STABILITY OF HECTORITE IN WEAKLY ACIDIC SOLUTIONS

Clay Minerals (1968) 7, 409. STABILITY OF HECTORITE IN WEAKLY ACIDIC SOLUTIONS III. ADSORPTION OF HEAVY METAL CATIONS AND HECTORITE SOLUBILITY KEVIN G. TILLER Divisin f Sils, C.S.I.R.O., Adelaide, Suth Australia (Received 15 March 1968) ABSTRACT: The heavy metal catins studied were adsrbed by hectrite suspended in 0-05 M calcium chlride as a ph-dependent reactin in the rder, Cu ~ Zn ~ (C, Mn) ~ Ni. The adsrptin f heavy metal catins was accmpanied by the remval frm slutin f silicic acid released by clay disslutin. It is prpsed that the reactin takes place n edge surfaces such that the clay lattice is effectively extended. This result cmplements ther wrk in which it has been shwn that the adsrptin f silicic acid by ther silicate clays results in an increased adsrptin f zinc, cbalt and nickel. The present study did nt prvide evidence f the exchange f zinc and ther catins fr lattice magnesium. The slubility f hectrite was decreased in the presence f heavy metal catins in the same rder that favured their adsrptin. It is prpsed that they limit the disslutin f hectrite in weakly acidic slutins by adsrptin n r near the sites mst prne t prtn attack, that is, the magnesium ins f the ctahedral layer expsed at edges and defects. Many f the factrs which affect the slubility f minerals, such as particle size, degree f crystallinity and chemical cmpsitin have been reviewed by Grim (1953). The wrk f Garrels (1960) and thers n mineral equilibria in aqueus slutins has brught attentin t the usefulness f thermdynamics in the study f the slubility f sme minerals. In tw recent studies, Tiller (1968a, b) has cnsidered the disslutin f hectrite in weakly acidic slutins frm the pint f view f the mechanism f the reactin and cnditins fr quasi-equilibrium. The derived equilibrium cnstant prvided a basis fr describing the equilibrium slubility fr a range f cncentratins f magnesium and hydrgen ins and silicic acid. Observatins during earlier studies (Tiller & Hdgsn, 1962) suggested

410 Kevin G. Tiller that the slubility f hectrite might als be reduced when cbalt r zinc were added in small amunts. The main aim f these studies is t investigate the mechanism f adsrptin f heavy metal catins by hectrite in the presence f 0"05 M calcium chlride. This will be achieved in tw ways. Firstly, general aspects f the reactin, such as the effect f ph, cncentratin in slutin, and differences between heavy metal catins, will be cnsidered. Secndly, recent studies f hectrite disslutin will be used as a basis fr investigating the rles f silicic acid and lattice magnesium in the adsrptin f zinc, cbalt and ther metal ins by hectrite since Elgabaly (1950) has already suggested that zinc fixatin by magnesium clays is due t the substitutin f zinc fr magnesium in lattice psitins. The latter experiments have been carried ut under cnditins fr which--in the absence f heavy metal catins--the stichimetry has been established and the prducts f disslutin f hectrite are knwn t remain in slutin. Hectrite was used as a mdel clay fr this reactin because these cnditins cannt be attained fr aluminus clays. The secnd aim is t study the effect f heavy metal catins n hectrite slubility, especially in relatin t their adsrptin by hectrite. Materials EXPERIMENTAL The preparatin f the hectrite suspensins was given in detail previusly (Tiller, 1968a). The hectrite used was separated (<2 t~) frm sample A.P.I. N. 34 supplied by Ward's Natural Science Establishment, Rchester, N.Y. and treated with ht ammnium chlride slutin t remve calcium carbnate. Calcium chlride slutins were prepared frm A.R. reagent and extracted with dithizne t remve traces f heavy metal impurities. Analytical methds Silicn was measured spectrphtmetrically using the reduced frm f a silicmlybdate cmplex. Magnesium, cpper, zinc, nickel, cbalt and manganese were determined by atmic absrptin. The ph f the clear supernatant slutin after centrifugatin was determined by glass electrde. General prcedure The clay samples, usually 50 mg (105 ~ C ven-dry basis), were pipetted frm a bulk suspensin and washed twice in 0.05 M calcium chlride befre being used in experiments. The reactins were carried ut by shaking 25 ml f suspensins in 0"05 M calcium chlride slutin in plyprpylene tubes at 20 + 1 ~ C. After shaking, the suspensins were centrifuged at abut 1200 g fr 20 minutes befre taking aliquts fr analysis. Details f the experiments are given in the apprpriate sectins f 'Results'.

Adsrptin Adsrptin and hectrite slubility 411 RESULTS The hectrite suspensins were shaken with variable amunts f acid fr five weeks t achieve quasi-equilibrium ver the desired range f final ph befre adding 5/~-mle f the required catin. The reactin was allwed t prceed fr a further tw weeks. After centrifugatin and analysis the amunt f catin adsrbed was calculated by the difference frm that initially added. The results fr the adsrptin f cpper, zinc, cbalt, and nickel ins by hectrite as a functin f ph are as shwn in Fig. 1. Other unpublished data shwed that the adsrptin f manganese ins by hectrite was similar t that f cbalt ins. The adsrptin f the heavy metal catin by nn-specific electrstatic adsrptin was minimized by the presence f excess calcium chlride. Catin cncentratins were chsens that precipitatin f the hydrxide wuld nt take place. Nevertheless, cpper may have precipitated in sme experiments in the samples f higher ph. The extent f the adsrptin f these catins was strngly dependent n ph and, fr a final cncentratin in slutin f abut 10-4 M, varied in the fllwing rder, Cu > Zn > (C, Mn)~ Ni. The weak adsrptin f nickel by clays, when cmpared with similar catins, will be cnsidered in later wrk. Tiller & Hdgsn (1962) have shwn previusly that cbalt and zinc were bund mre strngly by 8C 9 A Cu Zn C v 4C.el ~2c g I s g ph FIG. l. The adsrptin f cpper, zinc, cbalt and nickel ins by hectrite in 0"05 M calcium chlride as a functin f ph.

412 Kevin G. Tiller hectrite than by ther silicate clays and that their adsrptin by hectrite was accmpanied by the release f hydrgen ins. Sme data were als btained n the effect f cncentratin n the adsrptin f cbalt and zinc by hectrite in 0"05 M calcium chlride at ph 6"5-7.0. The experimental cnditins differed in the fllwing respects frm ther adsrptin experiments discussed abve. After equilibrium f the ph f the suspensins fr tw weeks, radiactive cbalt and zinc were added and allwed t react fr seven days at 25 ~ C. Prtins f the supernatant slutin and the clay, after washing nce with 0"05 M calcium chlride, were analysed fr cbalt and zinc by radiistpic methds. The results are shwn in Fig. 2. The small amunt f divalent cbalt and zinc adsrbed by nn-specific adsrptin, calculated as 2 /~-mles/g f clay at 2 x 10-4 M assuming that Ca 2+ (clay) Ca ~+ (slutin) C 2+ r Zn 2+ (clay) = C 2+ r Zn 2+ (sl~' was remved by the washing with calcium chlride. The amunts f cbalt and zinc adsrbed under these cnditins increased sharply with increasing slutin cncentratin up t values f abut 50 ff-mles/g f clay, when the cncentratin in the final slutin was abut 10-5 M. At higher cncentratins f cbalt and zinc in slutin the slpe f the istherms were markedly lwer, with the amunt f metal in adsrbed increasing by abut 0.3 ff-mles/g fr each further increment f 10-5 M in slutin cncentratin. 80 E 6C - ~d C 9 Zn 2C 8 i i i ~ I 2 4 6 I0 Cbalt and zinc in slutin (M xlo "4) FIG. 2. The adsrptin f cbalt and zinc by hectrite in 0"05 M calcium chlride as a functin f final cncentratin f cbalt and zinc in slutin.

