Natural Ferrihydrite As an Agent for Reducing Turbidity Caused by Suspended Clays

TECHNICAL REPORTS: SURFACE WATER QUALITY

Natural Ferrihydrite as an Agent for Reducing Turbidity Caused by Suspended Clays

F. E. Rhoton* USDA-ARS J. M. Bigham Ohio State University

Biologically impaired waters are often caused by the turbidity urbidity is a measure of water clarity and indicates the degree associated with elevated suspended sediment concentrations. Tto which light is scattered by suspended solids in a water Turbidity can be reduced by the addition of positively charged column. Suspended solids that contribute to turbidity include compounds that coagulate negatively charged particles in suspension, causing them to fl occulate. Th is research was clays, algae, and fecal materials that originate from soil erosion, conducted to determine the eff ectiveness of ferrihydrite, a poorly nutrient inputs, and waste discharge. As the turbidity of a water crystalline Fe oxide, as a fl occulating agent for suspended clays column increases, the depth of sunlight penetration becomes similar to those found in high-turbidity waters of the Mississippi impaired to the point that reductions can occur in photosynthesis −1 delta. Clay concentrations of 100 mg L from a Dubbs silt loam and associated dissolved oxygen content. Additionally, the (fi ne silty, mixed, active, thermic Typic Hapludalfs), a Forestdale silty clay loam (fi ne, smectitic, thermic Typic Hapludalfs), suspended solids absorb heat from sunlight to produce higher water and a Sharkey clay (very fi ne, smectitic, thermic Chromic temperatures that further reduce the dissolved oxygen content. −1 Epiaquerts) were suspended in 0.0005 mol L CaCl2 solutions In time, such waters can become unproductive with respect to at pH 5, 6, 7, or 8. Natural ferrihydrite with a zero point of populations of aquatic organisms, and the overall water quality charge at pH 5.8 was acquired from a drinking water treatment can be lowered for recreational uses and human consumption facility and mixed with the suspension at concentrations of 0, 10, 25, and 50 mg L−1. After settling periods of 24 and 48 h, (Knight et al., 2002). percent transmittance was measured at a wavelength of 420 nm Within this context, many regions of the USA contain water using a 3-mL sample collected at a depth of 2 cm. Th e greatest bodies that remain turbid for extended periods of time after runoff reductions in turbidity after 24-h equilibration were recorded events. One example is the Mississippi Delta, which lies within Ma- for the pH 5 suspensions of the Dubbs (31%) and Forestdale jor Land Resource Area 131, Southern Mississippi Valley Alluvium (37%) clays at a ferrihydrite concentration of 10 mg L−1 and for the Sharkey clay at a ferrihydrite concentration of 25 mg L−1 (USDA Soil Conservation Service, 1981). Th is region is character- (relative to the 0 ferrihydrite treatment). Water clarity for all ized by depressional backswamps, migrating streams with low fl ow samples further increased after 48 h. Th ese results indicate that rates, and oxbow lakes. Th e surrounding soils are very erosive and the eff ectiveness of ferrihydrite, as a means of reducing turbidity are developed in loamy or clayey textured sediments high in smec- associated with suspended clays, is greatest at pH values below tite. Th e combination of high clay content and smectitic mineral- its zero point of charge. ogy are primary factors contributing to turbidity-related problems in this region. Previous research conducted on Mississippi Delta oxbow lakes by Knight et al. (2002) has indicated that suspended sediment concentrations of 100 mg L−1 represent the breakpoint between impaired and unimpaired waters. Th e turbidity of water is normally reduced by the addition of electrolyte solutions that coagulate fi nely divided particles in suspension, causing them to fl occulate and settle more rapidly. Some researchers have investigated the use of calcium sulfate (gypsum) in solid and solution forms to resolve turbidity problems through fl occulation of sediment suspended in retention basins (Przepiora et al., 1997). In addition to gypsum, synthetic, poorly crystalline Fe oxides have proven to be eff ective for fl occulating soil clays Copyright © 2009 by the American Society of Agronomy, Crop Science Society of America, and Soil Science Society of America. All rights under laboratory conditions (Goldberg and Glaubig, 1987; Gold- reserved. No part of this periodical may be reproduced or transmitted berg et al., 1990; Kretzschmar et al., 1993). in any form or by any means, electronic or mechanical, including pho- Ferrihydrite (Fe HO ·4H O) is a poorly crystalline Fe oxide tocopying, recording, or any information storage and retrieval system, 5 8 2 without permission in writing from the publisher. formed by the rapid oxidation of Fe(II) in ground water on ex-

