Some Fagtors Affecting the Synthesis Of

Cawdlan Min*alogist VoI. ld w.4046 (1976)

SOME FAGTORSAFFECTING THE SYNTHESISOF GRYPTOCRYSTALLINE STRENGITEFROM AN AMORPHOUSPHOSPHATE COMPLEX

N. D. WARRT canada centre for Inhnd waters, P.o. Box 5050,Burlington, ontario, canada. JAMFS R. KRAMER Deportment of Geology, McMaster University, Eamilton, Ontafio, Canada.

ABsrR.Acr ments undertaken by Einsele (1938) showed that in the pH range betweetr Four experiments were initiated to examine the 5 and 8, signifi- effects of pH, Eh (dissolved oxygen), agitation and cant removal of inorganic phosphate from solu- coocentration of key reactants (Fe and p) on the tion by precipitation with amorphous iron oxide reaction of P with amorphous iron oxyhydroxide did occur. Subsequent studies (Williams er a/. under conditions encou.ntered in natural systems. l97la,b,c) have suggestedthat this reaction may pH and dissolved oxygen content are tle most im- be of considerable importance in natural wa- portant variables controlling initial p uptake in the ter system$. Mortimer (1971) found that the aqueous phase. The pH range (4-Z) is optimal for uptake and release of P in natural waters could uptake wher Fe greater is than 0.25 mg,zl and dis- be related to the oxidation state of the sedi- solved oxygen concentration is )l|Vo saturatiou. ments, Oxygen depletion resulted in increased Digestion of the resultant iron oxyhydroxide-phos- P concentrations phate under conditions of neutral pH, oxidizing Eh in the overlying water. and T: 100"C, produced a mixture of crypto- The thermodynamic properties of the iron crystalline strengite and phosphosiderite. This'end- phosphate system have been examined in detail product verifies the thermodynamic predictions of in a seriesof papers by Nriagu (I972a,b, 1974). Nriagu (1972b). He has summarized the available thermochenr- ical data from tle basic iron phosphateminerals REsurvr6 and has developed theoretical models of the solubilities and stabilities of these minerals in Quaue exp6rienoesont 6t6 entreprisesafin aqueous systems. d'examiner les effets du pH, (oxygdne du Eh dis- No complete sous), de I'agitation et de la experimental description of the concentration des phosphorus r6actifs cl6s (Fe et P) sur la r6action du p avec de reaction of with amorphous iron I'oxyhydroxyde ferreux amorphe dans des conditions oxides exists, although Patrick & Khalid (1974) rencontr6es dans des systdmes naturels. k pH et have provided good experimental information le contenu en oxygdne dissous sont les varianies les about five different soil types. plus importantes controllant la fixation du p ini- The effects of Eh and pH on the sorption of tial de la phase aqueuse. La port6e du pH (4-7) est P onto ferric oxyhydroxide motivated the work au maximum pur la fixation lorsque Fe est plus described here. These experiments de- grand were que 0.25 mg/l et que la concentratlon d'oxy- signed g€ne dissous est plus grande que l|Vo de satva- (i) to describe experimentally the effects of Ia digestion de I'oxyhydroxyde de phosphate !ion. Eh and pH in a simple aqueousFe-P ferreux dans des conditions de pH neutre, oxirdant sys- - Eh et ? 100'q a produit un m6lange de itren- tem, and gito c4ptocristalline et de phosphosiderite. Ce pro- (ii) to define the conditions of formation duit final conflrme les pr€dictions thermodynami- of a possible crystalline end-product of ques de Nriagu (1972b). this interaction. (Iraduit par le journal) ExPBRIMENTAL INrnonucrroN In all experiments the simplest possible chem- The reaction of phosphate ions with amor- ical systems were used so that extraneous chem- phous iron oxide has been suggested as a prob- ical species would not interfere with the pro- able inorganic control on tl.e availabilitv of cesses being investigated. The master variables phosphorus in water (Mortimer 1941). Eiperi- of Eh, pH, temperature and ionic concentrations of the elements involved were set at values approximating as possible rTo closely as those en- whom all correspondence should be sent. countered in the natural environment. 40 SOME FACTORS AFFECTING SYNTIIESIS OF STRENGITE 4T