Adsrptin and hectrite slubility 413 Hence the adsrptin f cbalt and zinc by hectrite at ph 6-5-7"0 in the presence f calcium chlride appraches a limit f abut 60-80 tl-mles/g which is much less than the capacity fr nn-specific electrstatic adsrptin (500 t~-mles/g fr divalent catins). This limiting value fr specific adsrptin which is lwer at lwer ph values, is f the same rder f magnitude as the number f magnesium atms expsed at edges f clay particles (Tiller, 1968a). Effect f adsrptin n the release f silicn and magnesium These and previus studies (Tiller, 1968a) shwed that the additin f acid t hectrite in the presence r absence f heavy metal catins, always resulted in the equimlar release f silicn and magnesium when cmpared with samples t which acid was nt added. This applied even fr quite small amunts f acid equivalent t the hydrgen ins released by catin adsrptin in these studies. It is assumed, therefre, that hydrgen ins released during the adsrptin f heavy metal catins will als release silicn and magnesium in equimlar amunts. If the nly result f the adsrptin f these heavy metal catins was the prductin f hydrgen ins, then the difference between the amunts f silicic acid and magnesium released (Mg-Si) wuld be independent f the amunt f heavy metal adsrbed even thugh the actual amunts f each f magnesium and silicic acid wuld have increased. These increases wuld bth be prprtinal t the amunt f hydrgen ins released. Any variatin in the value f (Mg-Si) can nly result frm an inequality in the net amunts f magnesium and silicic acid released due t ther mre specific effects assciated with the adsrptin f heavy metal catins. A quantitative assessment f these specific effects was made in the fllwing way. The heavy metal catins were added t clay suspensins t which acid was nt added s that the the disslutin f hectrite was minimized. By this means, the amunt f hydrgen ins prduced during adsrptin cntributed significantly t the ttal amunt f hectrite decmpsed. A. Heavy metals added initially In several experiments, varius cncentratins f cpper, cbalt and zinc were added t hectrite samples immediately after washing with calcium chlride and allwed t react fr fur r five weeks. The final amunts f magnesium ins and silicic acid in slutin and f metal ins adsrbed were determined. The variatin in the difference (Mg-Si) as a functin f metal adsrbed in each experiment is shwn in Fig. 3 fr three representative experiments. The results are interpreted as indicating that there is either a relative increase in magnesium released r relative decrease in silicn released crrespnding t 0"6-0"7 mles fr each mle f metal in adsrbed. Furthermre, examinatin f the actual cncentratins f each f silicic acid and magnesium in the final slutins f each experiment shwed that silicic acid always decreased but magnesium did nt increase appreciably as the amunt f metal adsrbed increased. When higher amunts f cpper were remved frm slutin, presumably by precipitatin, the difference (Mg-Si) was nt appreciably

414 Kevin G. Tiller +2 b5 9 Zn C LX Cu 9 ~ 9 A~ --" C g~ ~ -I t~ -2 Heavy metal catin adsrbed (pmle/5omg) ]ZIG. 3. The effect f the adsrptin f heavy metal catins n the difference between the amunts f magnesium and silicn released. Heavy metal catins were added initially. increased further. This suggests that nly adsrbed heavy metal catins are invlved in the relatinship f Fig. 3. B. Heavy metals added a#er equilibratin ] suspensins In rder t check that these results were nt partly due t an effect f heavy metal catins n the kinetics f silicn r magnesium release, the suspensins f hectrite were equilibrated fr nearly seven weeks befre their reactin with up t 5 c-mles f cbalt, zinc and ther catins fr a further tw weeks, Reference suspensins t which n heavy metal catins were added were als included in each experiment. At the end f the experiments, silicn and magnesium were determined in the supernatant slutins f the suspensins t which metal ins were added as well as f the reference suspensins shaken fr the same perid. The inclusin f reference suspensins permitted the results t be presented as ne curve. The data were assessed as abve except that the values f (Mg-Si) fr the apprpriate reference suspensins were subtracted frm the final value f (Mg-Si) crrespnding t each level f each catin added. These crrected values f (Mg-Si) are shwn in Fig. 4 in relatin t the amunts f cbalt, zinc, nickel and manganese adsrbed. As befre, the adsrptin f heavy metal catins has resulted in an inequality in the amunts f silicn and magnesium released which, when expressed as the difference (Mg-Si), is equivalent t 0-6-0-7 mles per mle f catin adsrbed. Examinatin f the actual amunts f magnesium and silicn in slutin at the end f these experiments, in cntrast t the cnsideratin f their difference (Mg-Si), shwed that the final amunts f silicn in slutin decreased as the