Published in J. Environ. Qual. 38:1887–1891 (2009). F.E. Rhoton, USDA-ARS National Sedimentation Lab., Oxford, MS 38655; J.M. Bigham, doi:10.2134/jeq2008.0454 School of Natural Resources, Ohio State Univ., Columbus, OH 43210. Received 22 Oct. 2008. *Corresponding author ([email protected]). Abbreviations: AAO, ammonium oxalate; BET, Brunauer–Emmett–Teller; CEC, cation © ASA, CSSA, SSSA exchange capacity; Fe , acid ammonium oxalate–extractable iron; %T, percent 677 S. Segoe Rd., Madison, WI 53711 USA o transmittance; ZPC, zero point of charge.

1887 posure to atmospheric conditions in streams (Rhoton et al., Netherlands) with Cu Kα radiation (35 kV, 20 mA). Ferrihydrite 2002) or during aeration at municipal water treatment facili- specimens were characterized for near total elemental composition ties where the precipitated ferrihydrite is fi ltered out before the by inductively coupled plasma optical emission spectroscopy after water is further treated for human consumption (Rhoton and digestion in aqua regia at 80°C for 1 h in a capped 250-mL poly- Bigham, 2005). Such precipitates are usually viewed as waste propylene bottle. Th e acid ammonium oxalate (AAO)-extractable products, but recent research has shown that iron-rich water Fe (Feo) content of the ferrihydrite was determined by the methods treatment residuals are eff ective scavengers of As and Pb (Beak of Schwertmann (1964). Quantitative color measurements were et al., 2006a, 2006b) as well as P (Rhoton and Bigham, 2005) made with a Minolta CR-200 Chroma Meter (Konica Minolta, due to their purity and high sorptive capacities. Ferrihydrite Tokyo, Japan). Th e zero point of charge (ZPC) of the ferrihydrite from a water treatment plant has also been used to increase the was determined by S. Goldberg (U.S. Salinity Lab, Riverside, CA) stability of surface soils for improved infi ltration and reduced using a Zeta-Meter 3.0 system (Zeta-Meter, Stanton, VA) follow- runoff (Rhoton et al., 2003). ing procedures outlined elsewhere (Goldberg et al., 1996). Because ferrihydrite has the ability to sorb chemical con- Suspended sediment columns were prepared by adding ali- taminants and improve soil aggregation, it follows that this quots of stock clay and ferrihydrite suspensions to 30-mL test −1 mineral might be eff ective as a fl occulant to reduce turbidity tubes containing 0.0005 mol L CaCl2 solutions at pH 5, 6, levels and improve water quality in lakes and reservoirs. Th e 7, and 8. Clay concentration was held constant at 100 mg L−1, objective of this research was to evaluate the eff ectiveness of a and ferrihydrite suspensions were added to yield concentra- highly reactive, naturally occurring ferrihydrite for fl occulating tions of 0, 10, 25, and 50 mg L−1. Each treatment was rep- suspended clays over a range of pH conditions. licated three times. Th e clay–ferrihydrite suspensions were thoroughly mixed and allowed to settle at room temperature Materials and Methods (25°C) for 24 h, after which the top 3 cm of suspension were Surface soil samples (0–7.5 cm) were collected from a Dubbs siphoned off with a J tube. Th e same procedure was repeated silt loam (fi ne silty, mixed, active, thermic Typic Hapludalfs), on a separate set of samples that were allowed to settle for 48 a Forestdale silty clay loam (fi ne, smectitic, thermic Typic En- h. All suspension samples were transferred to test tubes that doaqualfs), and a Sharkey clay (very fi ne, smectitic, thermic had a 16-mm path length. Percent transmittance (%T) was Chromic Epiaquerts). All