TABLEI. SUI'1ilARTOF3 EXPERIMEfiTSCARRIED OIII TO DEFINE POA-IRON OXIDE ASSOCIATION Exp€fl[Ent Purposeof Experirent KeyVariEbles Results NuIDer 0bserved I (l) To detemlne effect of pH on P04-3 uptake by mrphous ircn oxlde pH uptake dependenton pH. (ll) To detemlne the stablllty of the phosphateiron hydrcxlde cooplex pH Deflned pH range of stability of complex, at dlfferent DH's. ltttl To detemlne ilm of ructlon Tlm Reactlon coltplete t! ? ![i!-

2 A Effect of oxygenconc. on stabillty of complex Eh Conplexdlssolves f,hen dlssolved oxygen2!g 02lI. R rf orvoan h:;nteB svsten?hen dois comllr refom? Eh RefonB whenorJgen content 1-Ang0,ll.

Cancomplex fom a stable lnsoluble mlneral? Eh, pH, . Crystalllne prcduct fom and follm the conc. P0a-J themdJmMiq pred{ctlons of l{rlagu'

For exporlment 2A results rsad dlssolved orygon <2 mg.

The phosphonrs and ton concentrations em- had its pH raised to 7 or 8 for about 10 sec- ployed in these experiments closely approximate onds, then quickly had it lowered to 1.5 with a those ensountered in anoxic hypolimnetic wa- pre