Adsrptin and hectrite slubility 415 9 x a Zn C Ni Mn -6 E D- Z~ I i ~ + 4 Heavy mete l catin adsrbed (~mle/5omg) Fr. 4. The effect f the adsrptin f heavy metal catins n the difference between the amunts f magnesium and silicn released. Heavy metal catins were added after shaking hectrite suspensins fr 47 days. 1"5 ~6 ~ E C1. 1.r 9 Zn C x Ni ta Mn ~v c:. ~E _8 ~5 rim 0 ~,i Hevy metal catin adsrbed (ijmle/5omg) FIG. 5. The relatin between the amunts f heavy metal catins adsrbed and magnesium released.

416 Kevin G. Tiller amunt f heavy metal catin increased whereas the amunt f magnesium increased. Fig. 5 shws that the final amunt f magnesium increased abut 0.3-0"4 mles per mle f catin adsrbed. Slubility f hectrite In this paper the final cncentratins f magnesium and silicn in slutin are used as a measure f hectrite slubility at a defined ph except under cnditins where magnesium and silicn precipitate. The effect f heavy metal catins n the equilibrium slubility f hectrite in 0"05 M calcium chlride was assessed in the fllwing experiment. Five /~-mles f each heavy metal catin studied was added t suspensins cntaining variable amunts f hydrchlric acid. The suspensins were shaken fr 45 days, centrifuged and the supernatant slutin sampled fr the measurement f ph and magnesium and silicn cncentratin. The final cncentratins f silicn and magnesium are shwn as a functin f ph in Fig. 6. The deficit in the amunt f silicn released assciated with the adsrptin f heavy metals, discussed abve, is small in relatin t the amunt released by the acid added. It wuld nt affect significantly the final cncentratin f silicn in slutin except at the lwest pints f each curve where acid was nt added. The essentially identical trends in slubility shwn in bth figures (apart frm the pints f lwest ph in Fig. 6(a) which are due t the presence f amrphus silica) suggest that the slubility data reflect prperties f the-hectrite nly and nt experimental artefacts. Cnditins were chsen t ensure that precipitatin f metal hydrxides, with the pssible exceptin f that f cpper at the pint f highest ph, did nt ccur. The slubility f hectrite is decreased by the additin f these heavy metal E._8 16 2 () m, --~ IO.~_ a [] CCI 2 nly x C0C12+ Ni A CCI 2 + C 9 CaCl2+ Zn - - ~ [] CaCt2+ Cu x [] x?~ g 5 16 2 p:, I (b) x q~ w e "N ~_ IO -4 ~ _ E 10 4 k~ ph ph FIG. 6. The effect f heavy metal catins n hectrite slubility--(a) the final cncentratin f silicn and (b) magnesium in slutin as a functin f ph. Nte that the rdinate scales are lgarithmic.

