263Food, Agriculture & Environment, Vol.1 (3&4), August-December 2003

WFL PublisherScience and Technology www.world-food.netFood, Agriculture & Environment Vol.1(3&4) : 263-269. 2003

Competitive anion sorption effects on dairy wastewater dissolved phosphorus extractionwith zeolite-based sorbents

Thanh H. DaoUnited States Department of Agriculture, Agricultural Research Service, Beltsville Agricultural Research Center, Animal Manure and

By-Products Laboratory, 10300 Baltimore Ave, Beltsville, MD 20705, USA. e-mail: thdao@anri.barc.usda.gov

Received 22 November 2002, accepted 18 July 2003.

AbstractDairy wastewater is often used to irrigate field crops. Soluble and colloidal phosphorus (P) must be removed from the supernatant liquid to avoid furtherP loading of high-P fields. Information is needed on P sorption capacity of natural and synthetic zeolites and fly ash in a complex wastewater and onhow the spent products release sorbed PO4-P. Sorption isotherms were determined in single and multi-anion standard solutions and dairy wastewater toquantify the sorption capacity of modified zeolites and fly ash and increase our understanding of underlying mechanisms of oxyanion retention.Solution anion concentrations were determined by high-performance ion chromatography. The results show that natural zeolites have negligibleaffinity for NO3

- or PO43- anions. Surfactant-modified (SMZ) and synthetic (SZBP) zeolites and fly ash exhibit significant capacities to bind PO4-P.

Phosphate sorption on SMZ and SZBP was described by the Langmuir equation, with sorption maxima, Smax, averaging 0.71 and 0.31 mmol g-1,respectively. Class C fly ash strongly sorbs and removes PO4-P from solution. Sorption maxima increase by 4-fold and Langmuir K constants indicatea higher bonding energy than those of SMZ and SZBP. Competitive sorption is evident in PO4-P sorption from mixed solutions of SO4-S, NO3-N, andPO4-P. All sorbents removed dissolved PO4-P from multi-ion dairy wastewater suspensions containing 10 to 100 g solid L-1. The order of efficacy is flyash > SZBP > SMZ. Although differences in affinity and desorption exist, the zeolite-based sorbents prove valuable as temporary sinks and offerpromise in the development of reversible recovery treatments of P-laden animal wastewater.

Key words: Phosphorus, sorption-desorption, zeolite, dairy manure, dairy wastewater, fly ash, surfactant-modified zeolite, P removal, P recovery.

IntroductionAnimal manure management became a major environmentalchallenge on the farm and for the entire watershed when animalproduction changes from a pastoral setting to a confined andconcentrated environment. Major changes have taken place inanimal agriculture in the US over the past several decades. Today’slivestock production occurs on fewer farms and ranches with highconcentration of animals. Confined animal feeding operations(CAFO) generate large volumes of feces, urine, bedding, spilledfeed, wash water, and other processing wastes that are potentiallyrecyclable into sources of plant nutrients, soil conditioners, andenergy-producing raw materials1, 2, 3, 4, 5. The mixture or manureoften contains high concentrations of unassimilated nutrients, inparticular phosphorus (P) because livestock rations are oftenformulated with excess nutrients, compared to basal nutritionalrequirements6, 7, 8. Mineral dietary P contributed to high levels ofdissolved-reactive P (DRP) in animal excreta9. Non-ruminantmonogastric livestock (Sus scrofa domesticus; Gallus gallus)10, 11

as well as ruminant livestock (Bos taurus)12 manure also containlarge quantities of feed organic P, particularly phytase-hydrolyzableP (Dao, T.H., A. Lugo-Ospina, J.B. Reeves, and H. Zhang. 2003.Wastewater chemistry and fractionation of bioactive phosphorusin dairy manure. [in review], personal communication).Consequently, substantial amounts of phytate-P (myo-inositolhexakis dihydrogenphosphate) in feed grains are essentially notavailable to the animal and potentially contribute to water pollutionrather than animal productivity. There is increasing interest on post-excretion treatments to chemically bind or remove DRP in manurebefore it is applied to agricultural fields. Many soils in watersheds

of intense animal production contain excessive levels of nutrients,especially P due to repeated heavy applications of animal manure1,

