w a t e r r e s e a r c h 5 1 ( 2 0 1 4 ) 2 5 6 e2 6 5
Available online at www.sciencedirect.com
ScienceDirect journal homepage: www.elsevier.com/locate/watres
A three-step test of phosphate sorption efficiency of potential agricultural drainage filter materials G. Lyngsie*, O.K. Borggaard, H.C.B. Hansen University of Copenhagen, Department of Plant and Environmental Sciences, Thorvaldsensvej 40, DK-1871 Frederiksberg C, Denmark
article info
abstract
Article history:
Phosphorus (P) eutrophication of lakes and streams, coming from drained farmlands, is a
Received 17 August 2013
serious problem in areas with intensive agriculture. Installation of P sorbing filters at drain
Received in revised form
outlets may be a solution. Efficient sorbents to be used for such filters must possess high P
24 October 2013
bonding affinity to retain ortho-phosphate (Pi) at low concentrations. In addition high P
Accepted 27 October 2013
sorption capacity, fast bonding and low desorption is necessary. In this study five potential
Available online 8 November 2013
filter materials (Filtralite-P, limestone, calcinated diatomaceous earth, shell-sand and iron-oxide based CFH) in four particle size intervals were investigated under field relevant P
Keywords:
concentrations (0e161 mM) and retentions times of 0e24 min. Of the five materials exam-
Water quality
ined, the results from P sorption and desorption studies clearly demonstrate that the iron
Eutrophication
based CFH is superior as a filter material compared to calcium based materials when tested
Iron oxides
against criteria for sorption affinity, capacity and stability. The finest CFH and Filtralite-P
Carbonates
fractions (0.05e0.5 mm) were best with P retention of 90% of Pi from an initial concen-
Farmland drainage
tration of 161 mM corresponding to 14.5 mmol/kg sorbed within 24 min. They were further capable to retain 90% of Pi from an initially 16 mM solution within 1½ min. However, only the finest CFH fraction was also able to retain 90% of Pi sorbed from the 16 mM solution against 4 times desorption sequences with 6 mM KNO3. Among the materials investigated, the finest CFH fraction is therefore the only suitable filter material, when very fast and strong bonding of high Pi concentrations is needed, e.g. in drains under P rich soils during extreme weather conditions. ª 2013 Elsevier Ltd. All rights reserved.
1.
Introduction
Soils in intensively farmed countries may be sources of phosphorus (P) due to decades of surplus application of P in organic and inorganic fertilisers (Heal et al., 2005; Delgado and Scalenghe, 2008; Buda et al., 2012). However, a P-enriched soil only becomes an environmental problem when connected to the aquatic environment by an effective transport pathway such as artificial drains (Heathwaite et al., 2003). Tile drains
and ditches collect and direct the diffuse P contribution (loss) from fields to recipient waters and act as highways for both soluble and particulate P (Ule´n et al., 2007). Despite substantial efforts over many years to reduce this transport, leaching of P from agricultural land to the aquatic environment is still a serious and costly problem in many parts of Europe and elsewhere (Delgado and Scalenghe, 2008; Ballantine and Tanner, 2010; Buda et al., 2012). Thus, to reach the goal of good water quality as stated in the EU Water Framework
* Corresponding author. E-mail addresses: [email protected], [email protected] (G. Lyngsie). 0043-1354/$ e see front matter ª 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.watres.2013.10.061
w a t e r r e s e a r c h 5 1 ( 2 0 1 4 ) 2 5 6 e2 6 5
Directive (WFD) requires a substantial reduction of the diffuse P loss from farmland in many parts of Europe (Søndergaard et al., 2005; Kaasik et al., 2008). While a range of P mitigation options have been tested for surface transport (Hoffmann et al., 2009), this is not the case for subsurface transport (Kro¨ger et al., 2008). However, by installing a filter construction at the end of a drainage pipe, Penn et al. (2007) demonstrated an immediate reduction of drainage P leaching. This end-of-pipe approach could be carried out in connection with a small-scale constructed wetland (Reinhardt et al., 2005) or with flow-through filter structures in ditches (Penn et al., 2007). A great variety of different types of filters and filter materials for P retention have been described for retention of high P concentrations in wastewaters (Johansson Westholm, 2006; Penn et al., 2007; Cucarella and Renman, 2009; Vohla et al., 2011). However, it is