given in Table 2. Almost all P is removed from, solution in the pH range 5.5-7.5 when no solid is present initially. Outside this pH range, considerable P remains in solution. The results from the second set of solutions. .e= 75 of Experiment No. 1 (rilith initial iron oxide present) are also shown Figure In pH a) in 1.. the region between 3 and 6, more than 95Vo of the total phosphate is seen to have been removed in the second set of solutions, whereas. E50 none IMaslost in the first set. The only differ- R ence between the groups is that the second set E contains solid particles of amorphous iron oxide in the pH range between 4 and 6, whereas the R25 first set does not. Evidently, the solid is capable d of removing more than 95Vo of. the phosphate from the aqueous1fiase. In terms of concentra* tions, the amount of total phosphate was re- duced to less than 1O pg POa/l from 24O p,g 0 POrll. We can conclude that particulate amor- 23456789 phous oxyhydroxide phosphate pH ferric scavenges very efficiently from aqueous solution in the Fto. 1. P remaining in solution vs pH. Solid line pH range between 4 and 7. is the result for no precipitate existing below Inefficient phosphate removal at pH values pH : 6; dashed line precipitate is result for greater than 7.5 is seen in both experimental initially present. When iron-oxide precipitate is sets. pronounced, present, uptake of P onto the solid occurs down This becomes so thrt at a to pH : 4; in addition, P uptake is not as pH of 9, only 50/o of the P has been removed efficient above pH 7 for both experiments as from solution. Since the solubility of the amor- in the neutral pH range. Initial Fe : 25 me/|, phous ferric oxide complex does not increase initial P : 2L6 v,g POE/|, Sec Table 4 for appreciably at this pH (Stumm & Morgan 1970), other details. the removal of phosphate by the complex must not proceed very well at pH's greater than 7.5. (1969) After digestion, the solid material was fil- Parks has shown that the Z.P.C. of tered on a O.45 pcm Millipore filter, dried in air, iron oxide particles in aqueous solution is 8.5. examined microscopically and x-rayed with a Below this pH, the oxide is positively charged Guinier diffraction camera. and easily sorbs negatively-charged phosphate ions. As this pH value is approached, howevqr, positive METHoDs the surface charge on the iron oxide particles decreasesand these particles become All glassware was washed thoroughly in sul- less efficient in attracting the phosphate. The furic acid and allowed to soak in 1.0 N acid for at least 2 days prior to its use in any experiments. TABLE2. SUIT.IARYOFEXPERII4ENT I. Total phosphate was Inltlal Flnal Total Injtial Total Inltlal Eh (mv) g poa-3 determined using the pH pH Fe (ms/l) Po4-3(!s/r) Remaini ng molybdenum-blue method with extraction in isobutanol after Sutherland et a!. 2.'t7 2.99 2.5 362 100.0 {1966). The 2.13 4.05 357 100.0 sensitivity of this method is about O,5 pC W"/1. 2.20 4.97 ,q 368 100.0 2.04 5.97 216 Jbo 4.q fron was measured spectrophotometiically as 2.07 6.99 2.5 216 3tJ Z.a rJs orthophenathroline 2.11 7.99 216 367 14.0 complexn after Shapiro 2.15 9.0? 216 374 46.5 & Brannock (1962). All standard solutions *ere 2.1-?.2 3.00 216 460-350 100.0 prepared using reagent-grade chemicals and ta distilled-deionized 5.01 2.5 216 il ?a water. 216 " 4.4 6.03 2.5 ' Reagents used in the experiments are: Fe(S) 6.98 tq 216 4.5 8.00 216 ' 14.0 in HCl, NaOH, HCl and KHfOn. 9.01 , 47.0 2.l l 4.01 365 1.5 Rrsur,rs AND DrscussroN ,io 5.00 361 l.0 'Effect Figure L shows the removal of p as a func- of varying pH on uptakeand release of P on ircn oxldes Flrst set are results wlth no lnltlal solidl secondset are tion of pH (Experiment l). Actual data are results for initlal Fe-oxldeprecipitate. SOMB FACTORS AFFECTING SYNTITESIS OF STRENGITE 43 amount of phosphate in solution then increases TABLE3. SIJIO4ARYOF EXPERI}IENT2. relative to the amounts in solution when ad- Total Inltlal Total Inltlal Eh il P04-3 Rminlng sorption is proceeding at full capacity, i.e., Fe (mg/l) m -3 r-rrl ln [ater (5) at pH's (7.5. BATCHA 2.5 216 390 6.3 0 The results of the cenhifuging experiment 2.5 216 390 6,3 0 2.5 216 370 6.9 0 (solutions C-1 and C-2, Experiment 1, Table 2) 2.5 216 192 8.6 95 the adsorption 216 195 8.7 97 show tlat the reaction t:me for 2.5 216 190 9.0 95 to occur is less than 120 seconds. This repre- MTCH B 216 270 7.5 0 sents the total time required to form the ferric 2.5 216 272 7.3 0 2.5 216 273 7.1 0 phosphate complex and to centrifuge it out of 216 192 7.4 98 the top few mm of solution. This fast reaction 216 198 7.7 97 2.5 216 200 7.5 97 can be explained as a physical sorption teaction, 2.5 216 257 7.1 0 2.5 216 254 7.5 0 as a chemisorption teaction, or as a simple tc 216 249 7.8 0 chemical precipitation of the ferric phosphate -Effect complex. Unfortunately, no further differentia- of Eh upon retentlon of P on Fe-oxlde. tion as to which mechanism is operative in this : system can be made using the results of Experi- TABLE4. 5UI,t{ARYOr eXPgntMeNrS] ment 1. FinaI KH2P04 KI2PO4 Final'P04-3Concen- pH Experiment 2 relates P removal to the oxida- as SoIld as Llquld tration' mles/l tion state of the solution. The influence of Eh 6.8 141 YA< No 3.9 x l0-5 on phosphate complex is found ia 71 372 Yes No 3.9 x l0-1 the ferric 368 No Yes 3.9 x l0-? Table 3. Here we see that lowering the Eh of 363 No Yes 3,9 x l0-a Yes 3.9 x lo-3 tho system to between I92 and 2O0 mV from 7.0 359 No tEffect the orisral 370 mV resulted in the disappear- of phosphateaddltion on Fe(oH3)dlagenesjs. ance of the complex and an increase in phosphate concentration in the solution. After oxy- TABLE5, R[suLTsor gxptrtMg{rno. g'. gen had been allowed to re-enter the vessel, it Dlgestlon Dlgestlon .Solld KH2P04 Dissolved Final P0a-3 was noted that the precipitate reappeared and Pmduct Tlre Added Klr2P04Added that the phosphate had been removed again ff[:11""*, Amrphous 2 meks x 3.9 x l0-9 from its soluble phase. This phenomenon is Prcduct 5? 2 ueeks x 3.9 x l0-: identical to that described by Mortimer (1971) AmrDhous 2 weeks x 3.9 x lo-t ArBrDhous 2 weeks x 3.9 x l0-1 for natural systerns. Prcdlct S 2 weeks x 3.9 x l0-r The Eh value of I92 mY in this experiment 'Effect is equivalent to a dissolved oxygen concenEa- of phosphateaddltion on Fe(oH)3diagenesls. tion of about O.2 mgll. This tepresents about ln Tabfe3, tor g/l read ug/|, 2/o of. the oxygen that was originally in the solution. The first reappearance of precipitate identifiable crystalline product. These conditions was confirmed at an Eh of.249 mV; this cor- are: lPOa = 3.9 X 10i moles POns/L pH = responds to a dissolved oxygen content of I.2 7.0, Eh : 359 mV, digestion time : 18 days, mg Oz/l or LSVo saturation. temperature = 90"C. Dissolved-oxygen values measured by Mor- The product was identified by its x-ray pat- timer (1971) as being necessaryfor the release tern recorded with the Guinier diffraction ca- of phosphate and iron into water from lacus- mera. After the major reflections were indexed, trine sediments ranged between I and 2 mg the product was identified as a combination of Orl1. Comparison of his values with those of cryplocrystalline strengite, FePO+'2HzO -and this study (0.2-1..2 mg O"/1) provides good phosphosiderite, FePOa'2HsO (Iable 6). These agreement between laboratory measurements minerals are chemically identical but struc- and field observations. turally dimorphic, their sole difference being in The third experiment provides two notable the tilt of their phosphate tetrahedra with re- results. The first is tlat a crystalline ferric spect to their metal-oxygen octahedra. For phosphate mineral can form from the amorph- structural details of these rninerals the publica- ous ferric phosphate complex; the second is tions of Moore (1966) and Borensztajn (1966, that its conditions of formation confor'm to should be consulted. Table 6 provides a brief those defined thermodynamically by Nriagu structural comparison of these minerals- (1972b). The findings of experiment 3 are The conditions of formation described above summarized in Table 5. (Figure 2) fall within the thermodynamic boun- As seen in the Table, only one set of condi- daries of strengite as defined by Nriagu (1972a). tions results in the formation of a uniquely In the natural environment, strengite is not 44 THE CANADIAN MINERALOGIST