2, 13, 14, 15. A promising technology to sequester manure-P and otherorganic nutrients is the separation of liquid manure into particulateand liquid fractions. Conventionally solid-liquid mechanicalseparation is achieved by coagulation, flotation, sedimentation,filtration, and screening 16. The solids can be composted and land-applied. The liquid phase is used for irrigation or depending uponhow much treatment and clarification is made, the water is reusedin animal production. Soluble and colloidal P must be removedfrom the liquid phase to avoid further loading and P buildup inhigh-P fields. Drinking water-treatment polymers have beensuggested to enhance mechanical solid-liquid separation andclarification of animal wastewaters17,18. Combinations of organiccoagulants and mineral P immobilization chemicals were showneffective in phase separation and liquid-phase PO4-P removal18.Fly ash induced particulate destabilization at rates e” 50 g L-1 andreduced solution-phase DRP at rates > 1 g L-1 by 52 and 71 % indairy manure suspensions containing 30 and 100 mg solid L-1,respectively. Aluminum and Fe salts also lowered DRP at rates <10 g L-1 and higher concentrations re-dispersed particulates andincreased DRP due to increased suspensions’ acidity and electricalconductivity.

Dissolved P immobilization chemicals that have been examined,included metal salts, water-treatment residuals, coal-combustionash, and industrial by–products19, 20, 21, 22, 23. Co-blending P-immobilizing mineral by-products and animal manure can reducemanure DRP concentrations but has raised concerns about the

264 Food, Agriculture & Environment, Vol.1 (3&4), August-December 2003

availability of P when the amended manure solids are reused inagronomic production21. Extracts of soils amended with fly ash-and caliche-treated manure at the rate of 22 Mg ha-1 had significantlylower Mehlich-3 extractable P than those of soils amended withuntreated manure22. Co-blending water treatment sludge withbiosolids increased P sorption and decreased P availability tosorghum sudangrass [Sorghum bicolor (L.) Moench] 24. Chemicalimmobilization and precipitation, however, defer P removal fromthe dilute liquid-phase to a more concentrated solid-phase. Anion-exchangers present an alternate approach to P recovery instead ofP immobilization and information is needed on their efficacy in ananimal wastewater environment. Materials possessing suchproperties include natural zeolites that are composed primarily ofclinoptilolite. Natural zeolites are hydrated aluminosilicate mineralscharacterized by cage-like structures. They have large internal andexternal surface areas, and high cation exchange capacities becauseof the permanent negative charges arising from isomorphicsubstitutions in the crystal structure. Zeolite surface chemistryresembles that of smectite clays. Zeolite surface areas range up to800 m2 g-1, and cation exchange capacities typically vary from 25to 300 cmol kg-1 25. Natural zeolites can occur as millimeter- orgreater-sized aggregates and are free of shrink-swell behavior.Zeolites exhibit superior hydraulic characteristics and are suitablefor use in filtration systems26. External cation exchange capacitieshave been determined for a few natural zeolites and typically rangefrom 10 to 50 percent of the total cation exchange capacity27. Bothnatural and synthetic zeolites are used as sorbents, ion exchangers,and molecular sieves in industrial applications and agriculture.Natural zeolites have been used in animal feeding operations tocontrol odors and to reduce diarrhea in cattle sheep, and swine28.Zeolites typically have a high affinity for ammonium and havebeen used to lower these concentrations in drinking water andwastewaters29. The low cost of natural zeolites ($50-65 Mg-1) makestheir use attractive in domestic wastewater treatments. Naturalzeolites, however, have little affinity for inorganic anions such asnitrate or phosphate. Treatment of natural zeolites with cationicsurfactants such as hexadecyl trimethyl ammonium bromide(HDTMA) changes their surface chemistry, and the electrical chargereversal induced by the surfactant bilayer allows the retention ofinorganic anions by ion exchange30. In groundwater treatment,modified zeolites were shown to remove anionic environmentalcontaminants that include phenolate, arsenate, selenate, andchromate30, 31, 32. Other mineral by-products exhibiting zeoliticcharacteristics and behavior include fly ash from coal combustionin electricity generation. During combustion, fly ash, bottom ash,and boiler slag are produced. Fly ash is a fraction of the ash streamcomposed of particles between 0.001 to 0.1 mm and is carried fromthe boiler in the flue gas. Annual production of coal combustionby-products was estimated at 75 million Mg and about 20% of theash produced are being used33. A large volume remains in stockpilesor is disposed in landfills. Fly ash Class C has been shown tostrongly bind PO4-P and offer the means for reducing animalmanure-P solubility and the risks of offsite movement fromuncovered manure storage areas and land application of stockpiledor composted manure22. Little is known about how natural zeolite,modified natural zeolite and synthetic zeolite, would behave in acomplex and concentrated livestock wastewater, and whether andhow the spent product releases the retained P. Information is neededon similar wastestream by-products such as fly ash, whether natural,