questionable to directly transfer the experience obtained from these high P concentration studies to the low P concentrations in drainage water as the filter materials may behave differently at high and low solution concentrations (Agyei et al., 2002; ´ da´m et al., 2007). Therefore, the filter materials need to be A tested at low P concentrations and short reaction times relevant for cleaning P contaminated drainage water. P in drainage water may consist of P in dissolved organic matter, particulate P and dissolved ortho-phosphate (Pi). The focus in this investigation will be on Pi, which denotes inorganic phosphate irrespective of the species 3 2 ðH2 PO 4 ; HPO4 and=or PO4 Þ In natural, unpolluted areas in Denmark, the Pi concentration in base flow drainage water is typically 1.6 mM but the concentration can be up to 42 mM in farmland drains (Andersen et al., 2006) and Penn et al. (2007) found more than four times this concentration in Maryland, US. The high water flow during rainstorms and fast frost-thaw transitions is critical because of high or very high P leaching during such peak flows (Grant et al., 1996; Johnes, 2007). To effectively remove the Pi during peak flows with high Pi concentrations, the filter material must react fast and possess high Pi sorption stability and capacity. Even though fast and substantial Pi sorption is mandatory for the practical use of the filter materials, it is also important with a stable bonding of sorbed Pi to ensure that Pi is not desorbed when the sorption condition changes, e.g. because of decreasing Pi concentration in the drainage water (Grant et al., 1996). This, however, is often overlooked in studies focussing on sorption capacity (Klimeski et al., 2012). The Pi removal efficiency of a filter material is closely related to the content of various Al, Ca, Fe and Mg (hydr)oxides and carbonates (Johansson Westholm, 2006; Ballantine and Tanner, 2010; Vohla et al., 2011). In addition to the elemental composition, the specific surface area (SSA) is important as sorption normally increases at increasing SSA. Although it is outside the scope of this study, high hydraulic conductivity of the filter material is also essential for use in high-flow drainage filters. As a practical test of high Pi removal efficiency also under extreme conditions, we suggest the following three sorption/ desorption criteria: (i) A capacity to retain 90% Pi from 161 mM solution within 24 min; (ii) A reactivity resulting in retention of 90% Pi from 16 mM solution within 1½ min; (iii) A stability resulting in dissolution of 1e2 mm > 2e4 mm as Pi sorption is normally found to increase with increasing SSA for the same material (Ballantine and Tanner, 2010; Penn et al., 2011). SSA also differs among the materials with CFH and CDE having much larger SSAs than the other materials. The poorly ordered Fe oxides are responsible for the substantial CFH SSAs, whereas CDE has high SSA due to larger internal (N2 accessible) pores generated by the diatomite framework. The total element
composition of the Filtralite-P and CFH fractions (Supplemental data, Table S1) showed the finer Filtralite-P fractions were somewhat enriched in Ca and Mg but not in Al and Fe, whereas the three CFH fractions had the same composition. CaeMg enrichment in Filtralite-P finer fractions reflects the heterogeneity of the material with overrepresentation of the active sorbent (CaeMg compounds) in the smaller white particles at the expense of the Al/Fe-bearing matrix.
3.2.
Phosphate retention
3.2.1.
Phosphate sorption capacity
The 24-min Pi sorption isotherms for the five materials are very different with limited sorption to CDE, limestone and shell-sand; in fact CDE releases Pi at low concentration (Fig. 1). In contrast, CFH and Filtralite-P possess much higher Pi sorption capacity. Except CFH, the shape of the isotherms comprises two steps. For soils and various materials such as Al and Fe oxides, Pi sorption isotherms can often be fitted with the Langmuir equation resulting in sorption maximum and stability e.g. (Borggaard et al., 2005; Johansson Westholm, 2006; Vohla et al., 2011). This is not the case for the 2-steps isotherms in Fig. 1, i.e. Langmuirian sorption capacity and stability cannot be determined. Similar non-Langmuirian shaped sorption isotherms have also been reported for similar and other materials tested for their suitability as P filters and the shape of the curves has been ascribed to a combination of adsorption and precipitation reactions at low and high Pi concentrations, ´ da´m et al., 2007; Kaasik et al., 2008; Cucarella respectively (A and Renman, 2009). Therefore, it was decided to characterize the Pi retention capacity of all materials by means of Pi sorbed at initial Pi concentrations of 16 mM (S16) and 161 mM (S161). Sorption (S161) at the highest Pi concentration (161 mM) was chosen to show sorption at a high filter inlet concentration comparable to Pi concentrations reported for high Pi drainage waters (Penn et al., 2007; Chardon et al., 2012). The first test criterion was the material ability to remove at least 90% of Pi from the highest initial concentration within 24 min corresponding to sorption of 14.5 mmol/kg. As indicated in Fig. 1,
