Coagulation-Flocculation Technologies for Arsenic removal -A Review

 

Jeevan Jyoti Mohindru^{1, 2*}, Umesh Kumar Garg^{1, 3}, Rajni Gupta⁴

¹KG Punjab Technical University, Kapurthala, Punjab, India

²Department of Chemistry, DAV College, Amritsar, Punjab, India

³Guru Teg Bahadur Khalsa College of IT, Malout, Punjab, India

⁴SD College, Barnala, Punjab, India

*Corresponding Author E-mail: jjmdav@rediffmail.com

This review article presents in brief an overview of various technologies available based on coagulation-flocculation methods. Coagulation and flocculation are among the most employed and documented techniques for arsenic removal from water.  Arsenic is a human carcinogen in drinking water, having harmful effects on both human health and environment, even at low concentration. A limit of 0.01ppm (10µg/L) has been set by WHO as the safe limit in drinking water, above which it shows health hazards. In coagulation method the, positively charged ion of coagulants {e.g., aluminum sulphate (Al₂(SO₄)₃), ferric chloride (FeCl₃)} neutralize or reduce the negative charge of colloids, thereby causing coagulation of particles.  On the other hand in Flocculation the addition of an anionic flocculants leads to charge neutralization between the formed larger particles which ultimately change in to big flocs. As a result the dissolved arsenic is transformed into an insoluble solid. The precipitation or co precipitation with certain metal hydroxides can be done to remove arsenic from drinking water through sedimentation or filtration.  The pH of the medium plays an important role in deciding the suitability of a coagulant for the removal of Arsenic. Below pH 7.5, Al₂(SO4)₃, and FeCl₃ are equally effective in removing arsenic from water Between the two inorganic arsenic species, most researchers suggested that arsenate is more efficiently removed compared to arsenite and that FeCl₃ is a better coagulant than Al₂(SO₄)₃ at pH higher than 7.5.There is no effective treatment for arsenic toxicity. Removal of arsenic from water is the one and only method of preventing its toxicity. A great deal of research over recent decades has been done to lower the concentration of arsenic in drinking water and still there is a need to develop low cost viable techniques. Existing major arsenic removal technologies include oxidation, adsorption, precipitation, coagulation and membrane separation. This paper presents the review of current status of research in the area of arsenic removal from contaminated water with coagulation and flocculation methods.

 

 

 

INTRODUCTION:

Arsenic is a widely distributed element in nature¹ to which human beings are exposed mainly via oral intake of water and food.  It is a brittle crystalline solid {atomic weight 74.9; specific gravity 5.73, melting point 817◦C (at 28 atm), boiling point 613◦C} which exist in nature in the forms of arsenate ion (As (V)) and arsenite ion (As (III)). Arsenic in dispersed in water bodies and agricultural fields due to rain, wind and other natural effects, thereby leading to a high arsenic concentration in water. The common oxidation sates of Arsenic are −3, 0, +3 and +5 mainly in the form of arsenious acids, arsenic acids, arsenites, arsenates, methyl arsenic acid, dimethyl arsinic acid, arsine, etc. Arsenic (III) is a hard acid that complexes with oxides and nitrogen while Arsenic (V) is a soft acid that forms complexes with sulfides. Ground water mainly contains inorganic forms of Arsenic. Occurrence of arsenic in groundwater above the permissible value (>10 µ g/L WHO) is one of the wide spread problem owing to its toxicity and carcinogenicity. Exposure to elevated levels of Arsenic mainly due to drinking water is major health hazard in many countries around the world. Arsenic has been classified as Group A human carcinogen by International agencies like “The International Agency for Research on Cancer” and “United States Environmental Protection Agency” (USEPA) . A large no of  reports in literature, based on past and ongoing experience in various countries concerning the high risk of skin, bladder, lung, liver, and kidney cancer along with other noncancerous ailments that result from continued consumption of elevated levels of arsenic in drinking water.