likely to be a major constituent of lacustrine organic orthophosphate ions and amorphous sediments (f{riagu 1974) because (i) it is un- iron oxide in an isolated experimental system: stable relative to tinticite and cacoxenite, two ( 1) Phosphate removal from solution by other ferric phosphate minerals, and (ii) most amorphous iron oxide proceeds efficient- lacustrine sedimentary environments are reduc- ly only between pH values of. 4 and 7. ing a few mm below the sediment-water inter- In addition, solid particles of amorphous face. The formation of cryptocrystalline stren- iron oxide are necessaryfor the reaction gite in oxidized rnicrozones at the sedimenr- to proceed. water interface cannot be ruled out though and, (2) The reaction between the solution and rn fact, one occurrence of cryptocrystalline the solid ferric oxide is rapid; so rapid strengite has been reported by Halbach (L974). in fact, that it is complete in less than He identified the strengite in ferromanganese 120 seconds. concretions from the sediments of some Finnish (3) The iron oxide-phosphate complex is lakes. In this case, the formation of strengite stable in the pH range from 4 to at was attributed to the upward migration of fer- least 10, provided that the Eh is greater rous ions to the surface sediments, where they than 25Q mV. If thte redox potential were oxidized to ferric ions and reacted with becomesless than 200 mV, the complex the phosphateto form strengite. becomes unstable and dissolves. (4) This complex can be altered to crypto- Suuueny eNo CoNcrusroNs crystalline strengite and/ or phosphosiderite after a 2-week digestion at 90oC. These experiments have revealed the follow- These results are applicable to the short-terrn ing characteristics of the reaction between in- phosphorus budget of lakes that are shallow

X-EXPERIMENTAL @NDITIONSOF THIS STUDY STRENGITE

0.o

-o'1 .-' uJ -o.2 -0.3 iR -0.4 --;utto-

-0.6

%og*"-or* -g 6

Frc. 2. Stability relations of iron phosphates and hydroxides in the absence of HrS (after Nriagu lg1?a). SOME FACTORS AFFECTING SYNTHESIS OF STRENGITE 45

