Time con straints on ex per i men tal stud ies of lead apatites
Justyna TOPOLSKA1, *, Tomasz BAJDA1, Bartosz PUZIO1, Maciej MANECKI1 and Gabriela KOZUB-BUDZYÑ1
1 AGH Uni ver sity of Sci ence and Tech nol ogy, Fac ulty of Ge ol ogy, Geo phys ics and En vi ron men tal Pro tec tion, Al. Mickiewicza 30, 30-059, Kraków, Po land
Topolska, J., Bajda, T., Puzio, B., Manecki, M., Kozub-Budzyñ, G., 2019. Time con straints on ex per i men tal stud ies of lead apatites. Geo log i cal Quar terly, 63 (4): 721–728, doi: 10.7306/gq.1502
As so ci ate Ed i tor: Jacek Szczepañski
The pa per pres ents the re sults of ex per i men tal stud ies on the syn the sis and ther mo dy namic sta bil ity of se lected Pb-apatites in terms of cri te ria de ter min ing ter mi na tion of the ex per i ments. Based on the case study, we in di cate dif fi cul ties in ana lys ing the ob tained ex per i men tal data. Time-re solved sam pling of pre cip i tate formed dur ing a dropwise syn the sis of pyromorphite was per formed and the re sults were com pared to the lit er a ture data. It has been con cluded that the Ostwald rip en ing time for syn the sized sol ids de pends pri mar ily on the chem i cal com po si tion of the in tended Pb-ap a tite phase. We pre sented that het - er o ge ne ity of pre cip i tate af fects its dis so lu tion in terms of re peat abil ity of the re sults and equilibrating time. A unique 9-year-long ex per i ment on vanadinite sta bil ity at a pH range from 2.0–6.0 re vealed that among all tested dis so lu tion con di - tions only the re ac tions at the pH = 3.5 can per form as the ba sis for some ther mo dy namic cal cu la tions. It has been con cluded that the rate of phase tran si tions in the Pb-apatites group can be mis lead ing in terms of de ter min ing the equi lib rium of the sys tem, and the ex per i men tal setup de signed par tic u larly to pro vide re li able con trols in this as pect should be in volved.
Means in this re spect have been pro posed.
Key words: pyromorphite, vanadinite, ex per i men tal min er al ogy, dis so lu tion, sol u bil ity syn the sis.
INTRODUCTION
Apatites are a large group of ubiq ui tous min er als with broad ap pli ca tions. Their flex i ble struc ture can as sim i late nearly half of the el e ments from the Pe ri odic Ta ble (Pan and Fleet, 2002).
There fore, they show a wide range of crys tal forms and colours, and are of ten con fused with other min er als (Rakovan and Pasteris, 2015). As the ba sic com po nent of bones and teeth, apatites are lit er ally close to ev ery sin gle hu man be ing (LeGeros, 2008; Pasteris et al., 2008), whereas as one of the main ores of phos pho rus and REE, they also play an im por tant role in global econ omy (Filippelli, 2008). The abil ity of apatites to im mo bi lize met als in the struc ture and a wide range of en vi - ron men tal con di tions in which these phases are sta ble de ter - mine their pos si ble ap pli ca tion in tech nol o gies for ground wa ter and soil treat ment (Ma et al., 1993, 1995; Magalhães and Wil - liams, 2007).
Lead apatites with a gen eral for mula: Pb5(XO4)3Z (where X
= P, V, As and Z = Cl, OH, F, Br, I) are par tic u larly sig nif i cant in the field of en vi ron men tal pro tec tion. In duced pre cip i ta tion of these phases is the fun da men tal con cept of the in situ im mo bi li - za tion of Pb and As in con tam i nated soils and wastes (e.g., Cot - ter-Howells, 1996; Cot ter-Howells and Caporn, 1996; Manecki et al., 2000; Cam pos, 2002; Cao and Ma, 2004; Bajda et al., 2007). Op ti mi za tion of this treat ment method re quires ex ten sive re search on ap a tite phases in the field of their iden ti fi ca tion, crys tal li za tion, trans for ma tion, struc ture re fine ment and ther - mo dy namic sta bil ity (e.g., Ma et al., 1994a, b; Flis et al., 2010, 2011; Bajda, 2010; Baikie et al., 2014; Topolska et al., 2016;
Solecka et al., 2018; Puzio et al., 2018). Also, a num ber of pub li - ca tions on the in flu ence of mi cro or gan isms on the sta bil ity of Pb-apatites in the en vi ron ment has been pub lished (e.g., Manecki and Maurice, 2008; Park et al., 2011; Topolska et al., 2013, 2014; Ceci et al., 2015). The re search in di cates the lim i - ta tions of P-in duced soil remediation treat ments. Most of the stud ies on the im ple men ta tion of apatites as se ques tra tion agents are based on the meth ods typ i cal for ex per i men tal min - er al ogy, i.e. syn the siz ing crys tals with pa ram e ters sim i lar to those in the en vi ron ment and then dis solv ing them with a cho - sen method to de ter mine the sta bil ity and re ac tiv ity. In this re - gard, the key pa ram e ter stat ing on the re li abil ity of the abiotic lab o ra tory re sults and the pos si bil ity of re fer ring them to field
* Corresponding author, e-mail: topolska@agh.edu.pl Received: January 12, 2019; accepted: July 25, 2019; first published online: December 3, 2019
con di tions is the well-de ter mined equi lib rium be tween sol ids and so lu tions at the stage of syn the sis, as well as dur ing ther - mo dy namic stud ies.