three soils are representative of the read at a wavelength of 420 nm (Goldberg and Glaubig, 1987) Mississippi Delta Major Land Resource Area. Th e samples were with a Spectronic 1001 Spectrophotometer (Bausch & Lomb, air-dried in the laboratory, sieved to <2 mm, and dispersed by Rochester, NY). standard sonication procedures. Th e clay fractions (<2 μm) were Th e data were analyzed by the GLM procedure of SAS ver- separated from each soil by standard sedimentation procedures sion 8 (SAS Institute, 1999). Signifi cant diff erences in percent and concentrated using ceramic fi lter candles (pore size, 0.15 transmission as a function of treatment were determined from μm) attached to a vacuum source. Th e clays were then Ca satu- Duncan’s Multiple Range Test. −1 rated by three centrifugal washings with 1 mol L CaCl2. Excess electrolytes were removed by three centrifugal washings with Results and Discussion distilled water, followed by dialysis against distilled water using X-ray diff raction analyses of the soil clays selected for this SPECTRA/POR dialysis tubing, which had a 12,000 to 14,000 study indicated similar mineralogical compositions. Smectite molecular weight cutoff (Kretzschmar et al., 1993). Th e Ca-sat- was the dominant phase based on broad, 1.4-nm peaks that urated clays were freeze-dried and stored. Ferrihydrite samples expanded to roughly 1.7 nm after solvation of Ca-saturated were obtained as Fe oxide sludge from the Memphis, TN Water specimens with ethylene glycol (Fig. 1). Reductions in cation Operations Department, where water pumped from the Mem- exchange capacity (CEC) with K saturation, when compared phis Sand aquifer is aerated to precipitate the dissolved Fe. Th is with Ca (Table 1), suggested that 5 to 10% of the expandable Fe oxide (ferrihydrite) precipitate is then removed by fi ltering the layers had charge densities in the vermiculite range. All samples water through underground fi lter beds comprised of anthracite contained small amounts of kaolinite (0.72 nm) and clay mica before purifi cation treatments. Th e ferrihydrite suspension, thus (1.0 nm) (Fig. 1). Th e Sharkey clay had the highest surface area collected, was concentrated by repeated centrifugations and de- at 38 m2 g−1, followed by the Dubbs and Forestdale soil clays at cantations to produce a fi nal concentration of 100 g L−1. 35 and 30 m2 g−1, respectively. Th e distribution of CEC values Separate, untreated soil clay specimens were characterized for was consistent with the variation in surface area among the cation exchange capacity using Ca- and K-saturated specimens three clays (i.e., the CEC values increased with increasing sur- following the methods of Jaynes and Bigham (1986). Specifi c surface area). Based on the surface area and CEC data, the Shar- face area was determined by the Brunauer–Emmett–Teller (BET) key clay suspensions should be the most sensitive to changes in triple point method using a Micromeritics Flowsorb II 2300 sur- electrolyte or the addition of other fl occulating agents. face area analyzer (Micromeritics, Norcross, GA) with N2 as the Th e X-ray diff raction pattern of the yellowish-red (5.8 YR adsorbate. Mineralogy of the soil clays and ferrihydrite was de- 3.7/5.7) Fe oxide material collected from the Memphis water termined by standard X-ray diff raction methods using a Philips treatment plant consisted primarily of two broad bands at 0.15 Model APD 3520 X-ray diff raction unit (Philips, Eindhoven, the and 0.25 nm that are indicative of two-line ferrihydrite (Fig. 2).