@ uptake into particulate iron oxyhydroxide), TABLE6. C0llPARls0ll0F STREIIGITE(FeP04.2tl20)' PH0SPH0SIDERITE lakes tlat undergo periods of extreme oxygen (FeP0a.2HZ0),All0 CRYSTALLINEPR0DUCT DIGESIED AT pH 7 depletion @ release from sediments) and lakes subject high loadings phosporus Strenglte Phosphosiderlt€ pH 7 Dlgest to of iron and PDF IS-513 PDF l5-390 (phosphate-mineral formation). Anoxic conditions ((2 mg/l dissolved oxygen) are a major, I d(A) r d(A) I d(A) 80 40 6.51 80 5.50 though not unique, contributor to the release 60 60 4.90 60 4.90-4.95 80 4.69 50 4.69 of phosphorus from sediments; anoxic condi- 100 4.38 80 4.37 100 4.37-4.38 tions commonly occur in the hypolimnion of 40 4.12 4.12 60 ?oc io eutrophic lakes, such as in the central basin of 40 3.72 20 3.72 80 3,61 40 3.62 Lake Erie @urns et ol, 1,972), 20 3.29 l0 3.34 Lakes having their pH buffered above the pH 80 3.Il l0 3.20 3.tI 60 3.00 40 3.00 range of maximum P removal (pH>7) probably 60 2,95 40 a.to t 00 2.78 2.76-2.80 can attribute part of their labile P to the de- 10 creasing sorption of this nutrient by amorphous '10 l0 2.65 2.63 iron oxide above this pH. Most hard water (car- 80 2.54 60 2.57 6n 2.54-2.57 bonate) lakes have pH's about 8, so this mechan- 40 2.M5 l0 2.46 20 2.43-2.45 't0t0 2.400 ism is probably operative in them. 2.360 20 2.342 10 20 2.258 The formation of phosphate minerals should 40'10 2.I80 l0 2.220 be an important control on phosphate availabi- 2.134 l0 2.168 40'10 20 2.122 2.101 lity in fresh-water and sedimentary environ- 2.035 l0 2.O72 2.010 40 2.001 60 2.010 2.001 ments where total iron is in large excess over sulfur. In most natural environments, the phosphate minerals vivianite, lipscombite, beraunite and rockbridgeite should constitute the most 40.0 ml of a 12.0 mg POno/1 working solution stable mineral suite (Nriagu 1974), as opposed to 20O0 ml with D.D. water. This produced a to the strengite minerals synthesized in this stu- solution containing 24O 1.r,gPOa{,/I. The work- dy. Vivianite is probably the most common, and ing solution was prepared by dissolving 0.01569 has been identified in sediments from Lakes g of KHzPOa in 1000.O ml of D.D. Water. Superior, Erie and Ontario (Dell 1973). Experiment I A different application of these results could 1 were be in the field of nutrient removal from domes- The fourteen solutions of experiment of the tic wastewater. If solutions containing fenic prepared by mixing together 10.00 ml iron were applied to oxygenated wastewater iron standard Q5 mg Fe+a/l) with 90.O ml of (24O under near-neutral pH conditions, large amounts the phosphate standard 1tg POa€/l) in a prepared of soluble phosphate should be removed. In 250 ml beaker. All of the solutions this our experiments,more than 95% of the original for experiments 1 and 2 were prepared in phosphate was removed from solution. Com- manner. parablo results in domestic wastewater treat- ment plants would go a long way towards alle- RBnsRENces viating the problem of anthropogenic addition BonBNszre.rN,J. (1966): Structure crystalline de of phosphateto our lakes streams. and Ia m6tavarissite et de la m6tastrengite. BzIl. Soc. franc. Miner. Cryst, 89, 428438. APPENDIX BunNs, N. M. & Ross, C. (1972)z Project Hypo. Canada Centre Inland Waters Paper 6. Standard solutions Cennrrr, D. E. & Gooocelr, S. (1954): Sorption Tlvo standard solutions were employed in reactions and somd ecological imFlications. this research. The iron standard was prepared by Deep-SeaRes. 1, 22+U3. dissolving 1.2523 g of pure iron powder in 100 Dnr.r., C. L (1973): Vivianite: an authigenic ml of 12 N HCl. This solution was then diluted phosphate mineral rn Great Lakes sediments. with distilled-deionized @.D.) water to a final Proc. 16th Conf, Great Lakes Res., Internat. volume of 500 ml in a volumetric flask. 10.00 Assoc. Great Lakes Res., LO27-1028, ml of this stock solution were pipetted into a ErNsnr,e,W. (1938): Die Bildung und die Umbil- 1000 ml volumetric flask and diluted to volume dunger von Eisenphosphat und ihre dynami- with D.D. water. The resultant consentration of schen Bizeihungenanm Sediment und zum frein Fe+s in this solution was 25 mg Fe+a/l and jJs Wasser.Arch, Hydrobiol. 33, 364-387, pH was less than 1. HALBAcH,P. H. (1975): Migration and precipita- The phosphate standard was made by diluting tion of metals during fomration of freshwater 46 THB CANADIAN MINERALOGIST

