Removal of trace elements in three horizontal sub-surface flow constructed wetlands in the Czech Republic

Environmental Pollution 157 (2009) 1186–1194

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Removal of trace elements in three horizontal sub-surface flow constructed wetlands in the Czech Republic Lenka Kro¨pfelova´ a, Jan Vymazal a, b, *, Jaroslav Sˇvehla c, Jana Sˇtı´chova´ c a

´ 145, 379 01 Trˇebonˇ, Czech Republic ENKI o.p.s., Dukelska Czech University of Life Sciences in Prague, Faculty of Environmental Sciences,  ch 1, 281 63 Kostelec nad Cern mi lesy, Czech Republic ´meˇstı´ Smirˇicky Department of Landscape Ecology, Na y c  ´ Budeˇjovice, Faculty of Agriculture, Department of Chemistry, University of South Bohemia in Ceske  ´ 13, 370 05 Ceske ´ Budeˇjovice, Czech Republic Studenstka b

The paper describes the removal of trace elements in constructed wetlands treating municipal sewage.

a r t i c l e i n f o

a b s t r a c t

Article history: Received 24 August 2008 Received in revised form 6 December 2008 Accepted 9 December 2008

Between March 2006 and June 2008 removal of 34 trace elements was measured on a monthly basis at three horizontal-flow constructed wetlands in the Czech Republic designed to treat municipal wastewater. In general, the results indicated a very wide range of removal efficiencies among studied elements. The highest degree of removal (average of 90%) was found for aluminum. High average removal was also recorded for zinc (78%). Elements removed in the range of 50–75% were uranium, antimony, copper, lead, molybdenum, chromium, barium, iron and gallium. Removal of cadmium, tin, mercury, silver, selenium and nickel varied between 25 and 50%. Low retention (0–25%) was observed for vanadium, lithium, boron, cobalt and strontium. There were two elements (manganese and arsenic) for which average outflow concentrations were higher compared to inflow concentrations. Reduced manganese compounds are very soluble and therefore they are washed out under anaerobic conditions. Ó 2008 Elsevier Ltd. All rights reserved.

Keywords: Constructed wetlands Czech Republic Municipal wastewater Sub-surface flow Trace elements

1. Introduction Retention and/or release of trace elements in natural as well as constructed wetlands are affected by many factors (Vymazal and Kro¨pfelova´, 2008). However, the most important factors known to influence mobility of trace elements in wetlands are sulfide and Fe/ Mn hydrous oxide formation and dissolution (Khalid et al., 1978). Under reducing conditions many trace elements, including heavy metals, react with hydrogen sulfide to form highly insoluble metal sulfides (Krauskopf, 1956; Stumm and Morgan, 1981; Kosolapov et al., 2004) such as FeS2 (pyrite), FeS (pyrrhotite), PbS (galena), CdS, CuS (covellite), CuS2 (chalcocite), CuFeS2 (chalcopyrite), NiS or ZnS (sphalerite). These compounds are very stable and insoluble under anaerobic conditions. However, under oxidized conditions the oxidation of sulfide to sulfate will release these metals into the water (Gardiner, 1974; Engler and Patrick, 1975). This may occur, for example, as a consequence of oxygen release from plant roots in the rhizosphere (Engler and Patrick, 1975; Gambrell et al., ˇ , Czech * Corresponding author. ENKI o.p.s., Dukelska´ 145, 379 01 Trˇebon Republic. Tel.: þ420 233 350 180; fax: þ420 384 724 346. E-mail addresses: [email protected] (L. Kro¨pfelova´), [email protected] (J. Vymazal), [email protected] (J. Sˇvehla), [email protected] (J. Sˇtı´chova´). 0269-7491/$ – see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.envpol.2008.12.003

