ARTIFICIAL WETLANDS FOR ARSENIC REMOVAL ...

10th International Conference on Wetland Systems for Water Pollution Control

ARTIFICIAL WETLANDS FOR ARSENIC REMOVAL M.T. Alarcón-Herrera*, E. Flores-Tavizón*, I. R. Martín-Domínguez* and A. Benavides-Montoya* * Department of Environmental and Energy Research, Advanced Materials Research Center, Miguel de Cervantes 120, Complejo Industrial Chihuahua, 31109, Chihuahua, Chih., MX

ABSTRACT Elevated arsenic concentration in groundwater is becoming a worldwide problem. Removal of metals and metalloids from contaminated water through the use of artificial or constructed wetlands is an innovative treatment alternative with a high potential for the removal of arsenic. Thus, the objective of the present study was to analyze the arsenic removal efficiency of artificial wetland prototypes in laboratory conditions. The tests were made by triplicate in piston flux systems, with a sand support system between 5mm and 8mm in diameter and a hydraulic retention time of 8 days. The As concentration in the entrance flow was 1.12mg L-1. The results show that the average concentration in the exit flow during the time of the test was of 0.12mg L-1, which corresponds to a removal efficiency of 90% in the system. Even though As concentrations in the exit flow are 10 times those permitted for human consumption, the entrance concentrations were 10 times higher than the former. Considering entrance concentrations of approximately 100 µg L-1, there is a high potential for the removal efficiency of this system to be enough to reduce the exit flow concentrations to levels appropriate for human consumption (25µg L-1-10µg L-1). These results show the great application potential of artificial wetlands for the removal of arsenic from water. They can be mainly considered for application in rural areas, due to their low maintenance costs and minimum energy requirement, within the methods for the cleaning of water and the minimization of environmental problems caused by As contamination.

KEYWORDS Arsenic; constructed wetlands; removal INTRODUCTION Arsenic in groundwater poses one of the most important environmental health risks of the present century. In many parts of the world, groundwater resources naturally contain high levels of arsenic. These arsenic concentrations often exceed 50 µg L-1, the guideline value of the World Health Organization (WHO). Now that the maximum allowed level has been reduced to 10 µg L-1, the number of places that exceed regulations and the number of affected people have increased; therefore, new technologies should be developed in order to approach the problem. September 23-29, 2006 Lisbon, Portugal

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The consumption of arsenic-contaminated water causes different chronic health problems in the exposed population, including hyperkeratosis, hyper-pigmentation, peripheral vascular injuries, and several kinds of skin, lung, liver, kidney and bladder cancer. Realizing this and following the world health organization (WHO) guideline, the Mexican Government gradually reduced the allowable level of drinking water arsenic from 50 µg L-1 to 25 µg L-1, effective since 2005 (NOM-127-SSA1-2000). The various technologies reported in literature for the treatment of arsenic include conventional co-precipitation, lime softening, filtration, ion exchange, reverse osmosis and membrane filtration. Adsorption has been extensively used for arsenic removal as well, some of the frequently used adsorbents being iron oxide, manganese oxides and activated alumina. Bulky sludge, high energy requirements and cost, as well as the excessive use of chemicals, however, keep these techniques from being cost effective, especially al low scales and in rural communities. Thus, there is an urgent need for new low-cost water purification techniques (Jiang, 2001; EPA, 2002). Phytoremediation is a term applied to a group of technologies that use plants to reduce, remove, degrade, or immobilize contaminants, with the aim of restoring area sites to a condition useable for different applications. To date, phytoremediation efforts have focused on the use of plants to accelerate degradation of organic contaminants, or to remove hazardous heavy metals and metalloids from soils or water. Phytoremediation of contaminated sites is appealing because it is less expensive and more aesthetically pleasing to the public than alternate physical-chemical remediation strategies. The water phytoremediation process involves raising plants hydroponically and transplanting them into metal or metalloid-polluted waters, where they absorb and concentrate the contaminants in their roots and shoots. As they become saturated with the metal pollutants, either the roots or the whole plants are harvested for treatment and safe disposal. There are several species with promising results in arsenic phytoremediation. Although hyperaccumulators are mostly considered for the phytoextraction of As from soil and water in contaminated sites, accumulators and tolerators can be very useful in the phytostabilization and rhizofiltration of polluted soil and water. Artificial wetlands, also known as constructed wetlands (CW), are considered to be cost-effective technologies for water and wastewater treatment. They offer great advantages over conventional treatment systems because they operate on ambient solar Page 470


