Distribution of Aluminium Soluble and Insoluble Species in the System Basaluminite - Soil Solution

Transcription

1 Distributin f Aluminium Sluble and Insluble Species in the System Basaluminite - Sil Slutin OXANA SPINU 1 *, IGOR POVAR Institute f Chemistry, Academy f Sciences f Mldva, 3 Academiei str. Chisinau, MD2028, Republic f Mldva A nvel type f diagrams based n graphical and numerical methds, which quantitatively describe the distributin f sluble and insluble, inrganic and rganic, mnmeric and plymeric aluminium species in hetergeneus multicmpnent systems basaluminite sil slutin has been used. This apprach utilizes the thermdynamic relatinships tgether with riginal mass balance restraints, where the mineral phases are clearly expressed. The factrs influencing the distributin f sluble and insluble aluminium species in aquatic systems in a large range f ph values have been investigated. It has been fund that the SO 4 cncentratin has a remarkable influence n the distributin f aluminium hetergeneus speciatin. The develped new type f diagrams can be used t interpret data btained within the framewrk f water quality mnitring prgrams. Keywrds: basaluminite, cmplex frmatin, distributin diagram, Gibbs energy, hydrlysis. Identificatin f sluble chemical species f aluminum (Al) in the interstitial waters present a great interest, in particular, t assess the ptential txicity f sluble aluminum in the large cncentratins ver the plants [1], because nt all its sluble species are txic t the same extent. The in Al 3+, hydrxcmplexes Al(OH) 2+ and Al(OH) 2+ are the mst txic species, while the cmplexes Al - F - and Al - rganic ligand are less harmful. The cncentratin f sluble Al in the mst natural waters is very small ( M), in cnditins when the medium is almst neutral and cncentratins f cmplexing agents are negligible, being generally cntrlled by gibbsite [2]. Hwever, in sils which becme acidic due t the xidatin f pyrite r sulfur the in SO 4 is the dminant anin. Sil acidity can increase and as a result f atmspheric rainfall cntaining sulfur cmpunds. With increasing the SO 4 cncentratin the Al 3+ activity (r cncentratin) grws expnentially [3]. At the same time mdifies the relative stability f minerals in sils. In sulfate acidic sil slutin the Al xysulfate minerals, such as alungen Al 2 ) 3. 17(H 2 O), alunite KAl 3 (OH) 6 ) 2, jurbanite Al(OH) ). 5(H 2 O) and basaluminite Al 4 ). 5(H 2 O) [4-6] are mre stable than gibbsite. The identificatin f mineral species cntrlling Al slubility in acidic sil slutin, rich in sulfate presents a difficult prblem. Firstly, as has been shwn by X-ray diffractin, slids can be amrphus. Secndly, t distinguish between the adsrbed and structural sulfate is an additinal intricacy. In numerus studies the theretical calculatins have been carried ut in rder t determine the Al mineral species that frm in the acidic sulfate envirnments f sils. In [7] the authrs state that retentin f sulfate by acid sils is a cnsequence f the slubility f basaluminite and/r alunite. The basaluminite slubility and its cntrl ver the Al cncentratin in aqueus acid sulfate systems has been examined recently fr a series f natural acid waters [8,9]. Using Al, S and O, K XANES - spectrscpy, cmbined with the elemental analysis f the grundwater precipitates it has been demnstrated that fr ph values the Al cncentratin is cntrlled by less crystalline mineral - hydrbasaluminite (with an empiric frmula ) AL 4 ). 