1888 Journal of Environmental Quality • Volume 38 • September–October 2009 Fig. 1. X-ray diff raction patterns for the (a) Dubbs, (b) Forestdale, and (c) Sharkey soil clays. Small amounts of poorly crystalline goethite were also present, Table 1. Cation exchange capacity and surface area measurements for as was quartz from the fi lter beds. Quartz and other impurities the three clay specimens. comprised approximately 13% (wt/wt) of the sample based on Cation exchange capacity the content of acid-insoluble materials (Table 2). Most ferrihy- Clay Surface area Ca saturated K saturated m2 g−1 ––––––––––cmol kg-1–––––––––– drites are 100% soluble in acid solutions of AAO, and the Feo c is considered to be highly reactive. Carlson and Schwertmann Dubbs 35 46.5 41.4 (1981) described several natural ferrihydrites with 32 to 43% Forestdale 30 37.7 35.1 Sharkey 38 47.9 43.0 (wt/wt) Feo. By comparison, the Memphis water treatment sample yielded 43.6% Feo (Table 2). Th e sample also contained a variety of trace metals that were presumably adsorbed from the Memphis aquifer and/or anthracite fi lter bed material (Table 2). Th e BET surface area of the precipitate was 250 m2 g−1 and indicated that this sample had a very small particle size, which is typical of ferrihydrite. By comparison, the Sharkey soil clay, with a smectitic mineralogy and generally high content of fi ne clay (<0.2 μm), had a BET surface area that was fi ve times smaller than the ferrihydrite (Tables 1 and 2). Th e ZPC for the Memphis ferrihydrite of 5.77 (Table 2) is within the ZPC range of 7.5 to 5.3 reported for six naturally occurring ferrihydrites (Schwertmann and Fechter, 1982). Th e lower ZPC values were attributed to surface contamination created by increasing Si concentrations. Our results suggest that a net positive charge develops below an approximate pH of 5.8. Th e fl occulation data (Fig. 3) show that the highest %T af- Fig. 2. X-ray diff raction pattern for the ferrihydrite specimen showing ter 24 h settling time was obtained for the Sharkey clay (Fig. two-line ferrihydrite (0.25 and 1.5 nm), goethite (G), and quartz (Q). 3c) at pH 5, followed in order by the pH 6, 7, and 8 treat- Pettry and Switzer (1996), ferrihydrite (AAO-extractable Fe) is ments. A similar result was obtained across all ferrihydrite con- the dominant Fe oxide in surface horizons of the Sharkey soil. centrations. Th e maximum %T values for Sharkey, which oc- Th e Sharkey data also demonstrate the pH-dependent na- −1 curred at ferrihydrite concentrations of 25 mg L (83.5) and ture of the ferrihydrite surface charge. Specifi cally, %T de- −1 10 mg L (80.0), were not signifi cantly diff erent (p < 0.05) creased with each increase in pH above 5 due to sample disper- based on Duncan’s multiple range test. However, the 83.5%T sion resulting from the development of a net negative charge by value was signifi cantly greater than those recorded for higher the ferrihydrite as its ZPC (5.77) was exceeded. ferrihydrite concentrations and pH levels. Th e relatively high Th e Dubbs clay (Fig. 3a) exhibited a much lower %T after %T (75.4) recorded for the 0 ferrihydrite treatment at pH 5 24 h than Sharkey for the 0 ferrihydrite treatment at all pH suggested that the Sharkey clay had a signifi cant pH-dependent levels, which suggested a lower native pH-dependent charge. charge component that was native to the sample. According to Once ferrihydrite concentrations were increased, the changes

Rhoton & Bigham: Ferrihydrite Reduction of Turbidity 1889 Table 2. Selected physical and chemical properties of the ferrihydrite. Total elemental analysis Munsell color