Adsrptin and hectrite slubility 417 catins in the weakly acidic range, ph 5-7 fr the batch f clay used but nt at lwer ph values. The effectiveness f these catins in decreasing hectrite slubility is in the same rder as their reactivity with hectrite, that is, Cu > Zn > C ~ Ni. The amunts f heavy metal catins adsrbed by hectrite fr the pints f lwest slubility (<4 10-4 ~ Mg in Fig. 6(b) ranged in p~-mles/g frm 46-54 fr cbalt, 55-63 fr cpper, 56-61 fr zinc and 15-30 fr nickel. The imprtance f the amunt f heavy metal catin adsrbed was als studied by adding varying cncentratins f zinc t hectrite suspensins in calcium chlride. The slubility f hectrite decreased with increasing amunts f zinc absrbed. This wrk was extended t study the effect f zinc n the slubility f mntmrillnite (A.P.I. N. 25) but zinc has n effect fr ph values frm 4 t 6. GENERAL DISCUSSION AND CONCLUSIONS Adsrptin f heavy metal catins It has been shwn (Fig. 5) that the adsrptin f these catins increases the final cncentratins f magnesium in slutin when cmpared t reference samples withut heavy metal catins present. Magnesium released during the reactin with heavy metals culd result frm (i) the nrmal disslutin f clay due t the hydrgen ins present at the start f the reactin, (ii) the exchange f magnesium ins f the ctahedral layer by heavy metal catins, and (iii) the disslutin f the clay by hydrgen ins released during the adsrptin reactin. Cntributins frm (i) were largely allwed fr by sampling all suspensins at the same time. Negligible quantities f hydrgen ins were intrduced with the heavy metal catins. If the specific adsrptin f zinc and ther catins ccurred nly as described under (ii), a line f slpe equal t unity wuld have resulted in Fig. 5. Hwever the data f the figure culd be cnsistent with abut 0.4 f the ttal amunt f heavy metal catins having been adsrbed by direct exchange with magnesium ins f the lattice, prvided that the remaining 0"6 did nt cause the release f hydrgen ins and thence magnesium ins. Nevertheless, it has been shwn that hydrgen ins are released during the adsrptin f heavy metal catins by hectrite. Unpublished studies in this labratry have als established that the adsrptin f zinc by mntmrillnite frm very dilute zinc slutins in the presence f excess calcium r sdium salts invlves nly hydrlysed zinc ins. The evidence f Hdgsn et al. (1964) als favured the adsrptin f cbalt by clays in the presence f excess salt as the hydrlysed in. If this mechanism als applies t ther heavy metal catins, then the adsrptin f each mle f hydrlysed ins will cause 1 mle f hydrgen ins t appear in slutin due t simultaneus hydrlysis. These hydrgen ins will, in turn, yield 0"47 mles f magnesium n reactin with hectrite (Tiller, 1968a). In the absence f any specific effects f the heavy metal catins, it is assumed that an apprximately equal amunt f silicn wuld als be released. Hence all the additinal magnesium released during adsrptin can be explained by alternative (iii) withut the need t pstulate the specific exchange f zinc and ther catins fr magnesium in ctahedral lattice psitins which was suggested by

