of 100 mL on the shaker at the velocity of 400 rpm. Half of the sample (50 ml) was then passed through the column at the velocity of 2 mm/s and a blank (without iron filings) was considered for each sample. Then, the arsenic concentration of the samples was measured by the ICP system. Moreover, the electrical conductivity of the samples was measured using EC meter. After data collection the arsenic removal efficiency in any state was calculated. Mean and standard deviation of removal efficiencies were determined and the variables were compared with each other by means of paired t-test and one-way ANOVA.

Discussion of results and conclusions
Mixing
The results indicated that with the increase of the initial arsenic concentration, the removal efficiency also increased that was due to the oxidation of arsenite into the insoluble arsenate ion. Via analysis of iron filings by extraction with hydrochloric acid, Hsing et al. showed that almost 28% of arsenic has existed in the form of arsenate, which revealed that oxidation has also been effective in arsenic removal.

Initial Arsenic Concentration
The results demonstrated that with the increase of the initial arsenic concentration, the arsenic removal efficiency decreased. Yu Zhang achieved the adsorption capacity of 16 mg/g at the arsenic concentration of 1 mg/l, while Hsing reported the adsorption capacity of 7.5 at the arsenic concentration of 50 mg/l.

Iron Filings Dosage
Based on the obtained results, with the increase of the iron filings dosage, the arsenic removal efficiency increased as well. Tyrovola et al. showed that with the increase of the iron filings dose, the removal efficiency of arsenite ion increases.

Contact Time
The results showed with the increase of the contact time desorption can occur at various times. In the samples with high arsenic concentration and iron filings, due to ferric hydroxide sites on the iron filings and therefore higher adsorption, the desorption process occurred at longer contact times.

pH
The iron filings, unlike the other adsorbents, have a high affinity to the reaction with arsenic at the normal pH of water. Ramaswami et al. removed the arsenite by iron fillings at the pH of 7 with the efficiency of 95%.

The Magnetic Field
The results showed that the magnetic field reduced the arsenic level of the samples without iron fillings but increased the arsenic level of the samples with iron fillings. The ferric hydroxide ion was formed on the surface of the iron filings.
Sodium arsenite (NaAsO2) reacts with ferric hydroxide (Fe(OH)3) and forms ferric arsenite (Fe(AsO2)3) on the surface of the iron filings. Also, ferrous hydroxide ion is formed within the solution and can react with sodium arsenite and ferrous arsenite (Fe(AsO2)2) can be thus formed. Based on the physicochemical Hall Effect, when a multi-atomic ion placed within a fluid passes through the external magnetic field, the bond between the ions is weakened and they are dissociated and form cations and anions. When charged particles are placed in a magnetic field a force is applied by the magnetic field to the particle, which is called “Lorentz force”. After the blank or control sample (sodium arsenite) passed through the magnetic field, these two ions were dissociated based on the Hall Effect and finally reacted with the ferric hydroxide formed on the metal spiral. When the ferric arsenite ion passed through the magnetic field, the ions were dissociated and were affected by Lorentz force. Arsenite has one negative charge and ferric has three positive charges and since ferric ion has higher charge, more force is applied to it and it attaches to the spring inside the column. Arsenite was also affected by Lorentz force and reacted with the ferric formed on the spring, but as the dissociation level of ferric arsenite ion was more than its adsorption, the arsenite level in the outlet column increased. Ferric arsenite is insoluble and was not measured by the device. When the solutions were passed through the column, arsenite separated from ferric and changed into a solution which could be measured.

Electrical Conductivity (EC)
The results of this study demonstrated that the magnetic field increased EC. When the ions of a solution are exposed to the magnetic field, they are dissociated and the solution forms more ions and thus EC increases. Ma Wei showed that EC of the samples before and after the magnetic field were 0.22 and 0.27 S/m respectively.

Based on the results obtained from this study, it can be concluded that by applying a stronger magnetic field around the magnetic column, the arsenic in drinking water can be removed with high efficiency without adding any chemicals or adsorbents.


Keywords: Arsenite, Iron filings, Magnetic field, Magnetic column