synthetic, sorb and remove PO4-P, NO3-N and SO4-S from multi-ion mixtures and animal manure suspensions. The objectives ofthis study were to (i) quantify sorption capacity and increase ourunderstanding of underlying mechanisms of retention for oxyanionsby coal-combustion ash and synthetic, or surfactant-modifiednatural zeolites, and (ii) determine the effects of competing anionson PO4-P sorption and the potential of these sorbents for removingenvironmentally-sensitive anions from dairy wastewater to developreversible recovery systems for dissolved P in animal manure.

Materials and MethodsAnion sorbents: The natural zeolite was a clinoptilolite-rich tufffrom the St. Cloud mine near Winston, NM. The physical andchemical properties were previously described in detail30. Insummary, the St. Cloud zeolite consists of about 74 % clinoptilolite,5% smectite, 10% quartz and cristobalite, 10% feldspar, and 1%illite, based on internal standard X-ray diffraction analysis34. Theexternal cation-exchange capacity is 70 to 90 meq kg-1 while thetotal CEC is about 800 meq kg-1. The major exchangeable cationsare Ca2+ and K+. The external surface area of the St. Cloudclinoptilolite was 15.7 m2 g-1. Surfactant-modified zeolites (SMZ)were prepared from natural zeolite size fractions in the 0.4 to 1.4-mm range (14-40 mesh). The HDTMA surfactant was used tomodify the zeolite surface at a loading rate of 200 mmol kg-1 30, 32.In summary, the zeolite pellets were equilibrated with a 20 mMaqueous HDTMA solution at a solid-to-liquid ratio of 1:4. Themixture was shaken for 24 h at 30°C, followed by centrifugation,washing with deionized water, and air-dried before use. Thesynthetic zeolite by-product (SZBP) is a waste stream by-productof the industrial manufacture of synthetic zeolite at the W.R. Grace,Curtis Bay plant, near Baltimore, MD. Bulk samples of the wetfilter cake were obtained from the dewatering process of themanufacturing plant’s waste stream. The coal-combustion ash is aby-product of the electricity generation by the Harrington plantnear Amarillo, TX. Bulk samples of the hydrated ash were obtained,dried, and powdered. The coal-combustion ash is the same materialwith physical and chemical characteristics presented in previousstudies22.

Anion sorption-desorption isotherms: Anion sorption-desorptionby the mineral sorbents was determined by the batch equilibrationmethod as described by Dao and Lavy35. Sorbents (0.5 g) wereweighed into plastic culture tubes (16 by 125 mm). Solutions (1:20,w/v) of known concentrations (0, 0.25,0.5, 1, 2, 5, 10, 15, and 20mM) of PO4-P, NO3-N, and SO4-S were added to the tubescontaining the sorbents to determine sorption of individual anionsat 20°C. The sorbent-solution mixtures and triplicate tubescontaining P solutions alone were agitated on an end-over-endshaker for 16 h at room temperature. After centrifugation at 7000x g for 15 min, the supernatant solutions were filtered through 0.45-µm membranes, and P concentrations were determined aspreviously described. The amount of P sorbed was calculated asthe difference between the amounts of P in the standard solutionsin control tubes containing no sorbent and those in the equilibriumsolutions in tubes containing sorbents at the end of the equilibrationperiod. Competitive anion sorption was measured in equimolarmixtures of PO4-P and NO3-N (1:1), PO4-P, and SO4-S (1:1), andPO4-P, NO3-N, and SO4-S (1:1:1). Sorption isotherms were obtainedas previously described. Dairy manure suspensions containing 10to 100 g total solid L-1 were prepared from freshly reconstituted