Be sides their many un doubted ad van tages, the ex per i men - tal stud ies are ob vi ously lim ited by a num ber of lab o ra tory con - straints, among which the du ra tion of the ex per i ment seems to be one of the most fun da men tal. In this pa per, we pres ent our ob ser va tions on the role of ex per i men tal time in con duct ing re - search on the Pb-apatites prop er ties. Par tic u lar em pha sis was placed on its im pli ca tion on de ter min ing the equi lib rium of the sys tems dur ing syn the sis and sta bil ity ex per i ments.
METHODS
SYNTHESIS OF PYROMORPHITE
Syn the sis of the solid was car ried out at room tem per a ture by dropwise mix ing of the aque ous so lu tions of Pb(NO3)2, KCl, and K2HPO4 in mo lar pro por tions based on the stoichiometry of the pyromorphite (Pb:P:Cl = 5:3:1 mo lar ra tio). Sus pen sion of the form ing pre cip i tate was stirred for 5 hours us ing a me chan i - cal stir rer, and was al lowed to set tle. The pre cip i tate was sam - pled pe ri od i cally for the first 2 weeks and ana lysed with the use of Scan ning Elec tron Mi cros copy cou pled with En ergy Dispersive Spec tros copy (SEM-EDS). Af ter 2 weeks, the pre - cip i tate was sep a rated from the so lu tions by decantation, washed thor oughly sev eral times with redistilled wa ter and ac e - tone, and dried at 110°C. The syn thetic solid was char ac ter ized us ing SEM-EDS and wet chem i cal anal y sis. SEM-EDS ex am i - na tions were con ducted us ing Jeol 5200 at 25kV. For wet chem i cal anal y sis, small amount of the pre cip i tate was di gested in 0.02 M EDTA. The con cen tra tion of Pb in so lu tion was de ter - mined us ing atomic ab sorp tion spec tros copy (AAS), and that of P – us ing a colorimetric method of Lenoble et al. (2003). The mea sure ments were made with a SAVANTAA AAS spec trom e - ter and a HITACHI U-1800 UV-Vis spectro photometer. The syn the sis yielded fine, white pre cip i tates. Ex am i na tion by SEM-EDS re vealed par ti cles con tain ing Pb, P, O and Cl as ma - jor con stit u ents. Mor phol ogy in the form of hex ag o nal prisms, typ i cal for apatites, was ob served (Fig. 1). No sec ond ary phases were de tected. The re sults of wet chem i cal anal y sis of the solid yielded com po si tion of the pre cip i tate very close to the stoichiometry of pyromorphite: Pb5.09(PO4)2.94Cl. The de vi a tion from the ideal for mula was within an av er age an a lyt i cal er ror es - ti mated by trip li cates to 5% at the 95% con fi dence level.
SYNTHESIS OF VANADINITE
Syn the sis of vanadinite was car ried out at 90°C by dropwise mix ing of the aque ous so lu tions of Pb(NO3)2, KCl and NH4VO3
in mo lar pro por tions based on the stoichiometry of the vana - dinite (Pb:V:Cl = 5:3:1 mo lar ra tio). Sus pen sion of the form ing pre cip i tate was stirred for 5 hours us ing a me chan i cal stir rer, and was al lowed to set tle. Af ter 2 weeks, the pre cip i tate was sep a rated from the so lu tions by decantation, washed thor - oughly sev eral times with redistilled wa ter and ac e tone, and dried at 110°C. The syn thetic solid was ana lysed us ing X-ray pow der dif frac tion (XRD) and SEM-EDS. The XRD pat terns were col lected with a Philips PW 3020 X’Pert-APD diffra cto - meter sys tem (with a Cu an ode and a graph ite mono chro ma tor)
us ing a step-scan mode at a step size of 0.02 2Q and a rate of 1s per step. A FEI QUANTA 200 FEG was used for the SEM-EDS anal y sis at 15 kV. The syn the sis yielded a yel low ish ho mo ge neous pre cip i tate. The X-ray pow der dif frac tion data of the syn thetic vanadinite sam ple is pre sented in the Ap pen - dix 1*. The ana lysed phase was iden ti fied as pure vana dinite.