260
w a t e r r e s e a r c h 5 1 ( 2 0 1 4 ) 2 5 6 e2 6 5
Table 2 e Phosphate sorption (mean and SD) by the particle size fractions of the five filter materials from solutions with initially 16 mM Pi (S16) and 161 mM Pi (S161) together with pH and the specific surface area (SSA) of the various size fractions. Pi sorption expressed as mmol/kg and percent of added Pi. S16 and S161 were taken from isotherms in Fig. 1; sorption time 24 min and solution:solid ratio 100. Material
Particle size
S16
S161
mm
mmol/kg (%)
mmol/kg (%)
Artificial drainage water (ADW) as background electrolyte CDE 0.05e0.5 142 15 (9) 0.5e1 102 30 (6) 1e2 21 60 (1) 2e4 157 32 (10) CFH 0.05e0.5 1580 51 (98) 0.5e1 1570 32 (97) 1e2 1100 51 (68) Limestone 0.05e0.5 1320 208 (82) 0.5e1 258 34 (16) 1e2 188 19 (12) 195 76 (12) 2e4 0.05e0.5 1590 52 (99) Filtralite-P 0.5e1 1130 365 (70) 1e2 571 136 (35) 2e4 524 213 (33) Shell-sand 0.05e0.5 479 28 (30) 05e1 358 12 (22) 1e2 283 12 (18) 2e4 250 11 (16) Natural drainage water (NDW) as background electrolyte CFH 1e2 894 133 (47) Limestone 1e2 299 85 (16) 1e2 418 33 (22) Filtralite-P a
4020 325 3870 222 2150 952 2440 141 16,100 50 11,000 195 7930 140 4340 1860 2030 1410 2670 251 2230 276 16,200 12 14,900 839 10,200 2100 9430 21,400 3330 461 2800 45 2560 65 2450 60
pH
(25) (24) (13) (15) (100) (68) (49) (68) (13) (17) (14) (100) (92) (63) (59) (21) (17) (16) (15)
5550 273 (37) 1970 85 (12) 1980 96 (12)
SSAa m2/g
5.3 5.2 5.5 5.5 8.2 6.9 6.7 8.9 8.8 8.8 8.6 11.7 10.8 10.1 9.8 9.1 8.6 8.6 8.3
26 1 26 1 32 4 30 2 44 7 32 10 32 2 1.3 0.2
Available online at www.sciencedirect.com
ScienceDirect journal homepage: www.elsevier.com/locate/watres
A three-step test of phosphate sorption efficiency of potential agricultural drainage filter materials G. Lyngsie*, O.K. Borggaard, H.C.B. Hansen University of Copenhagen, Department of Plant and Environmental Sciences, Thorvaldsensvej 40, DK-1871 Frederiksberg C, Denmark
article info
abstract
Article history:
Phosphorus (P) eutrophication of lakes and streams, coming from drained farmlands, is a
Received 17 August 2013
serious problem in areas with intensive agriculture. Installation of P sorbing filters at drain
Received in revised form
outlets may be a solution. Efficient sorbents to be used for such filters must possess high P
24 October 2013
bonding affinity to retain ortho-phosphate (Pi) at low concentrations. In addition high P
Accepted 27 October 2013
sorption capacity, fast bonding and low desorption is necessary. In this study five potential
Available online 8 November 2013
filter materials (Filtralite-P, limestone, calcinated diatomaceous earth, shell-sand and iron-oxide based CFH) in four particle size intervals were investigated under field relevant P
Keywords:
concentrations (0e161 mM) and retentions times of 0e24 min. Of the five materials exam-
Water quality
ined, the results from P sorption and desorption studies clearly demonstrate that the iron
Eutrophication
based CFH is superior as a filter material compared to calcium based materials when tested
Iron oxides
against criteria for sorption affinity, capacity and stability. The finest CFH and Filtralite-P
Carbonates
fractions (0.05e0.5 mm) were best with P retention of 90% of Pi from an initial concen-
Farmland drainage
tration of 161 mM corresponding to 14.5 mmol/kg sorbed within 24 min. They were further capable to retain 90% of Pi from an initially 16 mM solution within 1½ min. However, only the finest CFH fraction was also able to retain 90% of Pi sorbed from the 16 mM solution against 4 times desorption sequences with 6 mM KNO3. Among the materials investigated, the finest CFH fraction is therefore the only suitable filter material, when very fast and strong bonding of high Pi concentrations is needed, e.g. in drains under P rich soils during extreme weather conditions. ª 2013 Elsevier Ltd. All rights reserved.
1.