energy, require low external energy input, can achieve high levels of treatment, and involve inexpensive technology easily operated and maintained. Constructed wetland models have been reported for removal of As and heavy metals from different wastewaters. The performance of As and heavy metal removal from artificial wastewater varied with the type of the constructed wetland. Subsurface wetlands and free surface wetlands removed heavy metals better than hydroponic systems and algae ponds (S. Buddhawong et al., 2004). Constructed wetland systems with a plants and a combination gravel/soil matrix have the potential to remove arsenic from contaminated water; therefore, the purpose of this study was to determine the system behavior in a prototype subsurface wetland. METHODS System description This study was conducted in a subsurface wetland prototype system, with three units operated in parallel under a continuous flow. The design of the prototype was created based on the hydraulic factors of the system, considering the minimum width-length ratio required for piston flux (3:1). The general characteristics of the design are presented in table 1. Planting in wetlands and plant adaptation The plant rhizomes were planted in the units, provided with nutritive solution for 47 days in order to allow for plant development. The prototypes (CW-1, CW-2, CW-3) were planted with S. Americanus. The 2 months of adaptation in the support medium allowed the plants to develop roots and invade the wetland. In order for the plants to develop, the wetlands were flooded with a modified Hoagland solution, a procedure repeated twice. Before the removal test was started, the wetlands were washed 3 times with water in order to displace the nutritive solution.

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Table 1. Characteristics of the CW-1, CW-2 and CW-3 test units.

Parameter Cell geometry: Length, L ; Width, W ; Height, Depth of the support medium, y Prototype slope, m Support medium Effective Size, D10 CW-1, CW-3 CW-2 CW-1, CW-3 Porosity, η CW-2 Density, ρ

Criterion 1.5 m; 0.5 m; 0.5 m 0.35 m 2% Coarse sand and gravelly sand < 2.38 mm and < 8 mm 31 % 35 % 1.43 g/cm3 100 m3/m2/d and 500 m3/m2/d

Hydraulic conductivit, k s , CW-1, CW-3 CW-2 Area of the transversal section, At

0.175 m2

Superficial area, As

0.75 m2

Flow, Q Hydraulic Retention Time, TRH

11.64 – 13.4 L/d 8 – 8.7 d

a.

Flow variations due to cell geometry differences during construction.

Operation of the Prototypes Periodically, 160 L of water with added arsenic at a concentration of 1mg of As L-1, in the form of sodium arsenate, were prepared and continuously flowed through the three units (CW-1, CW-2, CW-3) using peristaltic pumps. The hydraulic retention time of the solution in the wetlands was 8 days for CW-2 and 8.7 days for CW-1 and CW-3. The test period was 38 days, during which the units were operated at room temperature and humidity, between 34ºC and 35ºC, and 40% and 60% relative humidity. The intensity of the inner light of the prototype laboratory was between 380 and 700 µmol/m2s. Sampling

Water. The influent and effluent of the systems were monitored throughout the experiment, 100mL samples being taken from the different sampling points of each unit.

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Plants. At the end of the experiment, the wetland was divided into 3 sections and plants were extracted from each. Each plant was very carefully extracted, in order to retrieve the whole root that each individual had developed inside the support medium. Soil. Each unit was divided into 3 sections throughout the cells, and each section was divided in 3 parts itself. The extracted samples were prepared for digestion and analysis. Analytical determination All plants were carefully washed with tap water, distilled water and tridistilled water. They were then separated into aerial part and roots for the As determination. They were then dried at 60ºC until constant weight was achieved. Each dry sample was crushed, ground and sifted (100% –100 mallas, 150 µm) for homogenization before digestion. The latter process was exhaustive, being performed with nitric acid and hydrogen peroxide until the complete mineralization of the organic matter was reached. The soil and water samples were submitted to acid digestion following the EPA methodology (EPA-SW-846-3051 and 3015). Total As determinations were performed in a Inductively Coupled Plasma Optical Emission Spectometer (ICP/OES), manufactured by Termo Jarell Ash, model Iris/AP Duo. The samples with a As concentration under 80 µg L-1 were analyzed in an atomic adsorption spectrophotometry equipment model Avanta Sigma GBC, equipped with a GBC hydride generation system model HG 3000. The validation of the analytical method for the determination of total As was performed with certified standards (High Purity Standars, Cat No.10003-1, As 1000±3 µg mL-1 en 2% HNO3). For the plant samples, a certified tomato leaf standard was used (NIST 1573a). The precision was of 95.5%, uncertainty of 3.3%, error of 6.8%, and recovery of 112%. RESULTS AND DISCUSSION The removal of As in the 3 units was very similar (Figure 1). The variance analysis indicates that there are no significant differences between the values,( 95% confidence interval).


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1400

As Concentration µ g /L

1200 1000 Input CW-1

800

Output-CW-1 600

Input CW-2 Output-CW-2

400

Input CW-3 200

Output-CW-3

0 0

5

10

15

20

25

30

35

40

Time

Figure 1. As Concentration in the influent and effluent of the three test units.

As retention in plant tissues

Schoenoplectus americanus successfully developed in the test units with no apparent physical deterioration. The amounts of As retained in the plant structures at the end of the period were: 188, 128 and 203 mg per Kg of dry biomass for the plants of units CW-1, CW-2 and CW-3, respectively. The greatest retention occurred in the roots of unit CW-3 (52 mg Kg-1), which confirms along with previous studies that Schoenoplectus americanus is a tolerant plant. Arsenic retention in the medium (gravel) of the units Table 2 shows the amount of As accumulated per section and the total retained in the medium of each unit; an average of 431mg of As was accumulated per unit. Table 2. Distribution of As in the support medium (gravel) of the units at the end of the test period.

Section 1 Section 2 Section 3 Total

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As total (mg) CW-1

CW-2

CW-3

124.65 137.85 160.3 422.8 ± 18

142.34 166.89 128.83 438.1 ± 19

169.11 113.13 150.82 433.1 ± 28


General balance of As in the system In Table 3, the general balance of the amounts of arsenic retained in the 3 test units are presented. 86% of the As retention occurred in the medium (gravel). Plants retained only 0.9% of the arsenic; however, it is considered that they contributed, in conjunction with the microorganisms and the physical conditions created in the complex system, to promote the retention of this metalloid in the medium and result in a removal of 90%. Table 3. General balance of the amount of arsenic retained in the test units Unit

Influent

Gravel

Plants

422.8 438.06 433.06

4.69 2.6 5.98

Effluent

As, mg. CW-1 CW-2 CW-3

489.37 513.86 495.74

48.73 54.46 49.42

As retention % 90.04% 89.40% 90.03%

The results show that the average concentration in the exit flow during the time of the test was of 0.12mg L-1, which corresponds to a removal efficiency of 90% in the system. Even though As concentrations in the exit flow are 10 times those permitted for human consumption, the entrance concentrations were 100 times higher than the recommended value. Considering entrance concentrations of approximately 100 µg L-1 and the behavior of this system, there is a high potential for the increase of the removal efficiency at lower concentrations. Thus, the system’s removal efficiency has a high potential to be enough to reduce the exit flow concentrations to levels appropriate for human consumption (25µg L-1, Mexican regulations or WHO recommended values of 10µg L-1). CONCLUSIONS The artificial wetland prototype units of the submerged piston type, with gravelly sand transplanted with Schoenoplectus americanus, presented efficiency for the removal of As from water of 90%. Artificial wetland systems with a combination of plants tolerating specifically As and a gravel matrix have the potential to remove arsenic from contaminated water. The use of artificial wetlands as an alternative for the removal of arsenic from water is an important technique that has proven useful in prototype systems, and should be considered for pilot projects working at lower concentrations and aimed at treating water intended for human consumption.


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REFERENCES EPA (2002). Arsenic treatment technologies for Soil, waste, and Water. Solid waste and emergency response, EPA/542-R-02-004. U.S. Environmental Protection Agency: Washington, DC. Jiang, J.Q. (2001). Removing Arsenic from Groundwater for the Developing World. Water Sci. and Tech. 44 (6), 89-98. NOM-127-SSA1 (2000). Salud Ambiental, Agua para uso y Consumo Humano-Límites Permisibles de Calidad y Tratamientos a que Debe Someterse el Agua para su Potabilización. Norma Oficial Mexicana. S. Buddhawong, P. Kuschk, J. Mattusch, A. Wiessner, U. Stottmeister (2001). Removal of Arsenic and Zinc Using Different Laboratory Model Wetland Systems. Engineering in Life Sciences, 5( 3), 247 – 252. SW-846 EPA Method 3015 (1995) Microwave assisted acid digestion of aqueous sample and extracts In Test Methods for Evaluating Solid Waste, 3rd edition, 3rd update; U.S. Environmental Protection Agency, Washington, DC. SW-846 EPA Method 3051(1995) Microwave assisted acid digestion of sediments, sludges, soils, and oils In Test Methods for Evaluating Solid Waste, 3rd edition, 3rd update; U.S. Environmental Protection Agency, Washington, DC.

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10th International Conference on

Wetland Systems for Water Pollution Control September 23-29, 2006 Lisbon, Portugal

Volume I Pages 1 – 672 Consisting of: Key Note Papers Nutrients Removal Effluent Reuse and Pathogen Removal Heavy Metals Subsurface Flow Wetlands Systems (Oral Presentations) Hosted By:

Ministry for Environment, Spatial Planning and Regional Development


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ISBN: 989-20-0361-6 ISBN (13 dígitos): 978-989-20-0361-0 Depósito Legal: 247410/06 10th International Conference on Wetland Systems for Water Pollution Control, 23-29 September, Lisbon, Portugal. Vol. I Conference organized by MAOTDR (Ministério do Ambiente, do Ordenamento do Território e do Desenvolvimento Regional). Editorial Coordination: Veríssimo Dias and Jan Vymazal Editorial Secretariat: Eduardo Soutinho, Carla Canseiro, Bruno Correia, Ana Rita Gomes and Catarina Bicho. Copyright to MAOTDR 2006. Printed in PAC – Artes Gráficas, Lda. Rua Piteira Santos, 5 Parque Industrial de Vale Flores, Feijó, 2810-350 Almada, Portugal.

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Use of an Auto-Fluorescent E.Coli to Characterize Constructed Wetlands Pathogen Removal Efficiency............................................................................................................................................454 Sleytr, K.; Tietz, A.; Langergraber, G.; Haberl, R. Austria

Disposal of Pathogenic Organisms of the Sewage Treated with One Wetland of Type M.J.E.A® in Léon (Spain) ……….......…......................................................................................................................….455 Martínez, P. E. M.; Ansola, G.; Luis, E. Spain

Microbial Performance of a Combined Constructed Wetland Treating Diluted Wastewater.......................456 Tapia, R. R.; Torres, A.; Álvarez, J.A.; Bécares, E. Spain

IV Heavy Metals - Oral Presentations…………………………………………….…........………….…………457 Heavy Metals and Some Risk Elements in Plants Growing in a Constructed Wetland Receiving Municipal Wastewater……....................................................................................................……459

Vymazal, J.; Kröpfelová, L.; Švehla, J.; Chrastný, V. Czech Republic

Artificial Wetlands for Arsenic Removal……………………..........................................................………………….469 Alarcón-Herrera, M.T.; Flores-Tavizón, E.; Martín-Domínguez, I. R.; Benavides-Montoya, A. Mexico

Classical Subsurface Flow Wetland Optimization to Heavy Metal Removal……….............................……….477 García, T.; Angarit, S.M.; Rodriguez, M. S. Colombia

Heavy Metal and Nutrient Concentration Variation in Typha latifolia l. Phytomass in Wastewater Treatment Wetlands in Estonia............................................................................................................487 Maddinson, M.; Soosaar, K.; Lõhmus, K.; Mander, Ü. Estonia

Effects of Type of Flow, Plants and Addition of Organic Carbon in the Removal of Zinc and Chromium in Small-Scale Model Wetlands..........................................................................................................497 Paredes, D.; Vélez, M.E.; Kuschk, P.; Mueller, R. A. Germany

IV Heavy Metals - Poster Presentations…………………………………………...………………….….……...507 Biological Metal Removal in an Engineering Wetland System……………...........................................….………509

Mattes, A.; Higgins, J.; Gould, W.D. Canada

Study on Effective Desorbing Agents for the Recovery of Copper from Loaded Algae……......................…521 Freitas, O. M.; Delerue-Matos, C.; Boaventura, B. Portugal

Application of Response Surface Methodology to Adsoprtion of a Basic Dye by…....................................….531 Bentonitic Clay Santos, S. C. R.; Oliveira, A. F. M.; Boaventura, R. A. R. Portugal


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