15(H 2 O) that under cnditins f relatively lw humidity is very unstable and tends t dehydrate with frmatin f basaluminite. The same authrs have demnstrated that cncentratins f Al sluble within the ph range in acidic sulfate sils are cntrlled by a mixture f amrphus Al hydrxide and basaluminite. In the present paper it is investigated the distributin f sluble and insluble Al 3+ species f sil slutins in the hetergeneus multicmpnent system basaluminitesaturated aqueus slutin. Theretical part The equilibrium mdel is based n the fllwing assumptins: - the Al 3+ cncentratin in sil slutin is cntrlled by the basaluminite slubility: where K s represents the prduct f the mineral slubility f aluminum. Fr cnvenience, the charges f ins will be mitted. Therefre, the AL 2+ in cncentratin is: - the studied sil slutins are dilute slutins with a lw inic strength, s its influence can be neglected [10]. Cncmitantly with the prcess (1) it has als been taken int accunt a set f pssible equilibria in the system mineral-sil slutin, presented in the table 1. In the sil slutin Al is present in varius frms, including sluble hydrx-, flur- and sulfate cmplexes, as well as rganic cmplexes. There are als mre phsphate cmplexes and plymeric species f Al. The rganic acid cntained in the sil can be cnsidered as a triprtic acid H 3 Org[1,3]. It has been taken int accunt the frmatin f tw cmplexes f aluminium with ligand Org 3-, AlOrg and AlHOrg +. As plynuclear species f aluminium the (1) (2) * xana_spinu@yah.cm; Tel.(+373)

2 plynuclear Al hydrxcmplexes will be cnsidered (dimer Al 2 (OH) 2 4+ and trimer Al 3 (OH) 4 5+ ). Plymeric species with large mlecular mass Al i (OH) j, where i = 6, 8 and 13 will nt be taken int accunt, since they are basically nt frmed in the sil slutin because f lw plymerizatin kinetics. Cncmitantly, the rganic acids in sil slutin inhibit their frmatin [1,3]. Aluminum phsphate cmplexes can be represented in the frm f AlH 2 PO The Al silicate cmplexes are ignred because f their weak cmplexatin prperties. The mass balance (MB) cnditins fr the examined systems can be frmulated n the basis f the methd f residual cncentratins (RC) [11,12]: The quantities C i and C i r (ml/l) in the equatins (3)- (7) represent respectively the analytical and residuals cncentratins f the cmpnent i in the cnsidered hetergeneus system, while C i symblizes its quantity in precipitate in ne liter f slutin [13-15]. At the same time, within the CR methd the fllwing relatinships result: C OH = - C H and Cr OH r = -C Hr. In the equatin (8) C H dentes the excess f the H + ins in respect t hydrxyl ins in the tw-phase mixture C H = -C OH [14]. Based n the stichimetric cnditins f the cnsidered minerals with a general cmpsitin, Al 4 SO 4. 5H 2 O the fllwing relatins can be derived: The slid phase Al 4 SO 4. 5 H 2 O is stable if G S,tt > 0. The cnditin G S,tt = 0 crrespnds t the beginning f disslutin/precipitatin f basaluminite. Using the cmbined equatins f LAM and MB, the distributin diagrams f the aluminum in bth in the slid and liquid (species in aqueus slutin) phases, s called the diagrams f hetergeneus chemical equilibria (DHCE) can be calculated [2,12]. Fr the studied hetergeneus system the fllwing equatin fr calculatin f partial mlar fractins f species, γ i, where the reactins listed in table 1 are take place cncmitantly: (8) The Al 3+ equilibrium cncentratin is determined by the relatin (2). Frm the MB cnditins (3)-(7), fr a specific ph value, taking int accunt the relatins (10) and the equatins f the law f mass actin (LMA) deduced frm the table 1, we btain a system f 5 nnlinear equatins with 5 unknwn variables, [F], [Org], [SO 4 ], [PO 4 ] and C i, respectively, which can be easily slved by using numeric cmputerized methds 13-15]. Next, the residual cncentratins f the i cmpnent are calculated, C ir, knwing the initial cmpsitin f the hetergeneus mixture, in ther wrds, the quantities C i. Within the thermdynamic apprach elabrated earlier [16-19] and extended in this paper n the slid phase