Fig. 1. SEM mi cro pho to graphs of syn thetic pyromorphite
A – pre cip i tate aged for 48h, B – pre cip i tate aged for 1 week, C – pre cip i tate aged for 2 weeks
* Sup ple men tary data as so ci ated with this ar ti cle can be found, in the on line ver sion, at doi: 10.7306/gq.1502
The SEM-EDS anal y sis of the pre cip i tates con firmed the XRD re sults, yield ing fine crys tals with the size ~£2 um (Fig. 2) and the com po si tion close to vanadinite: Pb5(VO4)2.95Cl0.95 con sid er - ing the 10% an a lyt i cal er ror es ti mated based on rep li cates.
DISSOLUTION OF VANADINITE
Dis so lu tion ex per i ments were per formed in trip li cates in 250-mL polycarbonate bot tles im mersed in a closed wa ter bath with ther mo static con trol at 25°C. Equal por tions (150 mg) of syn thetic vanadinite were washed thor oughly with redistilled wa ter and in tro duced as a sus pen sion to the bot tles con tain ing 250 mL of 0.05 M KNO3 so lu tion. To tally, there were 7 sets of the ex per i ment with the pH ad justed, us ing 0.1 M HNO3, to: 2.0, 2.5, 3.0, 3.5, 4.0, 5.0 and 6.0. The dis so lu tion was car ried out for 8 months. Dur ing this time, the bot tles were man u ally stirred at least two times a week, and the so lu tion sam ples were pe ri - od i cally with drawn and fil tered us ing a 0.2 mm polycarbonate fil - ter. The [Cl] evo lu tion pat terns from the dis so lu tion ex per i ments were ob served to de ter mine the equi lib rium in the sus pen sions.
Cl con cen tra tion was mea sured us ing a turbidimetric method with sil ver ni trate. The sys tem was con sid ered in equi lib rium when at least three con sec u tive sam ples showed iden ti cal [Cl], with an ex per i men tal er ror ex pressed as stan dard de vi a tion of trip li cate re sults. The re sid ual sol ids were sam pled, washed with ac e tone, air-dried, and char ac ter ized with SEM-EDS. Ad - di tion ally, the re sid uum from the ex per i ment at pH = 3.0 was ex - am ined with XRD. Af ter this stage of dis so lu tion ex per i ment, the bot tles with the sus pen sions were trans ferred to a room tem - per a ture of 22 ±2°C and left for 9 years. The re ac tors were oc - ca sion ally mixed and checked for leaks. At the end of the ex per - i ments, the con cen tra tion of Cl in so lu tions was mea sured by the turbidimetric method with sil ver ni trate and the re sid ual sol - ids were ana lysed with the use of SEM-EDS (A FEI QUANTA 200 FEG, at 15 kV).
RESULTS AND DISCUSSION
TIME-DEPENDENT SYNTHESIS AND ITS IMPLICATIONS ON DISSOLUTION EXPERIMENTS
Among all Pb-apatites, pyromorphite is a min eral of the best-known prop er ties, as in di cated by the num ber of pub li ca - tions (e.g., Ma et al., 1994a, b; Flis et al., 2010, 2011; Bajda, 2010; Baikie et al., 2014; Topolska et al., 2016). Scheckel and Ryan (2002) re ported on the ne ces sity of 24-hour-long ag ing of the pyromorphite pre cip i tate formed dur ing dropwise syn - the sis. The re sults of our ex per i ments con firmed their gen eral con clu sions; how ever, ac cord ing to our ob ser va tions in the con text of the en tire Pb-ap a tite group, the pro cess seems to be more com plex. Fig ures 1 show the changes in the crys tal size of the syn the sized pyromorphite pre cip i tate with pro - longed time of ag ing. The sam ples were col lected 48 h (Fig.
1A), 1 week (Fig. 1B) and 2 weeks (Fig. 1C) af ter the syn the sis was com pleted. The SEM im ages clearly show that the num - ber of fine, long, thread-like crys tals de creased with time, trans form ing into stubby crys tals of even di men sions ac cord - ing to the Ostwald rip en ing mech a nism (Sordyl et al., 2017).