265Food, Agriculture & Environment, Vol.1 (3&4), August-December 2003

manure and appropriate dilution with tap water18. The three sorbents(0.5 g) were added to aliquots of the supernatant of the dairy manuresuspensions that were isolated by centrifugation (1:20, w/v).Sorption or reduction in manure dissolved PO4-P by the mineralsorbents was characterized in batch equilibration as previouslydescribed. Phosphorus desorption was determined by repeateddilution and batch equilibration in deionized water. Followingsorption measurements, the solution-phase was removed andreplaced with an equivalent volume of deionized water. The mixturewas gently shaken on a gyratory shaker (50 oscillations min-1) for16 h at 20oC. At the end of the equilibration period, the manuresupernatant and sorbent mixtures were centrifuged at 7,000 x g for15 min. The supernatant solutions were filtered through 0.45-µmmembranes, and P concentrations were determined as previouslydescribed. The amount of P desorbed was calculated from thechanges in solution P concentrations for each successiveequilibration. Anion sorption and desorption data were describedusing the Langmuir sorption model:

S = bKCe/(1+KCe)

that can be rearranged to the following linearized form,

Kb1 C S

C ee += bwhere Ce = equilibrium solution-phase P concentration (mM), S =equilibrium sorbed-phase concentration, (mmol kg-1 soil), b =sorption maximum (mmol kg-1 soil), and K= constant related tobonding energy (L mmol-1).

Solution-phase anion concentrations were determined by high-performance ion chromatography12. In summary, PO4-Pconcentrations of suspension aliquots were determined using ananion-exchange column (IC-Pak HC) and pre-column on a Waters2690 system (Waters Corp., Milford, MA)†. The high-performanceliquid chromatograph is equipped with UV (Model PDA 996) andelectrical conductivity (Model W432) detectors. The eluent was aborate-gluconate-acetonitrile solution, pH 8.5, and pumped at aflow rate of 1.5 mL min-1. The sorption and desorption experimentswere established according to a randomized complete block designwhere sorbents and P concentration treatments were replicated threetimes. Differences in treatment main effects and interactions weredetected following analysis of variance and the Duncan multiplerange test at the 0.05 probability level using the Statistical AnalysisSystem36. Sorption and desorption isotherm data were also fittedto the Langmuir sorption equation and isotherm comparisons12 weremade using the General Linear Model procedure.

Results and DiscussionNatural zeolites have negligible affinity for anions such as NO3

-

or PO43- (Fig. 1). Permanent negative surface charges result in

repulsion and negative sorption of anionic species. Surfactant-modified zeolites, SZBP, and the coal-combustion ash, in contrast,possess significant capacities to sorb and bind PO4-P in aqueoussolutions (Fig. 2). The treatment of natural zeolites with HDTMAcationic surfactant causes a surface electrical charge reversal thatallows the retention of NO3-N and PO4-P anions by SMZ. The

Langmuir sorption model described phosphate sorption by SMZand SZBP, with Smax constants averaging 0.712 and 0.308 mmolg-1, respectively. Retention mechanisms may be attributed to anionexchange and precipitation, as PO4-P isotherm behavior wasindicative of secondary precipitation reactions. It will be shownin later discussion that precipitation processes become moreprominent in removing PO4-P from solution when the ion exchangecapacity is near saturation, whether by PO4-P or other anions. Ahigh Ca2+ content of the untreated zeolite34 made it likely thatsorption was not the only mechanism of P removal from solution38.Critical ion activity products of calcium phosphates (i.e.,hydroxyapatite log Ksp ranges from = 10.93 39 to 14.46 40) wereexceeded at low P concentrations, yielding the “pseudo sorption”behavior of PO4-P on SMZ. The coal-combustion ash stronglysorbs PO4-P anions and removes PO4-P completely from thesolution phase (Fig. 2). Not only did sorption maximum increaseby 4-fold (i.e., 2.60 mmol g-1), the magnitude of the K constantalso indicated a higher bonding energy. Surface functional groupsof ash particles such as silanol groups and metal hydroxyl groupsare postulated to have reacted with anions as follows:

~ XOH + H2PO42- ⇔ ~ XOPO3H

- + OH-

where X = cation, i.e., Si 4+, Ca2+, Al3+, Fe2+, Fe3+. Phosphate-P islikely retained by such chemisorption mechanisms as unhydratedClass C bituminous coal-combustion ash contains SiO2 (647 gkg-1), Al2O3 (126 g kg-1), Fe2O3 (18.5 g kg-1), MgO (11 g kg-1), andCaO (33 g kg-1) 37.