with a mre cmplex cmpsitin, it can be demnstrated that the ttal Gibbs energy values G s,tt fr the examined cmplex prcesses, where disslutin-precipitatin reactin (1) and thse frm the table 1 take place cncmitantly, can be calculated with the fllwing equatin: In rder t calculate the Gibbs energy G s,tt referred t ne mle f aluminum, we will use the expressin: (9) (10) place cncmitantly (11) The index tt symblizes the sum f the partial mlar fractins f all the sluble species. Frm the expressins (3) (10), the fllwing relatin results: (12) After their uter appearance DHCE lk like diagrams f distributin f species in a hmgeneus system. The last, as a rule, are cnstructed in the crdinates γ i (ph), because in the absence f plynuclear cmplexes the partial mlar fractins f species γ i in aqueus slutin 251

3 Table 1 EQUILIBRIUM CONSTANTS USED IN THERMODYNAMIC CALCULATIONS (mre frequently, symblized by fr α) are functins nly n ph f slutin and d nt depend n the initial cncentratins f cmpnents. In the case f hetergeneus equilibria the partial mlar fractins f species additinally n the initial cmpsitin f the mixture and fr a fixed value f ph are functins f five variables. In this case it is ratinal t cnstruct the diagrams in crdinates, while the values f ther quantities are maintained cnstant. The prcedure fr cnstructing DHCE includes the next necessary steps: i) The calculatin f thermdynamic stability area f the mineral. Fr this purpse frm the equatin (9) r (10) the Gibbs energy values shall be determined. ii) The calculatin with equatins (11) the partial mlar fractins γ i f all the species cntaining the aluminum in within the thermdynamic stability area f basaluminite established in the previus stage. iii) In rder t cmplete the diagram, in the case f aqueus hmgeneus slutin, the partial mlar fractins shall be calculated frm the equatins used fr cnstructing usual distributin diagrams. In this case, under the cnditins f mnnuclear species frmatin the γ values d nt depend n the initial cncentratins f the cmpnents. Results and discussins It is well knwn that the ph value f saturated slutins is ne f the majr factrs ( master variable) that influences the distributin f aluminum in bth minerals and sluble inrganic and rganic species. The diagrams G S,tt (ph) allw easy the determinatin f thermdynamic stability area f basaluminite in functin n the initial cmpsitin f multicmpnent systems. In the calculatins carried ut in this paper the analytical cncentratins f aluminium, the inrganic ligands, flurine and phsphate ins as well as the rganic ligand have been kept cnstant, C Al = M, C F = M, C PO4 = M and C Org = M. The ph range f variatin was f The analysis f the data presented in figure 1 shws that the thermdynamic stability area f the mineral 252

4 becmes cnsiderably larger with increasing the ttal (analytical) cncentratin f the SO i in. The DHCE diagrams fr different cncentratins f sulfate are presented figure.5. These diagrams reflect explicitly the distributin f varius chemical species f aluminium in bth the slid and liquid phases. The partial mlar fractins f sluble aluminium species change substantially at the ph value f sedimentatin-slubilizatin f slid phase. Fr the studied cmpsitins f mixture, in acidic and neutral slutins, the equilibrium cncentratins f mnnuclear hydrxcmplexes are small, while the plynuclear species d nt almst frm within the entire range f ph and the cncentratin f sulfate. The cncentratin f the txic aluminum in decreases significantly with increasing C, and fr SO4 C = 1. SO M becmes insignificant. The predminant species in alkaline envirnments is the aninic hydrxcmplex Al(OH) 4-. With the increase f the cncentratin f sulfate ins the stability areas f ther sluble species