Dif fer ences be tween the pre cip i tate sam pled at hour 48 and af ter 1 week are not well vis i ble. Af ter 2 weeks (Fig. 1C), the pre cip i tate com prised mainly of crys tals ~>5 mm in size; how - ever, some ad mix ture of fine crys tals re mained. This sug gests that pro posed by Scheckel and Ryan (2002), last ing 24 hours ag ing of pyromorphite syn the sis is not an uni ver sal method to ob tain a ho mo ge neous pre cip i tate of this min eral and it cer - tainly should not be used as a cri te rion for de ter min ing the ho - mo ge ne ity of syn the sized pre cip i tate for other Pb-apatites. In - stead, we pro pose to ob serve the Ostwald rip en ing pro cess for each syn the sis ex per i ment in di vid u ally. Fur ther more, as re - cently sug gested by Sordyl et al. (2017), the ag ing time of syn - the sized pre cip i tate is in di vid ual for each Pb-ap a tite phase and strongly de pends on its chem i cal com po si tion. As an ex - am ple con firm ing con clu sions pro posed in this study and by Sordyl et al. (2017), we would like to re call here the re sults pre - sented by our re search group (Flis et al., 2011) in the pa per on ther mo dy namic pa ram e ters of the pyromorphite-mimetite solid so lu tion se ries. Fig ure 3 shows se lected data re ported pre vi ously (Flis et al., 2011), how ever, with im proved res o lu - tion achieved by changes in the time scales. This en ables to con sider the ki netic as pects of the ex per i ment, which have not been dis cussed so far. The plot pres ents the time-de pend ent evo lu tion of Pb con cen tra tion in the so lu tion equilibrating with syn thetic: pyromorphite (Fig. 3A), solid so lu tion be tween pyromorphite and mimetite (Fig. 3B), and mimetite (Fig. 3C) dur ing the dis so lu tion ex per i ments de scribed in Flis et al.
(2011). Note that a rel a tively sig nif i cant and ir reg u lar fluc tu a - tion of the Pb con cen tra tion was ap par ent for the first 300 h.
Based on the re sults of the ex per i ments on the in flu ence of ag - ing time on the ho mo ge ne ity of syn the sized pre cip i tates, pre - sented in this study, it can be con cluded that the fluc tu a tions re sulted most likely from the fact that the dis solv ing pre cip i - tates con tained het er o ge neous crys tals ex hib it ing a dif fer ent rate of dis so lu tion. The syn the sized phases used in the ex per i - ments had been aged only for 14 days (re fer to Flis et al., 2011 for the SEM im ages). Fur ther more, even though all three ma - te ri als: pyromorphite, solid so lu tion and mimetite, were pre - pared ac cord ing to the same syn the sis pro ce dure, in clud ing Fig. 2. SEM mi cro pho to graph of syn thetic vanadinite
The pre cip i tate was aged for 2 weeks
the ag ing time, the pro por tion of small and ma ture crys tals in the fi nal pre cip i tates dif fered (re fer to Flis et al., 2011 for the SEM im ages) and de pended on the chem i cal com po si tion of the phase, which is con sis tent with con clu sions pro posed by Sordyl et al. (2017). We find that this ef fect had an im pli ca tion on the fur ther dis so lu tion pro cess and there fore in Fig ure 3, the in ten sity of ini tial fluc tu a tion of the Pb con cen tra tion is not the same for all phases. It seems that the mimetite pre cip i tate was the most ho mo ge neous, hence the ob served changes in dis solved Pb amounts were least in tense in its case and the ex per i ments yielded the most con sis tent re sults (small er ror bars de scrib ing trip li cates).
Bring ing the pre cip i tate to a ho mo ge neous form is par tic u - larly im por tant when con duct ing stud ies on the ki net ics of the dis so lu tion re ac tion (Scheckel and Ryan, 2002); it is a com - mon knowl edge that crys tals with a larger sur face-to-vol ume ra tio are prone to re act with the so lu tion. This phe nom e non is slightly less im por tant dur ing ex per i men tal stud ies on ther mo - dy namic prop er ties of Pb-apatites, how ever, as it re sults from our ex pe ri ence and is de scribed above, run ning the dis so lu - tion ex per i ments on a het er o ge neous solid might af fect re - peat abil ity of the re sults and lead to an in creased dis so lu tion rate in the ini tial steps of the re ac tion. This, in turn, low ers the qual ity of fi nal con clu sions and ex tends the equilibrating time for the so lu tion.
