Comparative assessment of filter media for arsenic removal from drinking water: Commercially...

COMPARATIVE ASSESSMENT OF FILTER MEDIA FOR ARSENIC REMOVAL FROM DRINKING WATER: COMMERCIALLY AVAILABLE

AND PERSPECTIVE HYBRID SORBENTS

Z. Maletskyi, T. Mitchenko, H. Shevchuk, P. Stender, Y. Kolomiyets National Technical University of Ukraine

37 Pobedi Av., Building 4, Room 117, 03056 Kiev, Ukraine, [email protected]

ABSTRACT

The toxicity of arsenic to humans is well known: ingestion in small amounts leads to the slow poisoning, called arsenicosis, and a dose of 0.2 g-As per kg of human weight can cause lethal decease.

Most of countries recently lowered the requirements for arsenic concentration in drinking water from 50 µg/L to 10 µg/L.

Several technologies were proposed in order to meet the new water quality criteria: chemical precipitation, sorption, coagulation, oxidation and membrane processes. Application of special iron-containing sorbents revealed to be the most promising for this scope.

The aim of the current work is to compare efficiency of different commercially available arsenic selective sorbents with specially developed hybrid sorbents.

Obtained results shown that static capacity of the studied sorbents aimed to As(III) and As(V) compounds is not equal. Commercial adsorbents have 2.5-3.8 times higher static capacities by As(V) than As(III), while hybrid sorbents revealed to be more similar considering both arsenic forms (relative preference 1.3-2.6 times).

At the same time commercially available adsorbents turned to be more effective in dynamic conditions.

Comparing efficiency of different adsorbents, ability to regeneration should be taken into account. In this way commercially available adsorbents are less efficient, since they are non-regenerative or can be regenerated only by the producer. While hybrid sorbents can be regenerated straight in the filter and therefore used in cycle.

KEYWORDS

Adsorption Filters, Processes, Drinking Water Filters, Filter Media, Filter Media Design, Filter Media Testing, Water Filters

11th World Filtration Congress - Session PL02 - Solid-Liquid Separation II

1 11th World Filtration Congress - April 16-20, 2012 - Graz - Austria

1. Introduction

Arsenic is a widespread element on Earth: its concentration in earth's crust reaches 2 mg/kg. Arsenic occurs in water, rocks and soils in four oxidation states: -3, 0, 3 and 5. The most stable forms of its presence in natural waters are As (III) – under reducing conditions and As (V) – under oxidizing conditions [1].

The toxicity of arsenic compounds has been known since ancient times. Average lethal dose (LD50) varies from 0.014 to 0.185 gram per kilogram of human weight for different arsenic species. Chronic arsenicosis starts aggravating at thousand times lower dose [2]. Inorganic arsenic is of biggest threat to human health. Exposure to arsenic fairly often occurs through drinking water [3]. The main source of arsenic in drinking water is dissolution of natural rocks, like Arsenopyrite, with natural artesian or ground water [4]. Arsenic concentration in surface waters is usually low, except the cases when surface water blends with ground water [5].

Continuous consumption of water with 15 µg/L of arsenic results in extremely negative health effects [6]: skin cancer, oncological diseases of genitourinary system and lungs, diseases of blood vessels and significantly increases risk of diabetes, hypertension, reproductive disorders and impaired intellectual development in children [7].

According with [5] 137 million people in 70 countries were exposed to arsenic through drinking water in 2002. Despite all these, information on any systematic research dedicated to arsenic occurrence in Ukraine cannot be found.

Since some effects of arsenic influence are irreversible, the basic measure of public health care is to prevent the appearance of its impurities in drinking water [8]. WHO and several national health care organizations, including Ukrainian Ministry of Health Protection, have lowered the maximum concentration level of arsenic in drinking water from 50 to 10 µg/L.

As a result of the global standards toughening, a number of municipal water treatment plants in different countries met the incompliance of drinking water quality with the new normative.

Screening of water quality in Ukraine has shown that arsenic can be a big issue, especially in rural areas. Thus, concentration above 10 µg/L has been detected in 70% of the analyzed samples: in 17 samples of artesian water it was in the range 20-83 µg/L, in 19 samples of ground water – 20-100 µg/L and in 5 samples of surface water – 10-85 µg/L.

Therefore there is an actual need in effective and safe technologies of deep water purification aimed to arsenic impurities. Several levels of demand should be considered: municipal water treatment plants, local systems with the capacity up to 100 m3/h and point-of-use filters.

The analysis of scientific publications shows that the most promising methods of arsenic removal are chemical and photochemical oxidation, ion exchange, adsorption and membrane treatment [9]. The latter two excel others in effectiveness and, to some extent, in environmental safety.