are substantially mdified. In the case C = 2.5. SO M and C = 1. SO M, in the range f ph variatin the cmplexes AlOrg and AlHOrg + prevail, while fr C = 1. SO M this range f ph narrws, and fr C = 1. SO M the frmatin f these cmplexes is blcked by the predminance f + aluminium cmplexes with the sulfate in, AlSO 4 and - Al ) 2 (fig.5). In figure 6 the variatin f the partial mlar fractin f slid phase - basaluminite is given, depending n the ph and C 0. As ne can see, the areas f the mineral SO4 existence, i.e. f the hetergeneus system cincide with the thermdynamic stability f this mineral (fig. 1), at the same time the γ S values increase with increasing the C 0 SO4 values. Fig. 3. The curves f distributin f sluble species and slid fr C SO4 = ml/l. Fig. 4. The curves f distributin f sluble species and slid fr C SO4 = ml/l. Fig. 1. Variatin f the Gibbs energy versus ph in the system basaluminite saturated aqueus slutin in functin n C SO4, ml/l. Curve 1 - C SO4 = ml/l, 2 - C SO4 = M, 3 - C SO4 = M, 4 - C SO4 = M Fig. 5. The curves f distributin f sluble species and slid fr C SO4 = ml/l. Fig. 2. The curves f distributin f sluble species and slid fr C SO4 = ml/l. Fig. 6. The partial mlar fractin f basaluminite versus ph fr different values f Curve 1 -, C SO4 = ml/l, 2 - C SO4 = M, 3 - C SO4 = M, 4 - C SO4 = M 253

5 Cnclusins Within the methd f residual cncentratins, using the mass balance cnditins that take int accunt all the pssible chemical species present bth in aqueus and slid phases, a methd f thermdynamic calculatin f cmplex chemical equilibria in sils cntaining mineralphase basaluminite in real cnditins has been develped. The areas f thermdynamic stability f basaluminite fr a wide range f the variatin f ph and different values f sulfate in have been determined. A new type f distributin diagrams, which describe quantitatively the distributin f sluble and insluble species f aluminium in hetergeneus multicmpnent system basaluminitesaturated aqueus slutin has been presented. In the studied system the SO ins play an imprtant rle in 4 regulating the distributin f sluble and insluble species. The increasing the SO cncentratin leads t decreasing 4 f residual cncentratin f aluminium in slutin, C r. The Al btained data are in gd agreement with the experimental data reprted previusly in the literature. References 1.BI, S., SHUQING, A., TANG, W., XUE, R., WEN, L., LIU, F., J. Inrg. Bichem., V. 87, 2001, p POVAR, I., RUSU, V., Can. J. Chem., V. 90, 2012, p DRISCOLL, C.T., SCHECHER, W.D., Envrn. Bichem. Health, V. 12, 1990, p JONES, A. M., COLLINS, R. N., WAITE, T. D., Gechim. Csmchim. Acta, V. 75, 2011, p NORDSTROM, D. K., Gechim. Csmchim. Acta, V. 46, 1982, p CORTECCI, G., BOSCHETTI, T., DINELLI, E., CABELLA, R., Water, Air, Sil Pllut., V. 192, 2008, p ADAMS, F., RAWAJFIH, Y., Sil Sci. Sc. Am. J., V. 41, 1977, p SÁNCHEZ-ESPAÑA, J, YUSTA I., DIEZ-ERCILLA, M., Appl. Gechem., V.26, 2011, p EDET, A. E., MERKEL, B. J., OFFIONG, O. E., Envirn. Gel., V. 45, 2004, p SCHECHER, W.D., DRISCOLL, C.T., Water Resur. Res., V. 24, 1988, p POVAR, I., Ukr. Khim. Zh., V. 60, 1994, p. 371 (In Russian). 12.POVAR, I., Russ. J. Inrg. Chem., V. 42, 1997, p. 607(Engl. Transl.). 13.POVAR, I., J. Anal. Chem., V. 53, 1998, p POVAR, I., Russ. J. Gen. Chem., V. 70, 2000, p POVAR, I., Can. J. Chem., V. 79, 2001, p FISHTIK, I., POVAR, I., VATAMAN, I., Zh. Obshch. Khim., V. 56, 1986, p. 739 (In Russian). 17.FISHTIK, I., POVAR, I., Zh. Obshch. Khim., V. 57, 1987, p. 31 (In Russian). 18.FISHTIK, I., POVAR, I., VATAMAN, I., Zh. Obshch. Khim., V. 57, 1987, p. 736 (In Russian). 19.POVAR, I., FISHTIK, I., VATAMAN, I., Izv. Akad. Nauk Mld. SSR, Ser. Bil. Khim. Nauk., V. 5, 1989, p. 57 (In Russian). Manuscript received: