N. Seelsaen*, R. McLaughlan**, S. Moore***, J.E. Ball** and R.M. Stuetz* *Centre for Water and Waste Technology, School of Civil and Environmental Engineering, The University of New South Wales, Sydney, NSW 2052, Australia (E-mail:
[email protected]) **Infrastructure and Environment Group, Faculty of Engineering, The University of Technology, Sydney, NSW 2007, Australia ***School of Civil and Environmental Engineering, The University of New South Wales, Sydney, NSW 2052, Australia Abstract Sorption experiments were used to assess the ability of various materials (sand, compost, packing wood, ash, zeolite, recycled glass and Enviro-media) to remove heavy metal contaminants typically found in stormwater. Compost was found to have the best physicochemical properties for sorption of metal ions (Cu, Zn and Pb) compared with sand, packing wood, ash, zeolite and Enviro-media. The compost sorption of these metal ions conformed to the linear form of the Langmuir adsorption equation with the Langmuir constants (qm) for Zn(II) being 11.2 mg/g at pH 5. However, compost was also found to leach a high concentration of dissolved organic carbon (DOC, 4.31 mg/g), compared with the other tested materials. Various combinations of sand, compost and other materials were observed to have excellent heavy metal removal (75 –96% of Zn and 90 –93% of Cu), with minimal DOC leaching (0.0013–2.43 mg/g). The sorption efficiency of the different Enviro-media mixes showed that a combination of traditional (sand) and alternative materials can be used as an effective medium for the treatment of dissolved metal contaminants commonly found in stormwater. The application of using recycled organic materials and other waste materials (such as recycled glass) also provides added value to the products life cycle. Keywords compost; recycled material; sand filtration; sorption; stormwater
Water Science & Technology Vol 54 No 6–7 pp 299–305 Q IWA Publishing 2006
Pollutant removal efficiency of alternative filtration media in stormwater treatment
Introduction
The quality of stormwater can vary significantly depending on the surrounding land use (commercial, agricultural and urban area). Stormwater contains significant concentrations of heavy metals, nutrients, suspended, colloidal and volatile fractions of inorganic and organic particulates, and other anthropogenic compounds (Sansalone, 1999). Although the potential exists for the occurrence of a variety of metal ions in stormwater, the metals most observed to have a negative impact on receiving waters are copper, lead and zinc (Marsalek et al., 1999; Roesner, 1999). The treatment of stormwater containing low concentrations of heavy metals can be achieved using various conventional and emerging methods (e.g. ion-exchange, electrolyte or liquid extraction, electrodialysis, precipitation, reverse osmosis), however, most of these available physicochemical techniques are either economically unfavourable, technically too complicated or too selective (Patterson and Minear, 1975; Brown et al., 2000a, b). Alternative low-cost approaches are needed, which could utilise alternative filtration media (Bailey et al. 1999; Sansalone, 1999; Brown et al., 2000a,b; Shukla et al., 2002). These include carbonaceous materials, waste by-products, weathered soils and agricultural/ natural products such as activated carbon, iron oxide-coated sand, zeolites, rice bran and hulls, soybean, peanut husk, saw dust, peat and leaves. These materials have the capacity to adsorb dissolved pollutants and have been used as an alternative filter media (US E.P.A., doi: 10.2166/wst.2006.617
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1997; Clark and Pitt, 1999; Recycled Organics Unit, 2002). The term Enviro-media used in this paper refers to an engineered infiltration treatment medium containing either selected organic matter or a blend of organics, minerals, specialised aggregates, soil and other ingredients. One concern about the use of compost-based materials is that it results in large amounts of dissolved organic carbon (DOC) that can be leached into the stormwater effluent (Seelsaen et al., 2004). A number of urban runoff management approaches such as porous pavement, detention/retention ponds, wetlands, partial exfiltration trenches, infiltration trenches and vegetated swales (Shutes et al., 1997; Sansalone, 1999) could utilise the application of alternative filtration media. Infiltration systems are generally composed of sand (Clark and Pitt, 1999), however, a variety of alternative sorbents such as zeolite, activated carbon and compost can be incorporated into the media to improve water quality by assimilating and transforming organic, inorganic and toxic constituents through processes such as infiltration, sorption, precipitation, and binding by organic colloidal material or adsorption of metal –ligand complexes (Davis et al., 2001). This paper investigates the possibility of utilising alternative filtration media for the sorption of heavy metals such as copper and zinc in association with other nutrients contaminants typically found in stormwater. The capacity of media such as sand, compost, packing wood, ash, zeolite and a combination of these materials were evaluated (using batch sorption tests) as an alternative to concrete sand which is traditionally specified for sand filters.