High degree of arsenic removal can be achieved with sorption technology. However commercially available adsorbents are expensive and nonregenerable or have to be regenerated only by the producer. Several materials can play role of arsenic selective sorbents: granular iron oxides and hydroxides, oxides of titanium, zirconium and



aluminum, lime coated with iron hydroxide [10] and materials of a new class – hybrid sorbents, obtained through modification of ion-exchange resin with Fe compounds [11].

On industrial scale membrane technologies surpass adsorptive by economic criteria [12], but yield in the degree of arsenic removal. According with [13] cellulose acetate membranes remove about 90% of As (V) and 70% of As (III). Selectivity of polyamide membranes is significantly higher at alkaline pH – more than 90% for both As valence forms, but at pH <7 it drops to 20% for As (III). This limits application of membranes in point-of-use filters rated at removal of toxic impurities. Another significant drawback is the formation of concentrate which contains several times more arsenic than the raw water. Discharge of such concentrate will aggravate the problem of arsenic dissemination in natural waters.

In our opinion, it is possible to overcome all the mentioned drawbacks by combining membrane and sorption technologies applying regenerable and cheap materials.

Recently we have developed a method of hybrid sorbents production by impregnating anion exchange resins with humic substances [14]. These sorbents are selective towards transition metals, especially to iron. We have also shown that exhaustion of anion exchanger in industrial water demineralization processes can be considered as the particular case of hybrid sorbent formation [15]. Since the exhausted anion exchange resins represent large-tonnage industrial wastes, requiring additional funds for disposal, their reuse in water treatment processes is an effective environmental measure.

It is reasonable to assume that surface of such iron selective sorbents can be easily covered with iron oxides and hydroxides [16]. Thereby a new function interface can be obtained. Since the high affinity of industrial arsenic selective adsorbents is grounded on the presence of iron compounds, proposed hybrid sorbents promise to be effective as well.

In a frame of current work comparison of static and dynamic sorption characteristics of industrial arsenic-selective adsorbents and experimental hybrid sorbents has been carried out.

2. Materials and Methods

Six commercially available (Table 1) and ten hybrid sorbents (Table 2) were chosen for the comparative research. Group of commercially available sorbents includes arsenic selective sorbents and sorption materials which due to their physico-chemical properties are capable of arsenite or arsenate retention.

Table 1 – Characteristics of commercially available sorbents

Designation Name, producer Description Static

capacity, g-As/L-sorb.

Arsenic-selective sorbents

S1 Adsorbsia GTO, Dow Chemical (USA)

Titanium oxide As (III) – 1.91

As (V) – 7.21

S2 Lewatit FO 36, LanXESS (Germany)

Macroporous weak base anion exchange resin containing iron oxide

As (III) – 2.5 As (V) – 6.0



S3 ArsenXnp, Purolite (USA)

SDVB co-polymer containing iron oxochloride

As (III) – 1.6 As (V) – 4.0

Non-selective sorbents

S4 Dowex MWA-1, Dow Chemical (USA)

Macroporous weak base anion exchange resin

Not specified (1.22)2

S5 Dowex Marathon 11 Dow Chemical (USA)

Gel microporous strong base anion exchange resin

Not specified (1.32)2

S6 Centaur, Calgon Carbon Corp. USA

Catalytic granular activated carbon

Not specified (462)3

1Maximum capacity defined at 50 µg-As/L and pH 7; 2Total exchange capacity, eq/L; 3Iodine adsorption, g-I/L-adsorbent.

Hybrid sorbents have been prepared following the next methods.

Method A. The substrate of the hybrid sorbent was treated with:

Natural artesian water containing Fe (II) compounds under dynamic conditions – Method A1;

FeCl3 solution under static conditions – Method A2; FeSO4 solution under static conditions – Method A3.

Treatment conditions of Method A have been described in detail in [17].

Method B. The substrate of the hybrid sorbent was impregnated with humic substances [16].

Method C. At first sage the substrate of the hybrid sorbent was impregnated with humic substances and then treated with natural artesian water containing Fe (II) compounds under dynamic conditions [18].

Sorption properties of the listed materials were studied under static and dynamic conditions using model solutions of As (III) and As (V). The solutions were prepared from the respective salts of analytical purity grade and ultrapure water. The concentration of model solutions was refined analytically before each experiment.

Table 2 – Characteristics of hybrid sorbents

Designation Substrate ModifierImmobilized

layerMethod of obtaining

HS1 S4 - Fe

7.07 g-Fe/L-sorb.Method A1

HS2 S4 - Fe


HS3 S5 - Fe


HS4 S4 NHS1 - Method BHS5 S5 NHS1 - Method B

HS6 S4 NHS1

Fe 4.99 g-Fe/L-sorb.