1980; Holmer et al., 1998; Wood and Shelley, 1999; Jacob and Otte, 2003). On the other hand, while hydrous Fe and Mn oxides adsorb or co-precipitate trace metals under oxidizing conditions, these metals may be released after Fe/Mn hydrous oxide reduction under reducing conditions (Brooks et al., 1968; Jenne, 1968). If hydrogen sulfide is present, then these metals can form insoluble sulfides. The effect of sulfide and oxides, and hydroxides of Fe and Mn in controlling the solubility of trace metals in the sediment–water system is greatly modified by the presence of organics (Morel et al., 1973; Gambrell, 1994). Under anaerobic conditions, organic compounds can bring about reductive dissolution of metal oxides from a higher to a lower oxidation state. This reduction has a dramatic impact on the solubility and speciation of metals. For example, Mn(III,IV), Fe(III), Co(III) and Ni(III) oxides, when reduced to divalent ions under anoxic conditions, show an increase in solubility by several orders of magnitude (Stone and Morgan, 1987). It is very difficult to single out the specific effects of certain sediment components in such a complex system (Khalid et al., 1978). Constructed wetlands have been quite often used to treat various coal mine drainage waters (Brodie et al., 1988; Wieder, 1989; Wood and Cook, 1992; Mitsch and Wise, 1998; Barton and Karathanasis, 1999; Ramı´rez Masferrer, 2002; Karathanasis and

´ et al. / Environmental Pollution 157 (2009) 1186–1194 ¨pfelova L. Kro

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Fig. 1. Constructed wetland Morˇina. Photo Jan Vymazal.

Johnson, 2003; Ziemkiewicz et al., 2003) where the contaminants of interest are typically pH, sulfate, aluminum, iron and manganese. Also, constructed wetlands have been used to treat drainage waters from uranium mines (Kiessig et al., 2003; Overall and Parry, 2004), gold mines (Bishay and Kadlec, 2005) or various heavy metal ore mines (Sobolewski, 1996; O’Sullivan et al., 2004). However, all these application were constructed wetlands with free water surface (FWS CWs) relying mostly on Fe/ Mn precipitation and adsorption/ co-precipitation of other elements. Constructed wetlands with horizontal sub-surface flow (HF CWs) are commonly used for various types of wastewater such as municipal sewage, wastewaters from food processing, abattoir, pulp and paper production, textile industry, agriculture or landfill leachate (Kadlec and Knight, 1996; Vymazal et al., 1998; Vymazal and Kro¨pfelova´, 2008). However, the use of HF CWs with trace elements as the major target of treatment is limited. Pantano et al. (2000) reported the use of HF CWs to treat mining impacted groundwater with elevated metal concentrations in Butte, MT, USA. The wetlands were effective in removing Cd, Zn and Cu while arsenic was released from the system and lead concentrations were not affected by the wetland. Gerth et al. (2005) used a hybrid FWS-HF CW to treat seepage water from a uranium mine in Achlema-Alberoda, Germany. The authors pointed out that different conditions are needed for removal of arsenic (aerobic) and uranium (anaerobic). The major purpose of this study was to evaluate the removal of trace elements from municipal sewage in three HF constructed

wetlands in the Czech Republic. Trace elements usually do not occur in elevated concentrations in sewage from small settlements but information on the treatment efficiency is missing in the literature.

2. Materials and methods HF constructed wetlands at Morˇina (Fig. 1), Brˇehov (Fig. 2) and Slavosˇovice (Fig. 3) have all been designed to treat municipal wastewater. Major design parameters together with the treatment performance for organics, suspended solids and sulfate are shown in Table 1. Time-proportional composite 24-hr samples (250 ml) of inflowing and outflowing water were manually taken on a monthly basis between March 2006 and June 2008 at all constructed wetlands. The samples were mineralized with HCl and HNO3 (APHA, 1998), diluted to 100 ml and analyzed for Ag, Al, As, B, Ba, Be, Cd, Co, Cr, Cu, Fe, Ga, Hf, Ir, Li, Mn, Mo, Ni, Pd, Pb, Pt, Rb, Rh, Ru, Sb, Se, Sn, Sr, Te, Tl, V, U and Zn using an Inductively Coupled Plasma Mass Spectrometer PQ-ExCell (VG-Thermo Elemental, Winsford, Cheshire, UK). Isobaric interferences on As and Se mass were eliminated by using Collision-Reaction Cell Technology (As) and by selecting the more suitable isotope of selenium (80Se). Stability of the signal (in counts per second) was maintained by the continual addition of internal standard (scandium, yttrium, indium and therbium). For all measurements presented, standard quality control (QC) was performed. Quality control samples consist of triplicate samples and spiked samples. For the evaluation of measurement precision and accuracy, the standard material ‘‘SPS-WW1 Batch 108 – Reference Material for Measurement of Elements in Wastewaters’’ from Spectrapure Standards a.s., Oslo, Norway, was used. The reproductibilities, expressed as a relative standard deviation of the QC samples, were less than 5%. The recoveries between found and certified values were for all measurements in the range of 85 – 110%. Samples for Hg were analyzed without mineralization with the cold vapor method by using the analyzer AMA 254 (ALTEC, Praha, Czech Republic).