 

 

Table 1 - Safe limits of Heavy metals in the water as prescribed by various agencies

Heavy metal	Safe Limit  (WHO standards)	Safe Limit  (Indian standards)	Safe Limit  (European standards)
Arsenic(As)	0.01 mg/l (ppm)	0.05 mg/l (ppm)	0.01 mg/l (ppm)
Cadmium(Cd)	0.003 mg/l (ppm)	0.01 mg/l (ppm)	0.005 mg/l (ppm)
Chromium(Cr)	0.05 mg/l (ppm)	0.05 mg/l (ppm)	0.05 mg/l (ppm)
Copper(Cu)	1.5 mg/l (ppm)	1.5 mg/l (ppm)	2.0 mg/l (ppm)
Iron(Fe)	---	1.0 mg/l (ppm)	0.2mg/l (ppm)
Nickel(Ni)	0.02 mg/l (ppm)	3.0 mg/l (ppm)	0.02 mg/l (ppm)
Manganese(Mn)	0.5 mg/l (ppm)	0.5 mg/l (ppm)	0.05 mg/l (ppm)
Lead(Pb)	0.01 mg/l (ppm)	0.1 mg/l (ppm)	0.01 mg/l (ppm)
Zinc(Zn)	3.0 mg/l (ppm)	10.0 mg/l (ppm)	----

 

 

There as a large number of technologies which are being used for arsenic removal from ground water based on various principles namely, oxidation²; chemical coagulation³ /Flocculation precipitation⁴; electro coagulation⁵; adsorption⁶, reverse osmosis⁷ ]; membrane filtration/ nano-filtration⁸ ; and ion exchange⁹. None of these technologies has been universally accepted both in terms of cost effectiveness as well as efficiency in removal of Arsenic. Coagulation/Flocculation with various chemical salts is usually associated with problems concerning disposal of the resulting waste sludge. Adsorption process remains ineffective in low concentration moreover it requires pre oxidation of As(III) to As(V) for effective removal and a frequent regeneration of adsorbent and disposal of spent adsorbent are the other disadvantages with adsorption process¹¹. Disposal of arsenic containing sludge produced in electro coagulation process generates a new problem. It requires air injection, high voltage (40 V) and high current (4A) for effective performance¹². Reverse osmosis, membrane filtration and ion exchange processes have high arsenic removal efficiency that can lower down arsenic concentration in treated water as low as 2-5 μg/L ¹³. In ion exchange process an electric potential in place of chemical reagents is used to elute ion exchange media ¹⁴. Arsenic removal technologies can be broadly classified in to four types viz Oxidation process, coagulation flocculation, Adsorption/ion exchange and membrane technologies.

 

Fig.1  Arsenic removal technologies

 

All these technologies have some advantages as well as disadvantages which make these technologies suitable based on their cost effectiveness, easy operation, greater efficiency, suitability in a particular region and reliability¹⁵.

 

Table 2-  Pros and Cons of various Arsenic removal technologies

 

Although all the above technologies have been utilized by different researchers but it is felt that coagulation flocculation is the most cost effective and easy to use method for arsenic removal¹⁶.

 

Arsenic Removal Technology	Removal Efficiency For  As(III)           For As(V)
Coagulation3,5	20-70%	70-100%
Ion Exchange7,8,9	20-40%	80-100%
Adsorption7,8,9	40-70%	80-100%
Reverse Osmosis10,11	70-80%	80-100%
Electro dialysis10,11	70-80%	80-100%

 

Arsenic Toxicity and Related Health Hazards:

Arsenic contamination of natural waters bodies has become a worldwide problem that affects over 40 million people in the World. Reported initially in Bangladesh¹⁶, where groundwater arsenic concentrations surpass 3.4 mg/L In Taiwan, artesian aquifers display concentrations above 1.8 mg/L. In Portugal, water sources that exhibit higher concentrations of arsenic (approximately 800 ppb for groundwater and 60 ppb for surface waters) are generally located in Trás-os-Montes and Alto Douro ¹⁷, where the presence of arsenic-rich quartz-sulphur minerals is very common. Minho, Beiras, Ribatejo and Alentejo are additional locations where the legal contaminant concentration (10 ppb) is now exceeded. All these observations allow the evaluation of the real exposure impacts on public health. A variety of ores of copper, gold, nickel, lead and zinc may act as anthropogenic sources of arsenic, other sources may include wool and cotton processing, insecticides and herbicides, additives to various metal alloys and also from the glass and semi-conductor industry. In general, groundwater exhibit higher concentrations of arsenic as compares to surface waters. However a polluted surface water by industrial or mining effluents¹⁸, or by geothermal waste, has high concentration of arsenic. Acute and sub acute poisoning results from ingestion of large quantities of arsenic with lower exposure time, whereas chronic poisoning occurs due to consumption of arsenic contaminated water for a long time period Most cases of acute arsenic poisoning occur from accidental ingestion of insecticides or pesticides, being urinary arsenic concentration the best indicator of recent ingestion (1–2 days). The lethal dose¹⁹of arsenic in acute poisoning ranges from 100 mg to 300 mg. Nonspecific gastrointestinal effects such as diarrhoea and cramping, haematological abnormalities including anaemia and leukaemia, peripheral neuropathy (similar to Guillain-Barré syndrome), renal failure, respiratory failure and pulmonary oedema are common features of acute poisoning, which may lead to shock, coma, and even death. Depending on the quantity consumed, death usually occurs within 24 hours to four days ²⁰. Metabolic changes (such as acidosis, hypoglycaemia and hypocalcaemia) with acute arsenic poisoning are also reported. Long term arsenic toxicity leads to multisystem disease and the most serious consequence is malignancy. The clinical features of arsenic toxicity vary between individuals, population groups, and geographic areas. It is unclear what factors determine the occurrence of a particular clinical manifestation or which body system is targeted. Thus in persons exposed to chronic arsenic poisoning, a wide range of clinical features are common. The onset is insidious with non-specific symptoms of abdominal pain, diarrhoea, and sore throat.

 

The skin is quite sensitive to arsenic and dermatological changes are a common feature related to long term exposure. Skin lesions (hyperkeratosis and depigmentation) have been observed even in cases of exposure to levels in the range of 5–10 ppb ²¹. Arsenic associated skin cancer, Bowen‘s disease, is an uncommon manifestation in Asians and may be due to the high skin melanin content and increased exposure to ultraviolet radiation. Arsenic may cause a basal cell carcinoma in a non-melanin pigmented skin. The latent period after exposure may be as long as 60 years. The conclusions of several epidemiological studies confirmed the potentially carcinogenic effect (skin, lung, bladder, kidney, liver, uterus and gallbladder) of a few inorganic species of arsenic when present in high concentrations.

 

This led the WHO to recommend in 1993 a more restrictive guidance value of 10 ppb as the quality standard for drinking water, a value that is five times lower that the previous recommended limit. This drastic reduction in the maximum arsenic limit issued by the WHO has an essentially preventive character, since that parametric value is not yet sufficiently supported by extensive and conclusive epidemiological studies that are urgently warranted. Later on, in 2003, the United States Environmental Protection Agency (USEPA) drafted a proposal issuing final guidelines for cancer risk assessment²². The revised document advocates the use of nonlinear relationship between arsenic carcinogenesis and its dose in drinking water. Since the removal of arsenic from raw water is often the only viable option in order to obtain safe drinking water, it is pertinent to globally intensify applied research efforts. These should address both the quantification of arsenic effects in health (toxicity levels) and the development of innovative technologies for arsenic removal that can be more efficient and sustainable, especially for small water supply systems in rural areas.

Arsenic Removal Through Coagulation/Flocculation Process:

Precipitation and coagulation methods for arsenic removal from water depend upon the co-precipitation of both water insoluble arsenates and inorganic oxides of other metals. In Arsenic contaminated water there is formation of water insoluble inorganic oxides by the hydrolysis of added coagulants such as alum (aluminum sulfate), ferric chloride or ferric sulfate. The coagulant must be uniformly mixed into the arsenic contaminated water in order to obtain maximum arsenic removal efficiency. The resulting gelatinous precipitate occludes water insoluble arsenic compounds such as arsenates into the structure. In addition, water soluble arsenic compounds such as arsenites can also be electrostaticaly bound to the external surface of the gelatinous precipitate.

 

The amount of coagulant used can be significantly reduced by the addition of polymers or colloidal clays during the mixing of the coagulant with the arsenic contaminated water²³. This can substantially reduce the operating cost of the arsenic removal system. Many aquifers where arsenic contamination is present also contain phosphates or silicates in the water. The presence of phosphates or silicates in the contaminated water reduces the efficiency of arsenic removal ²⁴ and this also must be taken into consideration when precipitation and coagulation is the chosen arsenic removal method. Gravitational means are usually employed to initially separate the insoluble gelatinous precipitate from the treated water. Subsequent to that, filtration is used to separate any small particles of precipitate not removed by gravitational means in order to maximize arsenic removal efficiency.