Method C

HS7 S4 MSHS2


Method C

HS8 S5 NHS1


Method C



HS9 S6 - Fe Method A1

HS10 S6 - Fe


1NHS – natural humic substances; 2MSHS – model solution of humic substances

The concentration of arsenic in source and treated water was determined by atomic absorption spectrometry with electrothermal atomization [19].Detection Limit for this method is 3 µg/L with a tolerance of ±30%. The measurements were performed on a two-beam optical system, spectrometer Perkin Elmer AAnalyst 300 equipped with a graphite furnace TGA-800. Sample atomization was carried out at a temperature of 2100 °C in a pyrolytic graphite cell. As a radiation source EDL was used (193.7 nm) under the operating current of 420 mA and slit 0.7 mm. The signal was recorded by the peak height. The final result was obtained by averaging data of three parallel measurements, considering that their relative deviation did not exceed 5%.

To determine the static capacity, 1 ml of the sorbent was placed to the model solution of As (III) or As (V).The concentration of the As (III) model solution was 64 µg-As/L, and As (V) – 111 µg-As/L. Sorbents were in contact with As solutions during 16 hours at constant shaking. Then the concentration of arsenic in the final solution and the sorbed amount were determined.

For experiments in dynamic conditions 20 ml of each sample was placed in a glass laboratory column with a diameter of 10 mm. Arsenic model solution was passing through the media at rate of 20 BV/h. The model solution contained 40 µg/L of As (III) and 60 µg/L of As (V). Filtrate was collected in portions of 2 liters to measure the concentration of arsenic. The experiment was finished after reaching the breakthrough concentration in the filtrate (10 µg/L).

The dynamic capacity was calculated as follows:

where E – dynamic capacity, g-As/L-sorb.;

W – amount of sorbent, L;

a – volume of treated water before the breakthrough point, BV (L-sol./L-sorb.);

C0 – initial arsenic concentration in model solution, g/L;

C(V) – function which describes dependence of the As concentration in filtrate from the passed volume.

3. Results discussion

All hybrid adsorbents (HS1-HS10) and commercially available sorption materials, which had been used as a substrate for hybrid sorbents (S4-S6), were studied under static conditions. The determined static capacities are shown in Figure 1.

Analysis of the obtained data shows that the weak base (S4) and strong base (S5) anion exchange resins exhibit high sorption affinity aimed to As (V), but hardly retain As (III).



Capacity of the hybrid sorbents, obtained by Method A (HS1, HS2 and HS3), significantly improves with respect to As (III). At the same time it is not typical for the sorbents obtained by Method B (HS4 and HS5).

The affinity of the hybrid sorbents, obtained by Method C, aimed to As (III) also increases, in comparison with the substrate materials (S4 and S5). Only HS7 does not change its capacity significantly. That is probably due to the low amount of immobilized iron (HS6 – 4.99 g/L, HS7 – 0.78 g/L, HS8 – 3.58 g/L).

Modification of catalytic activated carbon (S6) following the Method A1 leads to a reversal of the sorption affinity aimed to As (III) and As (V).

Cluster analysis allows dividing sorption materials into three groups according to their static capacities both by As (III) and As (V):

"High efficient": HS2, HS10; "Middle efficient": S6, HS1, HS3, HS6, HS8, HS9; “Low efficient”: S4, S5, HS4, HS5, HS7.

Based on the cluster analysis and taking into account limited stability of HS10, hybrid sorbent HS2 has been selected for further research.

Figure 1 - Static capacity of different sorbents aimed to As (III) and As (V)

Commercially available arsenic-selective adsorbents S1, S2, S3 and hybrid sorbents HS2 and HS6 were studied under dynamic conditions. The results of this investigation are presented in the form of breakthrough curves in Figure 2.



Figure 2 - Breakthrough curves of arsenic sorption by different sorbents

Based on the breakthrough curves, filter cycle duration and dynamic capacity were determined for each sorbent (Table 3).

Table 3 - Filter cycle duration and dynamic capacity of the sorbents

Designation Filter cycle duration,

BVDynamic capacity

g-As/L-sorb. S1 36,000 3.6 S2 18,900 1.9 S3 37,700 3.7

HS2 4,700 0.5 HS6 10,000 1.0

As can be deduced from the Table 3, the highest dynamic capacity belongs to the commercially available arsenic selective adsorbents S1 and S3. Hybrid sorbents HS2 and HS6 are substantially inferior in this sense.