In binary anion mixtures, NO3-N or SO4-S competes and reducesthe sorption of PO4-P by SMZ (Fig. 3A). Monovalent NO3

- wasfound more competitive than divalent SO4

2- at displacing PO4-P onthe sorbents’ exchange complex. The competition for sorption siteson SMZ and SZBP surfaces are even more pronounced when bothNO3-N and SO4-S are present in solution, indicating that PO4-Psorption mechanisms include ion exchange. The competitivenessof these anions is synergistic. Sorption of PO4-P in tertiary P, N,and S solutions is further suppressed, resulting in reductions inSmax equivalent to 1/3 of that when PO4-P is alone in solution (Fig.3A).

Multiple modes of PO4-P retention are evident for SZBP (Fig.3B). At low Ce (< 0.5 mM), PO4-P sorption follows the Langmuirmodel, where S increases nonlinearly toward an Smax. Differencesin the competitiveness of NO3-N and/or SO4-S in displacing PO4-Pwere small. However, as P concentration rose beyond this range,apparent PO4-P sorption again increased rapidly. These resultssuggest that the sharp increase in “apparent sorption” is presumablydue to precipitation reactions, yielding insoluble phosphates tocontinue removing PO4-P from solution. Fly ash, as shown in Fig.2, shows a very high affinity for PO4-P and reduces solution-phaseP to non-detectable levels (isotherms not drawn or shown). Surfacereactivity of the finely divided fly ash arises from a large surfacearea and high variable charge density. Therefore, thecompetitiveness of NO3-N and/or SO4-S in displacing PO4-P arenot observed or demonstrated.

All three sorbents retained the capacity to sorb PO4-P in dairywastewaters containing a wide range of total solids (Fig. 4). Thesolution chemistry of such mixtures is complex as anionic speciescan include SO4

2-, Cl-, CO32- HCO3

-, NO3-, NO2

-, aluminate, silicate,and last but not least, PO4-P. In such an admixture, thezeolites’capacity to react and bind with manure PO4-P remains

† The mention of a trade or manufacturer names is made for information only and does not imply an endorsement,recommendation, or exclusion by the USDA, Agricultural Research Service.

266 Food, Agriculture & Environment, Vol.1 (3&4), August-December 2003

unaltered. The order of efficacy, as shown previously for PO4-Pstandard solutions, is as follows: Fly ash > SZBP > SMZ. A sorption isotherm for native wastewater PO4-P was constructedfor the SMZ data that showed differential levels of PO4-P in theequilibrium-solution phase (Fig. 4A). The sorption isotherm isdescribed by the Langmuir equation (Smax = 3.16 mmol g-1 and K =0.85 L mmol-1). Manure PO4-P removal increases with wastewaterP content and sorption appears more significant in these complexsolutions than P sorption from mixed anion standard solutions. TheSMZ has the ability to sorb organic environmental contaminantsinto the organic hydrophobic core created by the surfactant tailgroups31. In the dairy wastewater, organic carbon and coordinationligands complexing aquo-metal ions (Al(H2O)6

3+ or Fe(H2O)63+ )

may partition in such as layer to add to the PO4-P sorption capacityof SMZ. However, the capacity to remove and sorb manure PO4-P is appreciable, amounting to approximately 100 g P kg-1 of SMZ.At the lowest sorbent amendment rate of 4 g L-1 suspension, thesorption capacity of SZBP (Fig. 4B) and fly ash (Fig. 4C) wasnever exceeded. Therefore, both sorbent matrices sorb andeffectively reduce manure dissolved P concentration to non-detectable levels.