418 Kevin G. Tiller Elgabaly (1950) fr the fixatin f zinc by magnesium clays. It is cncluded that direct exchange f zinc and ther similar catins fr magnesium in lattice psitins, if it ccurs at all, can play nly a minr rle in the prcess f fixatin at rm temperatures in an aqueus envirnment. The emphasis given in the literature t the latter mechanism invlving the exchange f lattice ins may be partly a reflectin f the prperty f sme magnesium minerals, particularly when grund, f having relatively high ph values in aqueus suspensins. The ph values attained can ften give rise t misleading results because f the pssibility f precipitatin f cmpunds f zinc, cbalt and similar ins. Previus wrk (Tiller, 1968a) has shwn that the silicn released during the disslutin f hectrite under the experimental cnditins used here was nt appreciably adsrbed by the clay. The adsrptin f Mg was minimized by using suspensins in M/20 calcium chlride. It is against this backgrund that the effect f the adsrptin f heavy metal catins n the final relative cncentratins f silicn and magnesium has been cnsidered. The results f Figs 3 and 4 culd have been interpreted either as a relative enrichment f magnesium r depletin f silicn, each with respect t the ther, equivalent t abut 0-6-0"7 mles per mle f heavy metal catin adsrbed. Mst data have als shwn an actual decrease in silicn cncentratin whereas sme data shwed smaller increases in the final cncentratins f magnesium. The increased magnesium cncentratins have been discussed and it is cncluded that they are largely assciated with nrmal disslutin by released hydrgen ins. Hence it is cncluded that the disparity in the relative amunts f silicn and magnesium released in the presence f heavy metal catins is largely due t the reactin f released silicic acid with adsrbed heavy metal catins. It is prpsed that heavy metal catins are adsrbed as hydrlysed ins at edge sites adjacent t the ctahedral layer catins and that this prvides adsrptin sites fr silicic acid such that, n the average, tw mlecules f silicic acid are bund at sites adjacent t the tetrahedral layer fr every three catins f zinc, cbalt etc. adsrbed. This is equivalent t a partial extensin f the clay lattice. Such an extensin wuld be imperfect with respect t hectrite because f the prprtin f the cnstituents invlved. In cmplementary studies, Tiller (1968c) has shwn that the adsrptin f silicic acid prvides additinal sites fr the adsrptin f sme heavy metal catins n aluminus layer-silicate clays but nt n alumina. This assciatin with silicic acid may stabilize the adsrptin f heavy metal catins by sil clays and thus make them less available t plants. The effect f heavy metal catins n the slubility f hectrite In the absence f added heavy metal catins, the slubility f hectrite is described by the equilibrium cnstant f the disslutin reactin (Tiller, 1968b). The results f Fig. 6 shws that the slubility at any particular ph value is decreased in the presence f heavy metal catins. The assciatin f silicic acid and heavy metal catins adsrbed n the edge surface f hectrite, as discussed abve, culd be cnsidered as a rudimentary layer

Adsrptin and hectrite slubility 419 f a separate metal silicate phase. Accrdingly, the slubility f hectrite in the presence f heavy metal catins culd be interpreted in terms f the stability f the crrespnding metal silicate. This pint f view may have nly limited relevance t the prblem f stability because the extent t which the catin studied favurs hectrite stability is nt related t its tendency t frm silicates either synthetically (Caill~re & H6nin, 1961) r in nature (Rankama & Sahama, 1950). The preferred interpretatin f the results is that heavy metals cntrl hectrite slubility by an adsrptin reactin. This arises frm the bservatin that their effect n hectrite slubility fllws the same rder as that favuring their adsrptin by hectrite. It is prpsed that the disslutin f hectrite cntinues until the cncentratin f hydrgen ins in slutin decreases t such a value that heavy metal catins can cmpete successfully fr adsrptin sites assciated with the expsed magnesium" ins f the crystal lattice. The adsrbed catins are cnsidered t prevent the displacement f magnesium ins frm lattice psitins by hydrgen ins. With the mre strngly bund catins, hectrite can resist disslutin at lwer ph values. ACKNOWLEDGMENTS The authr appreciated discussin with Dr J. F. Hdgsn, U.S.D.A., Ithaca, N.Y., during the preliminary stages f this prject and the technical assistance f Mrs C. Huwelling fr the experimental wrk. REFERENCES CAILL~RE S. H~NIN M.S. (1961) Cllq. Centre Nat. Rech. Sci. (Paris) N. 105, 31. ELGABALY M.M. (1950) Sil Sci. 69, 167. GARRELS R.M. (1960) Mineral Equilibria at Lw Temperature and Pressure. Harper, New Yrk. GRIM R.E. (1953) Clay Mineralgy. McGraw-Hill, New Yrk. HODGSON J.F., TILLER K.G. & FELLOWS M^RT~ (1964) Prc. Sil Sci. Sc. Am. 28, 42. RANKAMA K. & SAHAMA J.G. (1950) Gechemistry. University f Chicag Press. TILLER K.G. & HODGSON J.F. (1962) Clays Clay Miner. 9, 393. TILLER K.G. (1968a) Clay Miner. 7, 245. TILLER K.G. (1968b) Clay Miner. 7, 261. TILLER K.G. (1968c) Transactins f the Ninth Internatinal Cngress n Sil Science, Adelaide, Vl. 2, 567.