 

It is very important that the fluid velocity through the filter be low so that the smallest possible particles of precipitated arsenic are removed from the aqueous phase. In addition, it is very important for high arsenic removal that the gelatinous precipitate formed is not broken up into smaller particles by high velocities and turbulent flow areas that might be encountered in the system during the coagulant mixing, co-precipitation, and gravitational separation or filtration steps. Sand/anthracite filters have been found to be effective in removing traces of arsenic from groundwater when utilized as part of a precipitation based arsenic removal system that has an efficient gravity separator prior to the filter. Such a system has successfully removed arsenic from contaminated groundwater at an arsenic chemicals manufacturing facility to below 25 micrograms/liter when operated at low fluid velocity and with frequent filter backwash to prevent channeling.

 

Fig.2  Coagulation Filteration Procedure

 

 

Ferric Chloride (FeCl₃) as Coagulant:

Removal of both arsenite and arsenate present at different initial concentrations were evaluated for different doses of ferric chloride²⁵. After addition of a particular dose of a coagulant (ferric chloride), the water in the bucket was mixed with a wooden stick, first vigorously for about 30 to 60 seconds and then slowly (approximately one turn of the wooden stick per second) for about 90 seconds. Wooden stick, instead of a mechanical device, was used in order to mimic field condition in rural Bangladesh. The effect of mixing on floc formation and presence of residual alum/iron was evaluated by varying the duration of slow mixing, and the mixing procedure adopted was found to provide good results in terms of floc formation. After mixing, the flocs were allowed to settle for periods ranging from 30 minutes to 24 hours. Water samples were then collected with a pipette from a depth approximately 10 cm from the bottom of the bucket. The water samples were then tested for total arsenic. The removal efficiencies of As(V) and As(III) have been studied by many workers, which indicates that removal efficiency of As(V) was pH dependent, whereas As(III) removal efficiency was independent of pH. With increase in pH, the removal efficiency of As(V) decreased. The percentage removals of As(V) with 1 mg/L Fe(III) dosage at pHs 6.5, 7.5, and 8.5 were 75, 40,and 16%, respectively. At 2 mg/L Fe(III) dosage, approximately 20% of As(III) was removed irrespective of pH. The removal efficiency of As(V) was much higher than As(III) at all pH values.

 

Ferric Sulphate (Fe₂(SO₄)₃) as Coagulant:

The most frequently used technology for removing arsenic from drinking water²⁶ is precipitation/co-precipitation using ferrous sulphate followed by some form of settling and/or filtration. The process is commonly referred to as coagulation-filtration. This technology is capable of treating a wide range of influent concentrations to the revised MCL for arsenic. According to USEPA more than 52% of the identified applications of arsenic treatment technologies for water are based on coagulation. Many studies have been done to examine the efficiency of arsenic removal using coagulation with ferric and aluminum salts . Coagulant type and dosage, pH, composition of the water, and contaminant type have significant effects on removal efficiency. It is well accepted that removal of arsenate [As(V)] is much better than arsenite [As(III)] and that silica interferes with arsenic removal at higher pH . For better removal efficiency, pre-oxidation is necessary for As(III) removal.

 

Alum as Coagulant:

Alum is most widely used coagulant for water treatment in the USA²⁷, and it has been reported to be equally as effective as ferric iron for arsenic adsorption on a molar basis. Alum, like ferric chloride, is an effective coagulant at pH < 6.5, because it carries a strong cationic charge in this pH region. At pH > 6.5, alum is only weakly cationic, and becomes much less effective for adsorbing anions. Ahmed and Rahaman reported effective removal with alum coagulation in the pH range of 7.2-7.5. McNeil & Edwards reported that aluminum and iron flocs have equal adsorption capacity, but the aluminum hydroxide flocs with sorbed arsenic can pass through 0.45-μm filters and decrease apparent arsenic removal by alum coagulation. The removal efficiencies of As(V) and As(III) have been studued  as a function of coagulant dose and pH , these studies indicates that removal efficiency of As(V) was pH dependent, and no significant As(III) removal was observed at any pH. With increase in pH, the removal efficiency of As(V) decreased. The percentage removals of As(V) with 1 mg/L Al(III) dosage at pHs 6.5, 7.5, and 8.5 were 90, 50, and 30%, respectively, which were less than the removal for the same Fe(III) dose. When alum was used as coagulant, no significant As (III) removal was observed at any dose or pH. The removal efficiencies of As (V) and As(III) with alum were lower than with ferric chloride.