concretions in Finnish Lakes. Proc. 2nd Inter- diments: effects of aerobic and anaerobic con- nat. Syrnp. Envir. Biogeochera.(in press). ditions. Science 186, 53-55. I(RAwEn, J. R., Ar-r,nN, H. E., Beur.Ne, G. W. & Szurno, L. & BneNNocK, W. W. (1962): Rapid BunNs, N. M. (1970): Lake Erie time stucly changes of silicate, carbonate and phosphate (LETS). Canada Cewre Inland Waters, Bw- rocks. U.,1. Geol. Survey Bull. Ll44-4, lington, Ontario. $rurr,e, S. S., Srens, J. I(, Wu-r.rervrs, J. D. H., MoonB, P. B. (1966): The crystal structure of AnusrnoNc, D. E. & HAeRrs, R. F. (f971): metastrengite and its relationship to strengite Sorption of phosphate by lake sediments. ,Soil and phosphophyllite.Amet Mineral. 51, 168- Sci. Soc. Amer. 35,24+249, 176. SruMM, W. & MonceN, J. J. (1970)r Aquatic Monrrurn, C. H. (1941): The exchangeof dis- Chemistry. Wiley, 583 p. solved substancesbetween mud and water in Surrrnru,ar.rn,.J. C., Kneum, J. R., Nrcnor-s, L. & lakes. I. l. Ecol. 29, 28A-329. KuNra T. D. (1966): Mineral-water equilibria (1942): The exchange of dissolved in the Groat Lakes: silica and phosphorus. Univ. substancesbetween mud and water in lakes. Mich. Great Lakes Div. Publ, No. L5, 439445. n. Ecol. l. 30, L47-201. Wrrr-rervrs, J. D. H., Srens, J. K. & Hennrs, R. F. (1971): Chemical oxchangesbetween (L970): Adsorption and desorption of inorganic sediments and water iD the Great Lakes - phosphorus io a 0.1 M NaCl system. Environ. speculations on probable regulatory mechan- Sci. Tech. 4,417-519, isms. Limnol. Oceanog, L5, 387404, _, & Anrvrsrnorc, D. NRrAau,J, O. (L972a): Stability of vivianite and E. (l97la): Fractionation of inorganic phos- ion-pair formation in the system Fe(POr)- phate in calcareous lake sediments. SoiI Sci, Soc. H3PO4-H2O.Geochim. Cosmochim. Acta 36, Amer. Proc. 35, 250-255. 459-470. _, _, _, & -_ (1971b): (1972b): Solubility equilibrium of Characterization of inorganic phosphate in non- strengite.Amer, I. Sci. 272, 476-484. calcareous lake sediments. ,Soll ,Sci. Soc, Amer. & Drtr-, C. l, (1974): Diageneticforma- Proc. 35, 556-561. tion of iron phosphatesiD r€cent lake sedimenls. _, &_ (l97lc): Irvels Amer. Mineral. 59, 934-946. of native inorganic and total phosphorus in lake Perrg G. A. (1964): Aqueous surface chemistry sediments as related to other sediment Imrame- and complox oxide minerals in equilibrium con- ters. Environ. Sci. Tech, 5. ILl3-1120. cepts in natural water systems.Amer. Chem. Soc, Spec.Publ.67, 12I-160. ,PATRrc&W. H., Jr. & Kner-m, R. A (1924): Manuscript received June 1975, emended Novem- Phosphaterelease and sorptiotr by soils and se- ber 1975.