Introduction
Soils in intensively farmed countries may be sources of phosphorus (P) due to decades of surplus application of P in organic and inorganic fertilisers (Heal et al., 2005; Delgado and Scalenghe, 2008; Buda et al., 2012). However, a P-enriched soil only becomes an environmental problem when connected to the aquatic environment by an effective transport pathway such as artificial drains (Heathwaite et al., 2003). Tile drains
and ditches collect and direct the diffuse P contribution (loss) from fields to recipient waters and act as highways for both soluble and particulate P (Ule´n et al., 2007). Despite substantial efforts over many years to reduce this transport, leaching of P from agricultural land to the aquatic environment is still a serious and costly problem in many parts of Europe and elsewhere (Delgado and Scalenghe, 2008; Ballantine and Tanner, 2010; Buda et al., 2012). Thus, to reach the goal of good water quality as stated in the EU Water Framework
* Corresponding author. E-mail addresses: [email protected], [email protected] (G. Lyngsie). 0043-1354/$ e see front matter ª 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.watres.2013.10.061
w a t e r r e s e a r c h 5 1 ( 2 0 1 4 ) 2 5 6 e2 6 5
Directive (WFD) requires a substantial reduction of the diffuse P loss from farmland in many parts of Europe (Søndergaard et al., 2005; Kaasik et al., 2008). While a range of P mitigation options have been tested for surface transport (Hoffmann et al., 2009), this is not the case for subsurface transport (Kro¨ger et al., 2008). However, by installing a filter construction at the end of a drainage pipe, Penn et al. (2007) demonstrated an immediate reduction of drainage P leaching. This end-of-pipe approach could be carried out in connection with a small-scale constructed wetland (Reinhardt et al., 2005) or with flow-through filter structures in ditches (Penn et al., 2007). A great variety of different types of filters and filter materials for P retention have been described for retention of high P concentrations in wastewaters (Johansson Westholm, 2006; Penn et al., 2007; Cucarella and Renman, 2009; Vohla et al., 2011). However, it is questionable to directly transfer the experience obtained from these high P concentration studies to the low P concentrations in drainage water as the filter materials may behave differently at high and low solution concentrations (Agyei et al., 2002; ´ da´m et al., 2007). Therefore, the filter materials need to be A tested at low P concentrations and short reaction times relevant for cleaning P contaminated drainage water. P in drainage water may consist of P in dissolved organic matter, particulate P and dissolved ortho-phosphate (Pi). The focus in this investigation will be on Pi, which denotes inorganic phosphate irrespective of the species 3 2 ðH2 PO 4 ; HPO4 and=or PO4 Þ In natural, unpolluted areas in Denmark, the Pi concentration in base flow drainage water is typically 1.6 mM but the concentration can be up to 42 mM in farmland drains (Andersen et al., 2006) and Penn et al. (2007) found more than four times this concentration in Maryland, US. The high water flow during rainstorms and fast frost-thaw transitions is critical because of high or very high P leaching during such peak flows (Grant et al., 1996; Johnes, 2007). To effectively remove the Pi during peak flows with high Pi concentrations, the filter material must react fast and possess high Pi sorption stability and capacity. Even though fast and substantial Pi sorption is mandatory for the practical use of the filter materials, it is also important with a stable bonding of sorbed Pi to ensure that Pi is not desorbed when the sorption condition changes, e.g. because of decreasing Pi concentration in the drainage water (Grant et al., 1996). This, however, is often overlooked in studies focussing on sorption capacity (Klimeski et al., 2012). The Pi removal efficiency of a filter material is closely related to the content of various Al, Ca, Fe and Mg (hydr)oxides and carbonates (Johansson Westholm, 2006; Ballantine and Tanner, 2010; Vohla et al., 2011). In addition to the elemental composition, the specific surface area (SSA) is important as sorption normally increases at increasing SSA. Although it is outside the scope of this study, high hydraulic conductivity of the filter material is also essential for use in high-flow drainage filters. As a practical test of high Pi removal efficiency also under extreme conditions, we suggest the following three sorption/ desorption criteria: (i) A capacity to retain 90% Pi from 161 mM solution within 24 min; (ii) A reactivity resulting in retention of 90% Pi from 16 mM solution within 1½ min; (iii) A stability resulting in dissolution of 1e2 mm > 2e4 mm as Pi sorption is normally found to increase with increasing SSA for the same material (Ballantine and Tanner, 2010; Penn et al., 2011). SSA also differs among the materials with CFH and CDE having much larger SSAs than the other materials. The poorly ordered Fe oxides are responsible for the substantial CFH SSAs, whereas CDE has high SSA due to larger internal (N2 accessible) pores generated by the diatomite framework. The total element
composition of the Filtralite-P and CFH fractions (Supplemental data, Table S1) showed the finer Filtralite-P fractions were somewhat enriched in Ca and Mg but not in Al and Fe, whereas the three CFH fractions had the same composition. CaeMg enrichment in Filtralite-P finer fractions reflects the heterogeneity of the material with overrepresentation of the active sorbent (CaeMg compounds) in the smaller white particles at the expense of the Al/Fe-bearing matrix.