Materials and methods Materials
The maximum size of materials (sand, ash, zeolite, compost, packing wood and Enviromedia) used in this study was 4.75 mm. All material was passed through a 4.75 mm sieve, material remaining in the sieve was then shredded (Talon N11968) until 95% of the material passed through the sieve. Sand was washed several times with MilliQ water to remove easily leachable materials. The fine and course glass was unwashed and used as received from the recycling plant. The compost was sourced from kerbside (garden) waste. Prior to use, all materials were dried in the oven at 40 8C over night.
Synthetic stormwater
The constituents used within the synthetic stormwater (SSW) (Table 1) were adapted from a previously study (Davis et al., 2001). Metal concentrations of 50 times the average level found in stormwater were used to test the capacity of the materials. The concentrations used for nutrients and DOC were typical of values found in the published literature for Table 1 Synthetic stormwater constituents and concentration
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Pollutants
Chemical
Total phosphorous Organic nitrogen Nitrate nitrogen Copper Lead Zinc Dissolved organic carbon
Dibasic sodium phosphate (Na2HPO4) Glycine (NH2CH2COOH) Potassium nitrate (KNO3) Cupric sulphate (CuSO4) Lead chloride (PbCl2) Zinc chloride (ZnCl2) Humic acid (Fluka)
* , 50 times metals concentration typically found in stormwater
Concentration (mg/L)
0.6 4 2 5* 5* 27* 10
stormwater. Humic acid was included in the SSW to provide a source-dissolved organic carbon. All chemicals were of analytical reagent grade and working solutions were prepared by diluting the stock solution with MilliQ water and mixing together before use. Batch tests
N. Seelsaen et al.
Batch tests were performed in duplicate using shake flasks (250 mL), which were shaken on an orbital shaker at 200 rpm for 24 h. The aqueous phase was separated by centrifuging the mixture for 20 min at 3,000 rpm, the liquid was then filtered through a 0.45 mm cellulose acetate membrane filter (Advantec MFS). Control samples (i.e. SSW without sorbent) were operated in parallel with the test samples. Analytical procedures
All filter samples were preserved and stored in a cool room (4 8C) prior to analysis. pH was measured before and after leaching studies, prior to sample preservation. Metal ion concentrations (Cu, Zn and Pb) were analysed by an inductively coupled plasma optical emission spectrometer (ICP-OES) (Perkin Elmer Optima 3000 Radial View). Dissolved organic carbon (DOC) concentrations were determined using a Shimadzu 5000A Total Organic Carbon (TOC) analyser. Results and discussion Sorption efficiency of different materials
The amount of contaminant retained on the sorbent phase (Qf, mg/g) was calculated using the following mass balance Equation (Jang et al., 2005): mðQf 2 Qi Þ ¼ VðCi 2 C f Þ
ð1Þ
where Qi ¼ the initial metal ion concentration in sorbent is normally assumed to be zero. Thus: Qf ¼ VðCi 2 C f Þ=m
ð2Þ
where Ci ¼ initial concentration of contaminants the solution (mg/L); Cf ¼ final concentration of contaminants in solution (mg/L); V ¼ volume of the solution (L); m ¼ dry weight of the sorbent (g). The sorption efficiency of each material can be assessed from the following equation: % contaminant removal ¼ ½ðCi 2 Cf Þ=Ci £ 100
ð3Þ
Table 2 shows the calculated performance efficiency for Zn and Cu removal by sand, recycled glass, ash, zeolite, compost, packing wood and Enviro-media. Compost, packing wood and Enviro-media resulted in the highest average removal efficiency for both Table 2 Metal removed (%) for Cu and Zn and DOC (%) concentrations using different filtration media Sorbent