 

Zirconium (IV) Chloride (ZrCl₄) as Coagulant:

The removal efficiencies of As(V) and As(III) using zirconium(IV) chloride²⁸ as a coagulant have been studied, such studies demonstrate that As(V) removal was highly pH dependent, whereas As(III) removal was independent of pH. The removal efficiency of As(V) during Zr(IV) coagulation was more influenced by pH than was Fe(III) coagulation. With increase in pH, the removal efficiency of As(V) decreased. The percentage removal of As(V) with 1 mg/L Zr(IV) dosage at pHs 6.5, 7.5, and 8.5 were 62, 28, and 8%, respectively, which were significantly less than the removal for the same Fe(III) dose. With 2 mg/L Zr(IV) dosage, approximately 8% of As(III) was removed irrespective of pH, which was significantly less than the removal for the same Fe(III) dose.

Zirconium (IV) oxy Chloride (ZrOCl₂) as Coagulant:

The removal efficiencies of As(V) and As(III) using zirconium (IV) oxychloride depend both dose and pH .As(V) removal was highly pH dependent²⁹ whereas As(III) removal was independent of pH. The effect of pH one removal efficiency of As(V) by zirconium (IV) oxychloride was more pronounced than ferric chloride. The percent removals of As(V) with 2 mg/L Zr(IV) dosage at pHs 6.5, 7.5, and 8.5 were 94, 59, and 23%, respectively, which were significantly less than the removals for the same Fe(III) doses. With 2 mg/L Zr(IV) dosage, approximately 8% of As(III) was removed irrespective of pH. So the removal efficiencies of As(V) and As(III) with zirconium(IV) oxychloride were lower than with ferric chloride. It can be observed from Figure 3.9 that the removal efficiency of As(V) was much higher than As(III) at all pH values.

 

Titanium (III) Chloride (TiCl₃) as Coagulant:

The removal efficiencies of As(V) and As(III) studies using TiCl₃  demonstrate that both As(V) and As(III) removal were highly pH dependent³⁰. The removal efficiency of As(V) decreased with increasing pH. The percent removals of As(V) with 1 mg/L Ti(III) dosage at pHs 6.5, 7.5, and 8.5 were 49, 37, and 30%, respectively, which were significantly less than the removals for the same Fe(III) dose. The removal efficiency of As(III) by Ti(III) increased with decreasing pH specifically for As(III) removal. The removal efficiencies were 42, 32, and 27% at pH 6.5, 7.5 and 8.5, respectively for a dose of 2 mg/L Ti(III). Thus the removal efficiency with titanium (III) chloride was higher than ferric chloride, which had a removal efficiency of 26% and was pH independentfor all dosages. In summary, the removal efficiency of As(V) during titanium(III) chloride coagulation was lower than during ferric chloride coagulation while the removal efficiency of As(III) during titanium(III) chloride coagulation was higher than that of ferric chloride.

 

Titanium (IV) Chloride (TiCl₄) as Coagulant:

The removal efficiencies of As(V) and As(III) in using titanium (IV) chloride shows  that As(V) removal was highly pH dependent³¹, whereas As(III) removal was independent of pH. With increase in pH, the removal efficiency of As(V) decreased. The percent removals of As(V) with 1 mg/L Ti(IV) dosage at pHs 6.5, 7.5, and 8.5 were 54, 40, and 26%, respectively, which were significantly less than the removal for the same Fe(III) dose. With 2 mg/L Ti(IV) dosage, approximately 26% of As(III) was removed irrespective of pH, which was the same with Fe(III). So the removal efficiencies of As(V) in the presence of titanium (IV) chloride were lower than in the presence of ferric chloride, but the removal efficiency of As(III) was similar to that of ferric chloride. It can be observed from Figure 3.5 that the removal efficiency of As(V) was much higher than As(III) at all pH values.