3.2.
Phosphate retention
3.2.1.
Phosphate sorption capacity
The 24-min Pi sorption isotherms for the five materials are very different with limited sorption to CDE, limestone and shell-sand; in fact CDE releases Pi at low concentration (Fig. 1). In contrast, CFH and Filtralite-P possess much higher Pi sorption capacity. Except CFH, the shape of the isotherms comprises two steps. For soils and various materials such as Al and Fe oxides, Pi sorption isotherms can often be fitted with the Langmuir equation resulting in sorption maximum and stability e.g. (Borggaard et al., 2005; Johansson Westholm, 2006; Vohla et al., 2011). This is not the case for the 2-steps isotherms in Fig. 1, i.e. Langmuirian sorption capacity and stability cannot be determined. Similar non-Langmuirian shaped sorption isotherms have also been reported for similar and other materials tested for their suitability as P filters and the shape of the curves has been ascribed to a combination of adsorption and precipitation reactions at low and high Pi concentrations, ´ da´m et al., 2007; Kaasik et al., 2008; Cucarella respectively (A and Renman, 2009). Therefore, it was decided to characterize the Pi retention capacity of all materials by means of Pi sorbed at initial Pi concentrations of 16 mM (S16) and 161 mM (S161). Sorption (S161) at the highest Pi concentration (161 mM) was chosen to show sorption at a high filter inlet concentration comparable to Pi concentrations reported for high Pi drainage waters (Penn et al., 2007; Chardon et al., 2012). The first test criterion was the material ability to remove at least 90% of Pi from the highest initial concentration within 24 min corresponding to sorption of 14.5 mmol/kg. As indicated in Fig. 1,
260
w a t e r r e s e a r c h 5 1 ( 2 0 1 4 ) 2 5 6 e2 6 5
Table 2 e Phosphate sorption (mean and SD) by the particle size fractions of the five filter materials from solutions with initially 16 mM Pi (S16) and 161 mM Pi (S161) together with pH and the specific surface area (SSA) of the various size fractions. Pi sorption expressed as mmol/kg and percent of added Pi. S16 and S161 were taken from isotherms in Fig. 1; sorption time 24 min and solution:solid ratio 100. Material
Particle size
S16
S161
mm
mmol/kg (%)
mmol/kg (%)
Artificial drainage water (ADW) as background electrolyte CDE 0.05e0.5 142 15 (9) 0.5e1 102 30 (6) 1e2 21 60 (1) 2e4 157 32 (10) CFH 0.05e0.5 1580 51 (98) 0.5e1 1570 32 (97) 1e2 1100 51 (68) Limestone 0.05e0.5 1320 208 (82) 0.5e1 258 34 (16) 1e2 188 19 (12) 195 76 (12) 2e4 0.05e0.5 1590 52 (99) Filtralite-P 0.5e1 1130 365 (70) 1e2 571 136 (35) 2e4 524 213 (33) Shell-sand 0.05e0.5 479 28 (30) 05e1 358 12 (22) 1e2 283 12 (18) 2e4 250 11 (16) Natural drainage water (NDW) as background electrolyte CFH 1e2 894 133 (47) Limestone 1e2 299 85 (16) 1e2 418 33 (22) Filtralite-P a
4020 325 3870 222 2150 952 2440 141 16,100 50 11,000 195 7930 140 4340 1860 2030 1410 2670 251 2230 276 16,200 12 14,900 839 10,200 2100 9430 21,400 3330 461 2800 45 2560 65 2450 60
pH
(25) (24) (13) (15) (100) (68) (49) (68) (13) (17) (14) (100) (92) (63) (59) (21) (17) (16) (15)
5550 273 (37) 1970 85 (12) 1980 96 (12)
SSAa m2/g
5.3 5.2 5.5 5.5 8.2 6.9 6.7 8.9 8.8 8.8 8.6 11.7 10.8 10.1 9.8 9.1 8.6 8.6 8.3
26 1 26 1 32 4 30 2 44 7 32 10 32 2 1.3 0.2