Compost Packing wood Enviro-media Ash Zeolite Fine glass Sand Coarse glass
Metal removal
DOC leached
Zn (%)
Cu (%)
Leaching (%)
Sorbed (%)
97 90 75 51 95 69 16 16
92 88 90 97 52 39 29 26
2419 3197 36 – – 371 – 28
– – 48 10 – 27 –
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metals. On the media where polymeric materials are lignin, tannins or other phenolic compounds are present (compost, wood, Enviro-media) ion exchange is expected to be an important mechanism for the removal of these dissolved metals (Yu et al., 2001). While these polymeric materials have the ability to sorbs metal ions, they also readily leach dissolved organic carbon into the solution at very high concentrations (Table 2). Ash and zeolite showed moderate sorption for either Cu or Zn, respectively, both materials also sorbed DOC, again the most likely mechanism for the metal removal would be ion exchange (Erdem et al., 2004). X-ray fluorescence spectrometry (XRF) of the materials showed that zeolite is composed of Al, Ca and Na whereas ash contains Si, Al and K as the main component. The increased concentration of sodium (0.96 ^ 0.03 mg/g, unreported data) in the zeolite batch test, supports the hypothesis that ion exchange is the principle mechanism for metal removal. Other materials showed varied metal removal. Fine glass had moderate sorption of zinc and copper, whereas coarse glass and sand only sorbed zinc and copper slightly. This reduction in sorption capacity for the different sized glass could have resulted from increased precipitation for the fine glass materials compared with the coarse glass, as these materials were used without cleaning, potentially causing an increasing solution DOC which could have changed in the solution pH after being in contact with glass. Zinc isotherm models
Compost and zeolite were selected to quantify the sorption capacity for the removal of Zn. Langmuir and the Freundlich Equations (Yu et al., 2001, Jang et al., 2005) were fitted to the experimental data (Table 3). These equilibrium isotherms were transformed into the following linearised forms: Langmuir model : C=q ¼ 1=ðK L qm Þ þ C=qm
ð4Þ
Freundlich model : log q ¼ log K f þ 1=n log C
ð5Þ
where C (mg/L) ¼ final equilibrium concentration; KL(1/mg) ¼ Langmuir isotherm constant related to the equilibrium constant or binding energy; qm (mg/g) ¼ amount of sorption corresponding to complete surface coverage; Kf (L/g) ¼ Freundlich empirical constant related to the sorption capacity; and n ¼ sorption intensity. Figures 1 and 2 show that there is a strong positive relationship for the sorption data and that the compost is better fitted to the Langmuir isotherm than the zeolite. The greater conformity of the sorption data to the Langmuir isotherm implies that the sorption of metal ions on the compost and zeolite follows either in a monolayer or a one-directional process (Jang et al., 2005). An essential characteristic of the Langmuir equation is that it can be expressed in terms of a dimensionless constant, equilibrium parameter (RL,I), which can be used to predict whether a sorption system is favourable or unfavourable in batch processes (Yu et al., 2001; Lin and Juang, 2002). The parameter is described by the following equation: RL;I ¼
1 ð1 þ K L;I C i Þ
ð6Þ
Table 3 Comparison of Langmuir and Freundlich constants for the sorption of Zn by compost and zeolite Sorbent
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Compost Zeolite
Langmuir model
Freundlich model
qm (mg/g)
KL
RL,I 5 100– 1500 mg/L
1/n
Kf
11.20 4.25
3.16 £ 1022 6.92 £ 1023
0.002 –0.235 0.10 –0.60
0.1429 0.1878
4.09 0.94
400 300 C /q (g/L)
y = 0.2355x + 34.044 R2 = 0.9585
Compost Zeolite
200
y= 0.0893x + 2.8299 R2 = 0.9929
0 0
500
1000
1500
C (mg/L)