On the other hand, SMZ was the sorbent that gives up sorbedPO4-P most readily (Fig. 4A), followed by fly ash that graduallydesorbs manure P after four cycles of dilution and batchequilibration with deionized water (Fig. 4C). Meanwhile, the SZBPretains manure PO4-P more tenaciously than SMZ or fly ash (Fig.4B). Manure PO4-P forms linkages that are largely not disruptedby repeated dilution and the strong sorption of PO4-P suggestsocclusion and chemisorption by the hydrogel formed by the blendof aluminate and silicate in SZBP. Surface complexation andprecipitation as amorphous aluminum phosphates can alsocontribute to the stability of the SZBP-PO4-P product. The extractionof sorbed P was carried out with water as the solvent although theextraction efficiency can be greatly improved by using a salt ormild acidic solutions. The relative ease of extractability of P fromthe spent sorbents still suggests circumstances for beneficial reusein production agriculture. For example, the P-enriched sorbentscan be used as a controlled-release nutrient source with predictablerelease characteristics.

ConclusionsThe experimental results suggest that modified natural and syntheticzeolites, and Class C coal-combustion ash have significant capacityto sorb PO4-P. Potential retention mechanisms include anionexchange and surface complexation. Fly ash functional groupssuch as silanol groups and metal hydroxyl groups strongly sorbPO4

3- to reduce solution PO4-P and suppress P solubility in theparticulate fraction, as our previous research has shown18. Thesecapabilities make fly ash a very versatile P sorbent for low costreclamation and treatment processes and reversible P recoverysystems. In mixed solutions of SO4-S, NO3-N, and PO4-P and indairy wastewater, competitive anion sorption is evident in PO4-Psorption onto the mineral sorbents. Differences in affinity andpotential to release sorbed P exist between the natural and syntheticzeolites and fly ash. These mineral sorbents may prove valuableas temporary sinks in reaction beds to treat animal wastewaters.Sorption of inorganic oxyanions is reversible and suggests that thespent sorbents can be further used in agricultural production.Manure processing technologies based on the retention and release

capacity of these mineral materials offer promise in the developmentof resource recovery approaches that are reversible for lowering Pcontent from liquid manure suspensions.

AcknowledgmentsThe author sincerely acknowledges the technical assistance of T.Wong and A. Pang in the conduct of this study. The SMZ has beenkindly provided by R.S. Bowman, New Mexico Institute of Miningand Technology, Socorro, NM and the SZBP by R. Harrison, W.R.Grace & Co., Curtis Bay Plant, Baltimore, MD.

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39 van der Houwen, J.A.M., and Valsami-Jones, E. 2001. The applicationof calcium phosphate precipitation chemistry to phosphorus recovery:The influence of organic ligands. Environ. Tech. 22:1325-1335.

40 Lindsay, W.L. 1979. Chemical equilibria in soils. John Wiley & Sons,New York, NY.

Figure 1. Negative anion sorption of NO3-N and PO4-P on natural zeolites. Dashed lines aresorption isotherms fitted to observed sorption data using the Langmuir equation, S = bKCe/(1+KCe), where Ce = equilibrium solution-phase concentration (mM), S = equilibrium sorbed-phase concentration, (mmol g-1 ), and b and K= constants.

268 Food, Agriculture & Environment, Vol.1 (3&4), August-December 2003

Figure 2. Phosphate-P sorption solids by surfactant-modified zeolite (SMZ), a syntheticzeolite by-product (SZBP), and Class C fly ash. Dashed lines are sorption isothermsfitted to observed sorption data using the Langmuir equation, S = bKCe/(1+KCe), whereCe = equilibrium solution-phase concentration (mM), S = equilibrium sorbed-phaseconcentration, (mmol g-1 ), and b and K= constants.

Figure 3. Effects of single and multi-anions on PO4-P sorption by surfactant-modified zeolite (A) and a synthetic zeolite by-product (B). Dashedlines are normalized sorption isotherms fitted to observed sorption data using the Langmuir equation, S = bKCe/(1+KCe), where Ce = equilibriumsolution-phase concentration (mM), S = equilibrium sorbed-phase concentration, (mmol g-1 ), and b and K= constants.

269Food, Agriculture & Environment, Vol.1 (3&4), August-December 2003

Figure 4. Phosphate-P rem

oval from dairy w

astewaters containing from

10 to 100 g L-1 of total solids by surfactant-m

odified zeolite(SM

Z), a synthetic zeolite by-product (SZBP), and C

lass C fly ash and PO

4 -P desorption from the spent sorbents.