4. Conclusions

It is reasonable to compare static and dynamic characteristics of studied sorption materials with their ability to regeneration and frequency of waste deposits formation during water treatment processes (Table 4).

Table 4 - Comparison of different sorption materials

Designation

Ratio of As (V) / As(III)

static capacities

Dynamic capacity g-As/L-sorb.

Ability to regeneration

Frequency of waste formation

S1 3.8 3.6 Not able In each cycle



S2 2.4 1.9 Not able In each cycle

S3 2.5 3.7 Can be performed by manufacturer

After the loss of mechanical

stability

HS2 1.3 0.5 Possible, directly

in filter


stability

HS6 2.6 1.0 Possible, directly

in filter


stability

Therefore, if the water contains both As (III) and As (V) impurities, the next sorption materials should be considered for treatment:

Commercially available arsenic selective adsorbents S3 or S1 – for water treatment systems designed to work in nonregenerable mode. These can be the combined systems where adsorption treatment is applied after the step of membrane purification;

Hybrid sorbent HS6 – for water treatment systems designed to operate in a cycle mode "sorption-regeneration."

Current Research was supported by Ukrainian Ministry of Science and Education according with Project "Research and development of efficient resource-saving technologies for deep water purification considering arsenic compounds" (Topic #2363-p, State registration #0110U002384, KVNTD #I.2 15.17.21.

Literature

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2. Oregon Department of Human Services. 2002. Health effects information. Arsenic. Oregon: Office of Environmental Public Health, 2002.

3. Arsenic poisoning from groundwater in West Bengal. Saha, K. C. 1998. 4, 1998, Breakthough, Volume 7, p. 5-14.

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6. Wang L., Huang J. 1994. Chronic arsenism from drinking water in some areas of Xinjiang, China. Edited by Jerome O. Arsenic in the Environment. Nriagu: 1994, Volume 2.

7. WHO, Executive board. 2006. EB118/14. Switzerland: World Health Organization, 2006.

8. WHO. 2003. Arsenic in Drinking-water. Geneva: World Health Organization, 2003.



9. Johnson R., Heijnen H. 2001. Safe Water Technology for Arsenic Removal. Edited by A. Feroze et al. Technologies for Arsenic Removal from Drinking Water. Bangladesh: Mati ar Manush, 2001.

10. Mechanisms of Arsenic Removal From Water. W., Hoell. 2009. Izmir, Turkey: Belediyesi, 2009. Proceedings of The International Congress on Production of Safe Water.

11. Arsenic removal using a polymeric/inorganic hybrid sorbent. DeMarco M., SenGupta A. 2003. 37, б.м.: Elsevier, 2003, Water Research, p. 164-176.

12. International Consultants Inc. 200. Technologies and costs for removal of arsenic from drinking water: EPA, 200. EPA 815-R-00-028.

13. Ravenscroft P., Brammer H., Richards K. 2011. Arsenic Pollution: A Global Synthesis: John Wiley & Sons, 2011. 1444355465, 9781444355468.

14. Formation and properties of hybrid sorbents based on anion exchange resins and humic substances. Maletskyi Z., Mitchenko Т., Makarova N., Hoell W. 2010. Kiev: NaUKMA, 2010, Scientific notes of National Kiev Mohyla Academy. Chemical technologies and science, Volume 92, p. 69-75.

15. Properties of Anion Exchange Resins exhausted by Humic Compounds. Maletskyi Z., Mitchenko T., Makarova N., Hoell W. 2011: DWT, 2011, Desalination and Water Treatment Journal, p. 78-83.

16. Hybrid sorbents based on exhausted anion exchange resins for iron removal. Maletskyi Z., Mitchenko T., Höll W., Makarova N., Sus M. 2009. Baden-Baden, Germany: EDS, 2009. Proceedings of the Desalination for the Environment Conference. p. 54-55.

17. Development of Novel Multifunctional Sorbents on a Basis of Ion Exchange Resin Exhausted in Processes of Water demineralization. Mitchenko T., Makarova N., Schevchuk H. 2007: Japan Association of Ion Exchange, 2007, Journal of Ion Exchange, Volume 18, p. 412-417.

18. Eliminating Arsenic from RO concentrates by the reuse of anion exchange resins. Maletskyi Z., Mitchenko T., Makarova N., Shevchuk E. 2010. Tel Aviv, Israel: EDS, 2010. Proceedings of the International Conference EuroMed 2010. p. 85-87.

19. ASTM International. 2003. Standard Test Methods for Arsenic in Water. ASTM 2003, Sections 25-33. D 2972 – 03.



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