Surface area ZPC† pH Feo Al P Cd Ni Hg Cu Pb Zn AIR‡ Hue Value Chroma m2 g−1 –––––––g kg−1––––––– –––––––––––––––––mg kg−1––––––––––––––––– % 250 5.77 6.8 436 13.6 4.5 28.8 8 0.5 329 49 64 13.4 5.8 YR 3.7 5.7 † Zero point of charge. ‡ Acid insoluble residue. in %T as a function of pH were very similar to Sharkey. Th e highest %T (63.8) was recorded at pH 5 for the 10 mg L−1 ferrihydrite treatment, which was signifi cantly greater (p < 0.05) than at higher pH levels. Th ere were no signifi cant diff erences in %T between the ferrihydrite concentrations within a pH level. Th e generally sharp decrease in %T that occurred as solution pH was increased again indicated greater colloidal stability as the suspended colloids became more negatively charged. Th e Forestdale clay (Fig. 3b) at 24 h showed little variability in %T as a function of pH at the 0 ferrihydrite treatment, averag- ing approximately 43. As ferrihydrite concentrations increased, only the pH 5 treatment at 10 mg L−1 showed a signifi cant (p < 0.05) change in %T, increasing from 41.4 to 56.2. Apparently, this soil clay was less reactive than the previous two due to a smaller CEC (Table 1), which limited its reactivity with the ferrihydrite, and thus its fl occulation, even at the lowest pH. Th e %T measured after the 48-h settling period (Fig. 4) was similar to the 24-h data with respect to the eff ect of pH on ferrihydrite–clay interactions. Th e value of 95.3 recorded for the Sharkey clay (Fig. 4c) at pH 5 and a ferrihydrite concentration of 25 mg L−1 was signifi cantly (p < 0.05) greater than the %T values for all other pH levels. Th ere were no signifi cant diff erences between ferrihydrite treatments at pH 5, but all were signifi cantly greater than the 0 treatment. In all cases for the Sharkey clay, the diff erences in %T were much less within a pH level compared with the 24-h readings, and in all but one case (pH 6, 25 mg L−1) the ferrihydrite treatments resulted in a signifi cant increase in %T compared with the 0 treatment. For the Dubbs and Forestdale clays (Fig. 4a and 4b), the highest %T was also recorded for the pH 5 treatment and a ferrihydrite concentration of 25 mg L−1, and in both cases these values were signifi cantly greater than those recorded for other pH treatments. Th e similarity of results at pH 5 across all clay samples after 48 h (as compared with 24 h) is probably explained by a slower reaction time for the Dubbs and Forestdale clays due to their smaller CECs and surface areas. Conclusions Th e results from this study indicate that ferrihydrite, obtained as a byproduct of the aeration of Fe-rich drinking water, can be used as a fl occulant for soil clays suspended in surface water bodies. Th e eff ectiveness of the ferrihydrite for these purposes is determined by the CEC of the suspended clays and by the pH of the suspension because ferrihydrite has a pH-dependent charge. Development of a net positive charge on the ferrihydrite increases its ability to fl occulate negatively charged clays to an extent that is directly proportional to the acidity of the suspension. −1 Fig. 3. Percent transmittance of (a) Dubbs, (b) Forestdale, and (c) Sharkey In this study, ferrihydrite concentrations of 10 to 25 mg L clays in suspension after equilibrating for 24 h at diff erent pH and signifi cantly reduced the turbidity of suspensions at pH 5 after ferrihydrite contents. Error bars represent SD.