Fig. 2. Constructed wetland Brˇehov. Photo Jan Vymazal.

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´ et al. / Environmental Pollution 157 (2009) 1186–1194 ¨ pfelova L. Kro

Fig. 3. Constructed wetland Slavosˇovice. Photo Jan Vymazal.

3. Results and discussion Out of 34 monitored elements, concentrations of nine elements (Be, Hf, Ir, Pd, Pt, Rh, Ru, Te and Tl) were mostly below the detection limits both in inflow and outflow, and therefore these elements were not included in the further evaluation. Removal of studied elements in all three constructed wetlands is presented in Tables 2–4 and average removal efficiencies are shown in Table 5. The results indicated that elements are retained with various efficiencies, ranging from 90% (aluminum) to 57% (arsenic). The highest removal was recorded in all three CWs for aluminum. High Al removal in HF CWs was also reported by Vymazal and Kra´sa (2003) and Lesage (2006) from the Czech Republic and Belgium, respectively. Vymazal and Kra´sa (2003) reported a decrease in Al concentration in CW at Nucˇice from 450 to <40 mg l1 (>91.1% removal). Lesage (2006) reported a decrease in Al concentration in HF CWs at Zemst-Kesterbeek from 1218 to 86 mg l1 resulting in 92.9% removal. Aluminum can form insoluble compounds through hydrolysis and/or oxidation which leads to the formation of a variety of oxides, oxyhydroxides and hydroxides, and these compounds are least soluble at a pH of 7.0, i.e. neutral range, which is common for municipal wastewaters. Also, zinc was retained effectively with an average retention of 78%. This is in accordance

with previously reported results from the Czech Republic (Vymazal and Kra´sa, 2003), Belgium (Lesage, 2006) or USA (Gersberg et al., 1984). High removal of zinc was also reported by Eckhardt et al. (1999) and Mæhlum et al. (1999) for HF CWs designed to treat landfill leachate in USA and Norway, respectively. Zinc forms very insoluble compounds with sulfide and carbonate, and also zinc is co-precipitated in Fe and Mn oxides (Stumm and Morgan, 1981). Zinc is also reported to be retained on iron plaques at the surface of plant roots (Otte et al., 1995). Despite wide variation in inflow concentrations, the outflow concentrations of both aluminum (Fig. 4) and zinc (Fig. 5) were quite stable during the 28-month period. Elements which were removed in the range of 50–75% were uranium (72%), antimony (72%), copper (67%), lead (63%), molybdenum (56%), chromium (55%), barium (54%), iron (53%) and gallium (51%). In the literature, there is no information available for removal of antimony, barium, molybdenum and gallium in HF CWs and, therefore, it is difficult to evaluate our results. Groudev et al. (2000) reported high removal of uranium from tailing ponds drainage water using natural wetland in Bulgaria. Also, Gerth et al. (2005) reported that constructed wetlands could be useful for treatment of seepage from uranium mining sites in Germany. In Fig. 6, removal of uranium in CW Brˇehov is shown. Inflow uranium

Table 1 Major design parameters and treatment performance during the period (2006–2008) of constructed wetlands Morˇina, Brˇehov and Slavosˇovice in the Czech Republic. Morˇina

Brˇehov

Slavosˇovice

Start of operation Design PEa Type of sewerage Pretreatment

7/2000 700 Separate Grit chamber, Imhoff tank

7/2001 150 Combined Grit chamber, Imhoff tank

Vegetated beds area (m2) Number of beds Sealing Specific area (m2 PE1) Average flow (m3 day1)b Hydraulic loading rate (cm day1) Filtration material (size in mm) Plantsc Treatment performance (in/out, mg l1)b BOD5 COD Suspended solids SO2 4

3520 4 (2 series in parallel) PVC 5.0 108 3.1 Crushed rock (4–8) Phragmites Phalaris

10/2003 100 Combined Grit chamber, septic tank 504 2 (parallel) PVC 5.0 16 3.2 Gravel (4–8) Phragmites Phalaris

318/47 716/134 398/10.5 141/56

190/19 480/117 176/20 37/21

78/13.2 200/66 54/18 41/14

a b c

PE, population equivalent. During the monitored period. Phragmites australis (Common reed), Phalaris arundinacea (Reed canarygrass).

983 2 (parallel) Clay 6.6 34 3.5 Gravel (3–20) Phragmites

´ et al. / Environmental Pollution 157 (2009) 1186–1194 ¨pfelova L. Kro Table 2 Removal of trace elements in constructed wetland Brˇehov during the period of March 2006 –June 2008.

Al Zn Sb Cu Pb Ba Cd U Cr Ga Mo Ag Fe Ni Se Hg Rb Sn Li Co V B Sr Mn As

Inflow (mg l1)

SD

Outflow (mg l1)

SD

Efficency (%)

3748 186 1.19 40.7 13.2 479 0.33 4.69 11.3 36.3 0.94 2.32 2417 22.4 1.25 0.18 203 1.59 7.3 2.46 6.34 110 348 202 2.45

4358 198 1.34 43.0 15.1 1390 0.29 2.49 11.2 59.6 0.80 4.46 2860 27.8 1.09 0.17 385 1.65 3.4 1.41 7.9 66 462 104 1.42

328 26 0.18 6.5 2.9 124 0.10 1.44 3.66 11.9 0.35 1.01 1072 12.1 0.71 0.11 143 1.14 5.4 1.87 4.85 86 334 245 3.80

233 23 0.26 8.3 3.8 52 0.12 1.41 3.15 5.88 0.27 1.63 1380 19.7 0.66 0.12 217 3.48 2.3 1.24 4.7 34 423 121 1.89

91.2 86.0 84.9 84.0 78.0 74.1 69.7 69.3 67.6 67.2 62.8 56.5 55.6 46.0 43.2 38.9 29.6 28.3 26.0 24.0 23.5 21.8 4.0 21.9 55.1

The values represent average values and standard deviations (SD) for the whole period.

concentrations were occasionally quite high, but uranium occurs naturally in this area as a consequence of the local geological conditions. Removal of copper, lead and chromium has been well reported. Obarska-Pempkowiak (2000, 2003) reported 58.2% removal of copper in Przywidz, Poland. Other studies from HF CWs, such as those by Junsan et al. (2000) from China, Lesage (2006) from Belgium or Gersberg et al. (1984) from the USA, indicated higher Cu removal (>90%). Copper forms very insoluble

Table 3 Removal of trace elements in constructed wetland Morˇina during the period of March 2006 –June 2008 (for details see Table 2).

Al Sn Zn Pb Sb Cd U Cu Mo Cr Ga Ba Fe Ni V Rb Se Hg Ag B Sr Li Co Mn As

Inflow (mg l1)

SD

Outflow (mg l1)

SD

Efficiency (%)

5658 5.55 232 15.6 0.64 0.32 1.93 25.2 1.18 6.75 7.78 68.5 930 17.5 2.96 31.6 0.96 0.17 1.16 210 840 15.1 1.31 85 1.15

9635 9.4 172 25.4 0.95 0.30 0.38 17.5 1.51 7.61 4.94 37.2 1140 30.4 4.11 42.9 0.85 0.21 0.73 151 1020 5.3 0.73 57 0.73

84 0.25 22 2.46 0.13 0.07 0.47 6.6 0.32 1.99 2.52 26.2 460 8.9 1.54 21.1 0.63 0.12 0.85 157 726 13.2 1.30 101 1.56

87 0.46 30 2.86 0.17 0.17 0.73 12.7 0.39 2.43 1.17 15.2 700 12.4 2.44 26.1 0.70 0.11 0.85 40 900 3.9 0.90 82 1.39

98.5 95.5 90.5 84.2 79.7 78.1 75.6 73.8 72.9 70.5 67.6 61.8 50.5 49.1 48.0 33.2 34.4 29.4 26.7 25.1 13.6 12.6 0.76 18.8 35.7

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compounds with sulfur, including both cupric (CuS) and cuprous (Cu2S) sulfides (Sobolewski, 1999). Copper is also strongly influenced by adsorption and/or co-precipitation with Fe and Mn oxides. In addition, Cu may be retained in wetlands by complexation by recalcitrant fractions of plant litter or other organic matter types (Du Laing et al., 2006). Lead can form insoluble sulfide (PbS) under anaerobic conditions, but carbonate precipitation may be effective for the removal of lead as well (Lin, 1995). Obarska-Pempkowiak (2000, 2003) reported a comparable removal (53.5%) of lead in HF CW in Przywidz, Poland. Lesage (2006) reported removal of Pb higher than 90% in two HF CWs in Belgium. Also Vymazal (2003) reported high Pb removal (98%) in Nucˇice, Czech Republic. The solubility of chromium is strongly dependent upon its oxidation state, Cr(VI) being more soluble than Cr(III). Cr(III), like other cationic metals, is rapidly adsorbed by Fe and Mn oxides in any clay minerals (Schroeder and Lee, 1975; Bartlett and Kimble, 1976; Dreiss, 1986). Average chromium removal amounted to 52%, which is comparable with removal of 55% in Zemst-Kesterbeek, Belgium, (Lesage, 2006) but much lower than 94% and 84% reported from Hasselt-Kiewit, Belgium (Lesage, 2006) and Putignano, Italy (Ranieri, 2004), respectively. Ranieri (2004) found out that about 90% of the inflow Cr was bound into sediments. For iron, only results from Morˇina and Brˇehov were taken into consideration. At Slavosˇovice, extremely high Fe concentrations were recorded in the ouflow (Table 4) because of leaching from clay material which is used to seal the beds. Fe content of the clay is about 22 g kg1 and laboratory experiments showed that under anaerobic conditions Fe is washed out from the clay (results not shown). An average removal based on two systems was 53%, but the results reported in the literature for HF CWs vary widely. Vymazal (2003) reported removal of 34% in CW Nucˇice in the Czech Republic. Mæhlum et al. (1999) reported removal of 8% and 80% for two landfill leachate CWs in Norway, while Eckhardt et al. (1999) reported Fe removal of 91% for a landfill leachate system in New York state, USA. On the other hand, Lesage (2006) reported an increase in Fe concentration from 718 mg l1 in the inflow to 1224 mg l1 at the outflow in Zemst-Kesterbeek, Belgium. Iron

Table 4 Removal of trace elements in constructed wetland Slavosˇovice during the period of March 2006 –June 2008 (for details see Table 2).

Al U Zn Sb Hg Cu Mo Ag Cr Se Ba Pb Li Ga Sn Rb V Sr Co Cd B Ni Mn As Fea a

Inflow (mg l1)

SD

Outflow (mg l1)

SD

Efficency (%)

938 2.94 72 0.15 0.19 25.2 0.40 1.93 2.88 0.65 72.2 3.66 4.65 7.26 0.50 64.6 2.90 278 1.22 0.10 76 5.84 192 1.39 980

1045 1.05 64 0.19 0.54 15.9 0.25 6.54 2.98 0.78 19.2 3.32 2.15 3.15 0.60 92 3.45 421 0.93 0.09 46 9.2 122 1.36 550

186 0.81 30 0.075 0.10 14.7 0.27 1.35 2.10 0.50 53.2 2.72 3.81 6.05 0.42 59.8 2.90 279 1.22 0.10 77.4 6.54 241 2.49 4077

1282 1.29 32 0.10 0.35 25.7 0.18 1.71 2.70 0.64 23.5 3.60 2.10 3.08 0.54 78 3.68 373 1.24 0.11 31.2 9.3 155 1.12 3500

80.2 72.4 58.3 50.0 47.4 41.7 32.5 30.1 27.1 23.1 26.3 25.7 18.1 16.7 16.0 7.4 0 0 0 0 1.8 12.0 25.5 79.1 358

Filtration beds are sealed with clay with high Fe content (22 g kg1).

´ et al. / Environmental Pollution 157 (2009) 1186–1194 ¨ pfelova L. Kro

1190 Table 5 Average removal efficiency ( %) for studied elements. Element

Efficiency

Element

Efficiency

Element

Efficiency

Element

Efficiency

Aluminum Zinc Uranium Antimony Copper Lead Molybdenum

90.0 78.3 72.4 71.5 66.5 62.6 56.1

Chromium Barium Irona Gallium Cadmium Tin Mercury

55.1 54.1 53.1 50.5 49.3 46.6 38.6

Silver Selenium Nickel Vanadium Rubidium Lithium Boron

37.8 33.6 27.7 23.8 23.4 18.9 15.0

Cobalt Strontium Manganese Arsenic

8.3 5.9 22.1 56.6

The values represent average treatment efficiencies from three monitored systems. a Results from Slavosˇovice not taken into consideration (see text for explanation).

outwash from HF CWs filled with local sands and gravels seems to be a common phenomenon, especially in relation to P removal capacity of filter materials (Vohla et al., 2005). As mentioned above, when Fe is a target element for the removal, free water surface systems are more effective because of predominant aerobic conditions which enhance iron precipitation. Cadmium, tin, mercury, silver, selenium and nickel, were retained in the range of 25–50%. The results for tin, silver and selenium are reported probably for the first time for HF CWs. The results indicate only mild retention of these elements. Cadmium removal averaged 49%, a value slightly higher than removal reported by Obarska-Pempkowiak (2000, 2003) from Poland (51%) and lower than that reported by Vymazal (2003) from Nucˇice, Czech Republic (77%). Cadmium forms insoluble sulfides and carbonates and it also may co-precipitate with Al, Mn and Fe oxides. Mercury inflow concentrations in monitored CWs varied between 0.08–0.15 mg l1 and the average removal was rather low, 35.8%. This is substantially less than the removal values reported from Belgium – 87% and 84% in Hasselt-Kiewit and Zemst-Kesterbeek, respectively (Lesage, 2006). Nickel very often co-precipitates with iron and manganese hydroxides and oxyhydroxides. It also forms insoluble sulfide under anaerobic conditions and carbonate precipitation may be a substantial retention mechanisms as well (Vymazal and Kro¨pfelova´, 2008). Also for nickel, removal efficiencies in HF CWs vary widely, but in general higher removal values have been reported. Vymazal (2003) reported the removal of 93% in

16000 14000

Nucˇice, Czech Republic, Ranieri (2004) found removal of 77% in Putignano, Italy, and Lesage (2006) found similar removal of 81% in Zemst-Kesterbeek, Belgium. Eckhardt et al. (1999) reported removal of 50% in a CWs designed to treat landfill leachate in the USA. Removal of vanadium, rubidium, lithium, boron, cobalt and strontium was very low (Table 5). In the literature, there is no information on the removal of these elements in full-scale HF CWs and, therefore, it is not possible to make any comparisons. Interestingly, lithium was retained to some extent. This is surprising, as lithium is often used as tracer for hydraulics studies. Despite the fact that some elements were washed out of the system (negative removal) in individual systems (Tables 2–4), on average only manganese and arsenic exhibited negative removal (Table 5, Fig. 7). This has also been reported in the literature from other HF CWs. Lesage (2006) reported an increase of Mn concentration from 79 mg l1 to 169 mg l1 in Zemst-Kesterbeek and Vymazal (2005) reported an increase of Mn concentration from 109 mg l1 to 402 mg l1 in Morˇina. Similar results were reported for arsenic. Lesage (2006) reported an increase of As concentration in Zemst-Kesterbeek from 0.71 mg l1 to 5.6 mg l1 and in Hasselt-Kiewit from 3.0 mg l1 to 11.0 mg l1. Also, Byekwaso et al. (2002) reported release of arsenic from an HF bed treating an effluent from Kasese Cobalt Company’s cobalt recovery processing plant in Uganda. Transformations of Mn and As in wetland soils and sediments are controlled by oxidation– reduction conditions. Under aerobic conditions manganese (oxy-)

Inflow Outflow

10000 8000 6000 4000 2000 0 3. 0 4. 6 0 5. 6 0 6. 6 0 7. 6 0 8. 6 0 9. 6 0 10 6 . 11 06 . 12 06 .0 1. 6 0 2. 7 0 3. 7 07 4. 0 5. 7 0 6. 7 0 7. 7 0 8. 7 0 9. 7 0 10 7 .0 11 7 . 12 07 .0 1. 7 0 2. 8 0 3. 8 0 4. 8 0 5. 8 08 6. 08

Al concentration (µg/l)

12000

Date of sampling Fig. 4. Aluminum retention in CW Brˇehov during the period March 2006 – June 2008.

´ et al. / Environmental Pollution 157 (2009) 1186–1194 ¨ pfelova L. Kro

1191

1100 1000

Inflow Outflow

Zn concentration (µg/l)

900 800 700 600 500 400 300 200 100 3. 06 4. 06 5. 06 6. 06 7. 06 8. 06 9. 0 10 6 .0 11 6 .0 12 6 .0 1. 6 07 4. 07 5. 07 6. 07 7. 07 8. 07 9. 0 10 7 .0 11 7 .0 12 7 .0 1. 7 08 2. 08 3. 08 4. 08 5. 08 6. 08

0

Date of sampling Fig. 5. Zinc retention in CW Brˇehov during the period March 2006 – June 2008.

hydroxides precipitate but under anoxic/anaerobic conditions Mn4þ is reduced to Mn2þ which is quite soluble. As manganese does not readily form an insoluble sulfide phase, it may be washed out of the system (Ponnamperuma et al., 1969; Stumm and Morgan, 1981; Hallberg and Johnson, 2005). In addition, the supplementary experiments revealed that manganese was easily washed out from the filtration material (crushed rock) in Morˇina. At higher soil redox levels (þ200 – þ500 mV), As5þ is the predominant As species. It may also co-precipitate with iron oxides (Otte et al., 1989; St-Cyr and Campbell, 1996). The reduction of As5þ to As3þ occurs at redox levels corresponding to the nitrate reducing zone of soils, characterized by a soil redox level of approximately þ200 – þ300 mV (DeLaune et al., 1998). Upon reduction, As3þ becomes the major As species in solution, and As solubility increases (Ferguson and Gavis, 1972; Pierce and Moore, 1982; Hess and Blanchar, 1976; Masscheleyn et al., 1991a,b). The possible explanation for higher concentrations of As in the outflow, seems to be the accumulation of As during the early operation of constructed wetlands when the beds are at least partially aerobic, but once the beds become more anoxic/anaerobic,

12.0

Inflow Outflow

10.0 8.0 6.0 4.0 2.0 0.0 3. 0 4. 6 0 5. 6 0 6. 6 0 7. 6 0 8. 6 0 9. 6 0 10 6 .0 11 6 . 12 06 .0 1. 6 0 2. 7 0 3. 7 0 4. 7 0 5. 7 0 6. 7 0 7. 7 0 8. 7 0 9. 7 0 10 7 .0 11 7 . 12 07 .0 1. 7 0 2. 8 0 3. 8 0 4. 8 0 5. 8 0 6. 8 08

Uranium concentration (µg/l)

14.0

As is reduced and washed out. The analysis of sediment within the filtration material in monitored as well as other constructed wetlands (results not shown) revealed relatively large arsenic pool in the sediments. When the constructed wetlands are compared (Tables 2–4), it is obvious that the removal efficiency was similar in Brˇehov and Morˇina, while in Slavosˇovice the removal efficiency was much lower. It is well known that inflow concentrations may affect the removal efficiency especially when adsorption is taken into account. Data in Tables 2–4 revealed that inflow concentrations of most elements were substantially higher in Morˇina and Brˇehov than in Slavosˇovice. Both Morˇina and Brˇehov constructed wetlands are located in areas with higher natural background concentrations of many trace elements due to the occurrence of various metal ores. In the Morˇina area, there are several ore mines that have been abandoned for centuries. Also, very low inflow concentrations of organics (BOD5, COD) and suspended solids, and lower build up of sediments in the filtration bed in Slavosˇovice may affect the removal efficiency. On average, sediment concentration (on dry

Sampling date Fig. 6. Uranium retention in CW Brˇehov during the period March 2006 – June 2008.

´ et al. / Environmental Pollution 157 (2009) 1186–1194 ¨ pfelova L. Kro

1192

12.0

As concentration (µg/l)

10.0

Inflow Outflow

8.0 6.0 4.0 2.0

3. 0 4. 6 0 5. 6 0 6. 6 0 7. 6 0 8. 6 0 9. 6 0 10 6 .0 11 6 . 12 06 .0 1. 6 0 2. 7 0 3. 7 0 4. 7 0 5. 7 0 6. 7 0 7. 7 0 8. 7 0 9. 7 0 10 7 .0 11 7 . 12 07 .0 1. 7 0 2. 8 0 3. 8 0 4. 8 0 5. 8 0 6. 8 08

0.0

Sampling date Fig. 7. Inflow and outflow As concentrations in CW Brˇehov during the period March 2006 – June 2008.

Table 6 Czech surface water quality standards (in mg/l) (Narˇı´zenı´ vla´dy, 2007). Element

Concentration Element

Iron 2000 Aluminum 1500 Manganese 500 Antimony 500 Boron 500 Zinc 160

Tin Nickel Uranium Vanadium Molybdenum Copper

Concentration Element 50 40 40 35 35 25

Concentration

Arsenic 20 Lead 14.4 Cobalt 7 Silver 7 Selenium 4 Mercury 0.1

mass basis) in Slavosovice was about 50% lower than in Brˇehov and Morˇina (results not shown). As a consequence, this may lead to lower retention of trace elements via complexation with organic compounds. However, the high sulfate reduction in all systems indicated that anaerobic conditions probably prevailed and formation of insoluble sulfides (indicated by the black color of the sediment) could have been the major retention mechanism. In Table 6, the Czech surface water quality standards are shown. The data in Tables 2–4 indicate that the average outflow concentrations of monitored elements, with two exceptions, are well below these standards. The first element with an outflow concentration that exceeded the surface water standard was iron in Slavosˇovice where it is leached out of the clay material which is used to seal the bed (Table 4). The other element concerned was mercury, which had average outflow concentrations either slightly exceeding (Tables 2 and 3) or leveling with the standards. However, it is important to realize that the flows of all receiving streams were substantially higher than the flow of wastewater and, therefore, a necessary dilution to acceptable mercury concentrations in the streams was provided. 4. Conclusions In three constructed wetlands with horizontal sub-surface flow in the Czech Republic, removal of 34 elements was monitored for 28 months. The results revealed a wide range of average removal efficiencies, ranging between 90% for aluminum and 57% for arsenic. High average removal was also recorded for zinc (78%), and elements removed on average in the range of 50–75% were uranium (72%), antimony (72%), copper (67%), lead (63%), molybdenum (56%), chromium (55%), barium (54%), iron (53%) and gallium (51%). Removal of cadmium, tin, mercury, silver, selenium, and nickel varied between 25 and 50%. Low retention (0–25%) was

observed for vanadium, rubidium, lithium, boron, cobalt and strontium. There were two elements – manganese and arsenic – for which average outflow concentrations were higher compared to inflow concentrations. Reduced manganese and arsenic compounds are very soluble and therefore they may be washed out under anaerobic conditions. In addition, manganese can apparently be washed out from the crushed rock. This phenomenon has also been described in many natural wetlands. The results indicated that three monitored HF constructed wetlands could be a very useful tool for the removal of trace elements such as aluminum, zinc or copper, but at the same time it seems that some trace elements, such as selenium and cobalt, are not retained efficiently. For several trace elements such as tin, barium, silver or vanadium there is a lack of results on their retention in HF CWs and therefore further research is needed to evaluate their retention in HF constructed wetlands. The results, however, cannot be generalized and to draw general trends it would be necessary to monitor more HF constructed wetlands with different design parameters. However, as the design of monitored HF CW is very common in the Czech Republic, the conclusions may have wider application there. Acknowledgements The study was supported by grants No. 206/06/0058 ‘‘Monitoring of Heavy Metals and Selected Risk Elements during Wastewater Treatment in Constructed Wetlands’’ from the Czech Science Foundation, No. 2B06023 ‘‘Development of Mass and Energy Flows Evaluation in Selected Ecosystems’’ and No. ZF JU-MSM 6007665806 ‘‘Sustainable Methods in Agricultural Operations in Submontane and Mountainous Regions Aimed at Harmonization of Their Production and Extraproduction Functions’’ from the Ministry of Education, Youth and Sport of the Czech Republic. References APHA, 1998. Standard Methods for the Examination of Water and Wastewater, 20th edition. American Public Health Association, Washington, DC. Bartlett, R.J., Kimble, J.M., 1976. Behavior of chromium in soils. I. Trivalent forms. Journal of Environmental Quality 5, 370–383. Barton, C.D., Karathanasis, A.D., 1999. Renovation of a failed constructed wetland treating acid mine drainage. Environmental Geology 39, 39–50. Bishay, F., Kadlec, R.H., 2005. Wetland treatment at Musselwhite mine, Ontario, Canada. In: Vymazal, J. (Ed.), Natural and Constructed Wetlands: Nutrients, Metals and Management. Backhuys Publishers, Leiden, The Netherlands, pp. 176–198.

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