 

Titanium (IV) Oxy Chloride (TiOCl₂) as Coagulant:

The removal efficiencies of As(V) and As(III) using titanium (IV) oxychloride coagulant  shows that coagulant dose and pH variations show that both As(V) and As(III) removal were pH dependent. The percent removals of As(V) with 2 mg/L Ti(IV) dosage at pHs 6.5, 7.5, and 8.5 were 54, 37, and 29%, respectively, which were significantly less than the removals for the same Fe(III) dose. The removal efficiency of As(III) was also pH dependent; it increased with increasing pH. The removal efficiencies of As(III) were 16, 20, and 26% at pH 6.5, 7.5 and 8.5, respectively for a dose of 2 mg/L Ti(IV), which shows that the removal efficiency was less than that observed for ferric chloride at pH 6.5 and 7.5 while it was the same at pH 8.5. So the removal efficiencies of As(V) by titanium (IV) oxychloride coagulation were lower than those for ferric chloride coagulation, while the removal efficiencies of As(III) were less than or equal to those of ferric chloride and that the removal efficiency of As(V) was higher than As(III) at all pH values.

 

Titanium Sulphate (Ti (SO₄)₂) as Coagulant:

The removal efficiencies of As(V) and As(III) using titanium (IV) suplphate coagulant  shows that coagulant dose and pH variations show that both As(V) and As(III) removal were pH dependent³³. The percent removals of As(V) with 2 mg/L Ti(IV) dosage at pHs 6.5, 7.5, and 8.5 were 90, 70, and 509%, respectively, which were significantly less than the removals for the same Fe(III) dose. The removal efficiency of As(III) was also pH dependent; it increased with increasing pH. The removal efficiencies of As(III) were 15, 10, and 5% at pH 6.5, 7.5 and 8.5, respectively for a dose of 2 mg/L Ti(IV) sulphate, which shows that the removal efficiency was less than that observed for ferric chloride at pH 6.5 and 7.5 while it was the same at pH 8.5. values.

 

Factors affecting arsenic removal by coagulation:

The efficiency of arsenic removal by coagulation filtration process depends on water quality and process conditions applied. The water quality includes pH, temperature, initial arsenic concentration and speciation and the presence of other competing ions³⁴. The process conditions include type and dose of coagulant, flocculation conditions and method of floc separation applied

 

Table 3-  operating conditions and removal efficiency of various coagulants in Arsenic removal

Coagulant/Flocculent	Operating Conditions			Removal Efficiency		Ref.
	pH	Initial conc.	Optimum dosage	For As(III)	For As(V)
Ferric Chloride	7.0	3mg/l	30mg/l	45%	75%	25
Ferric Sulphate	7.0	1mg/l	25mg/l	80%	60%	26
Alum	7.0	20µg/l	40mg/l	20%	90%	27
Zirconium (IV)Chloride	7.0	3mg/l	30mg/l	08%	55%	26,27
Zirconium (IV)Oxy Chloride	7.5	50µg/l	2mg/l	08%	59%	27
Titanium (III)Chloride	7.5	50µg/l	2mg/l	32%	75%	25,26,27
Titanium(IV) Chloride	7.5	50µg/l	2mg/l	26%	55%	26,27
Titanium  (IV)Oxy Chloride	7.5	50µg/l	2mg/l	37%	20%	26,27,28
Titanium Sulphate	7.0	1mg/l	25mg/l	90%	15%	29

 

 

Fig.2  Comparison of removal efficiecy of various coagulants for As(III) and As(V)

 

   Fig.3 Rmoval Efficiencies of various coagulants for As(III) and As(V) in water samples

 

Fig.3 Rmoval Efficiencies of various coagulants for As(III) at differnet pH

 

Fig.4 Rmoval Efficiencies of various coagulants for As(V) at differnet pH

Conclusion:

Arsenic has been serious polluters of water since Roman times and perhaps earlier. As arsenic in drinking water is having a major human impact in several locations many treatment technologies are available for arsenic removal but none of them is found to be completely applicable. Adsorption is a useful tool for controlling the extent of aqueous arsenic pollution

 

ACKNOWLEDGEMENT:

The authors are grateful to the authorities of IKG Punjab Technical University, Kapurthala and Guru Nanak Dev University, Amritsar for their support in analyzing the data. The authors are also thankful to Principal DAV College, Amritsar for providing the necessary lab facilities. A special thanks to University Grants Commission (New Delhi) for funding.

 

CONFLICT OF INTEREST:

The authors declare no conflict of interest.

 

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Received on 23.03.2017         Modified on 22.04.2017

Accepted on 14.05.2017         © AJRC All right reserved