1890 Journal of Environmental Quality • Volume 38 • September–October 2009 References Beak, D.G., N.T. Basta, K.G. Scheckel, and S.J. Traina. 2006a. Bioaccessibility of arsenic (V) bound to ferrihydrite using a simulated gastrointestinal system. Environ. Sci. Technol. 40:1364–1370. Beak, D.G., N.T. Basta, K.G. Scheckel, and S.J. Traina. 2006b. Bioaccessibility of lead sequestered to corundum and ferrihydrite in a simulated gastrointestinal system. J. Environ. Qual. 35:2075–2083. Carlson, L., and U. Schwertmann. 1981. Natural ferrihydrites in surface deposits from Finland and their association with silica. Geochim. Cosmochim. Acta 45:421–429. Goldberg, S., and R.A. Glaubig. 1987. Eff ect of saturating cation, pH, and aluminum and iron oxide on the fl occulation of kaolinite and montmorillonite. Clays Clay Miner. 35:220–227. Goldberg, S., B.S. Kapoor, and J.D. Rhoades. 1990. Eff ect of aluminum and iron oxides and organic matter on fl occulation and dispersion of arid zone soils. Soil Sci. 150:588–593. Goldberg, S., H.S. Forster, and C.L. Godfrey. 1996. Molybdenum adsorption on oxides, clay minerals, and soils. Soil Sci. Soc. Am. J. 60:425–432. Jaynes, W.F., and J.M. Bigham. 1986. Multiple cation-exchange capacity measurements on standard clays using a commercial mechanical extractor. Clays Clay Miner. 34:93–98. Knight, S.S., R.F. Cullum, T.D. Welch, and C.C. Cooper. 2002. Sediment- chlorophyll relationship in oxbow lakes in the Mississippi River alluvial plain. p. 76–81. In Watershed management to meet emerging TMDL Environmental Regulations Conference Proceedings. ASAE, St. Joseph, MI. Kretzschmar, R., W.P. Robarge, and S.B. Weed. 1993. Flocculation of kaolinitic soil clays: Eff ects of humic substance and iron oxides. Soil Sci. Soc. Am. J. 57:1277–1283. Pettry, D.E., and R.E. Switzer. 1996. Sharkey soils in Mississippi. Bull. 1057. Miss. Agric. Stn., Starkville. Przepiora, A., D. Hesterberg, J.E. Parsons, J.W. Gilliam, D.K. Cassel, and W. Faircloth. 1997. Calcium sulfate as a fl occulant to reduce sedimentation basin water turbidity. J. Environ. Qual. 26:1605–1611. Rhoton, F.E., J.M. Bigham, and D.L. Lindbo. 2002. Properties of iron oxides in streams draining the loess uplands of Mississippi. Appl. Geochem. 17:409–419. Rhoton, F.E., M.J.M. Romkens, J.M. Bigham, T.M. Zobeck, and D.R. Upchurch. 2003. Ferrihydrite infl uence on infi ltration, runoff , and soil loss. Soil Sci. Soc. Am. J. 67:1220–1226. Rhoton, F.E., and J.M. Bigham. 2005. Phosphate adsorption by ferrihydrite- amended soils. J. Environ. Qual. 34:890–896. SAS Institute. 1999. Th e SAS system for windows. SAS Inst., Cary, NC. Schwertmann, U. 1964. Diff erenzierung der eisenoxide des bodens durch photochemische extraction mit saurer ammoniumoxalatlösung. Z. Pfl anzernernähr. Bodenkde. 105:194–202. Schwertmann, U., and H. Fechter. 1982. Th e zero charge of natural and synthetic ferrihydrites and its relation to adsorbed silicate. Clay Miner. 17:471–476. USDA Soil Conservation Service. 1981. Land resource regions and major land resource areas of the United States. Agric. Handb. 296. U. S. Gov. Print. Offi ce, Washington, DC.

Fig. 4. Percent transmittance of (a) Dubbs, (b) Forestdale, and (c) Sharkey clays in suspension after equilibrating for 48 h at diff erent pH and ferrihydrite contents. Error bars represent SD. a 24-h equilibration period. After 48 h of equilibration, ferrihydrite concentrations of only10 mg L−1 resulted in signiﬁ - cant reductions in turbidity at the same pH. In addition to the ability of ferrihydrite to reduce turbidity levels, research has demonstrated that this Fe oxide mineral serves as a scavenger of P, As, Pb, and other heavy metals.

Rhoton & Bigham: Ferrihydrite Reduction of Turbidity 1891