N. Seelsaen et al.
100
Figure 1 Langmuir isotherm for Zn sorption for compost and zeolite (sorbent concentration: 5 g/200 mL; temperature: 25 8C; pH ¼ 5.0; reaction time 24 h)
where Ci ¼ highest initial solute concentration (mg/L); and KL,I ¼ Langmuir model (1) constant. The value of RL,I indicates whether the isotherm is irreversible (RL,I ¼ 0), favourable (0 , RL,I , 1), linear (RL,I ¼ 1) or unfavourable (RL,I . 1). The equilibrium parameters for compost and zeolite (Table 3) show that the sorption of Zn(II) on both compost and zeolite are favourable. Performance of different Enviro-media mixes
Various blends of Enviro-media comprising sand, compost, zeolite and ash were trialled. Table 4 shows the results of five different mixed-media combinations based on percentage of weight. The removal of Zn and Cu for the different mixes (M1, M2, M3, M4, M5) were similar, however, the amount of DOC leached depended on the amount of compost. Compost appears to play the most important role in heavy metal removal. Other sorbents such as zeolite and ash have less effect on heavy metal removal. From these results, it shows that the combination of sand, compost and other sorbent can be adjusted to create mixes which sorb heavy metals but also minimise DOC leaching. Stability of alternative materials
Table 5 shows the leaching characteristic of four different source materials using MilliQ water. Packing wood, compost and fine glass leached the most DOC, zinc, copper and lead, while coarse glass leached some metals and sand leaches very little DOC and some
Log q (mg/g)
100
Compost Zeolite y = 4.0862x0.1429 R2 =0.804
10
y = 0.9435x0.1878 R2 = 0.5961
1 1
10
100 Log C (mg/L)
1000
10000
Figure 2 Freundlich isotherm for Zn sorption for the compost and zeolite (sorbent concentration: 5 g/200 mL; temperature: 25 8C; pH ¼ 5.0; reaction time 24 h)
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Table 4 Performance of Enviro-media mixes in heavy metal removal and DOC leaching (sorbent concentration: 10 g/200 mL; temperature: 25 8C; pH ¼ 5.0; reaction time 24 h)
N. Seelsaen et al.
Enviro-media mixes*
DOC leaching
DOC
Zn sorbed
Zn removal
Cu sorbed
Cu removal
(percent by weight)
(mg/g)
leaching (%)
(mg/g)
(%)
(mg/g)
(%)
2.4 1.4 0.9 0.9 0.7
2116 1240 797 743 617
275.3 275.6 275.0 274.5 274.7
96.1 96.2 96.0 95.8 95.9
45.4 45.5 45.5 45.6 45.6
93.3 93.5 93.6 93.7 93.7
M1 M2 M3 M4 M5
(S50%:C50%) (S70%:C30%) (S70%:C20%:Z10%) (S60%:C20%:Z20%) (S60%:C20%:Z10%:A10%)
*S: sand, C: compost, Z: zeolite, A: ash Table 5 Leaching test of media by using MilliQ water Sorbent
DOC leaching (mg/g)
Zn leaching (mg/g)
Cu leaching (mg/g)
Pb leaching (mg/g)
4.59 4.31 0.58 0.20 0.02
13.5 2.3 4.8 1.3 0
2.2 2.2 5.1 1.1 0.2
2.3 nd 1.1 0 0
Packing wood Compost Fine glass Coarse glass Sand nd: non-detected
copper. This observation supports sorption studies (Table 2) and suggests that the source of the materials impact upon application of these materials and may be a point of concern for potential environmental applications. For example 1.88 mg/g of chromium (unreported data), which is often used as a wood preservative was leached from the packing wood. A significant difference is observed between the different sized glass (coarse and fine) which also suggests that process recycling is a likely source of potential contamination in addition to the material source. Conclusions
† Alternative filtration media such as compost and packing wood showed an excellent adsorption capacity for copper and zinc compared with concrete sand, a traditional infiltration medium. † A disadvantage with the application of using compost-based materials is the large amounts of DOC that can be potentially leached owing to composition of the organic compounds. † Recycled glass exhibited improved metal removal efficiency compared with sand, however, unwashed fine glass leached organic matter into solution. † A combination of alternative and traditional materials showed that improvement in the performance of traditional infiltration systems comprising just sand for stormwater treatment can be achieved. † The recycling of waste materials such as compost and glass for stormwater treatment provides added value to the products life cycle. † The source of the recycled materials is important in providing an efficient material for application in stormwater treatment. Acknowledgements
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The authors wish to acknowledge the financial support from the NSW Department of Environment and Conservation (DEC), Centre for Organic Resource Enterprises (CORE), and the Royal Thai Government. All test materials were supplied by CORE.
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