TIME AND THE EXPERIMENTAL THERMODYNAMICS
The ex per i men tally de ter mined ther mo dy namic pa ram e - ters of pyromorphite, mimetite and their in ter me di ate mem - bers have been pub lished in a num ber of pa pers (Bajda, 2010;
Flis et al., 2010; Topolska et al., 2016). Re search on the prop - er ties of vanadinite, the third end-mem ber in a solid so lu tion se ries of an ionic sub sti tu tions among Pb-apatites (Baker, 1966), is still on go ing and is more prob lem atic. The phos - phate, ar se nate and vanadate an ionic groups are sim i lar to each other in terms of stereochemistry. While the chem i cal re - ac tiv ity of As and P are com pa ra ble, V – be ing a tran si tion metal – ex hib its dif fer ent prop er ties. Un like phos phate or ar se - nate an ions, vanadate groups tend to poly mer ize in aque ous so lu tion (Schindler et al., 2000). Hence, a vanadinite Pb5(VO4)3Cl is prone to recrystallize into a chervetite Pb2V2O7
in a wide range of con di tions (Ev ans and Garrels, 1958). The recrystallization is slow and poorly un der stood in the con text of vanadinite sol u bil ity. Ex per i men tal de ter mi na tion of the min - eral ther mo dy namic pa ram e ters based on its in con gru ent dis - so lu tion is pos si ble if the ther mo dy nam ics of the ad di tional phase is known and the state of equi lib rium for both phases and the so lu tion is found. To de ter mine the equi lib rium con di - tions for the vanadinite-chervetite sys tem at 25°C, a se ries of dis so lu tion ex per i ments of syn thetic vanadinite at var i ous pH was per formed for over 9 years. The ex per i men tal time was di - vided into two parts. The 8-months pe riod re flected the stan - dard ex per i men tal time for this type of re search, dur ing which reg u lar anal y ses were per formed. Then, the ex per i ments were con tin ued for ad di tional 8.5 years, af ter which the so lu tions and sol ids were ana lysed again to eval u ate whether the con - clu sions from the short-term stud ies were rea son able. Fig ure 4 shows the evo lu tion of Cl con cen tra tion in the ex per i men tal so lu tions. The sta bi li za tion of Cl con cen tra tion was cho sen as a pa ram e ter de fin ing the equi lib rium in the sys tem, since it is a com po nent of vanadinite, but it does not build into the struc - ture of a po ten tially form ing chervetite. Fig ure 4 shows the Cl con cen tra tions in so lu tions at pH = 2.0, 2.5, 3.0 and 3.5 (note Fig. 3. The evo lu tion of Pb con cen tra tion in the dis so lu tion
ex per i ment of pyromorphite (A), pyromorphite-mimetite solid so lu tion (B) and mimetite (C)
Data points rep re sent the av er age of trip li cate ex per i ments, er ror bars are two stan dard de vi a tions
the break in the time scale sep a rat ing the re sults ob tained af - ter 8 months from those ob tained af ter 9 years). Fig ures 5 and 6 pro vide SEM mi cro pho to graphs of the sol ids af ter 8 months and 9 years of dis so lu tion, re spec tively. The EDS anal y sis (data not shown) in di cated that the fine hex ag o nal crys tals be - longed to the Cl-rich phase with the ionic pro por tions cor re - spond ing to vanadinite Pb5(VO4)3Cl, while the tab u lar ones were made of chervetite Pb2V2O7. These re sults were con - firmed by the XRD anal y sis (data pre sented in the Ap pen dix 1). All reg is tered peaks were iden ti fied as those from vana - dinite and chervetite ac cord ing to the re ferred data base.
DISSOLUTION OF VANADINITE: IMPLICATION OF THE RESULTS OF 8-MONTH RESEARCH
The ex per i ment was con ducted at a pH range from 2.0 to 6.0,how ever, dur ing the first 8 months, the con cen tra tion of Cl in the so lu tions with pH ³4.0 was be low the de tec tion limit. As pre sented in Fig ure 4, the con cen tra tion of Cl in all other ex - per i men tal so lu tions in di cated the po ten tial equi lib rium in the sys tem, i.e. the sam ples col lected in 2200 h, 3200 h and 4000 h of the ex per i ment yielded val ues iden ti cal within the ex per i - men tal er ror (ver i fied with the use of a Stu dent’s t-test, at a = 0.05). Three con sec u tive anal y ses per formed for a so lu tion over a pe riod of sev eral weeks, re veal ing sta tis ti cally iden ti cal con cen tra tion val ues, are usu ally a suf fi cient cri te rion to rec - og nize the equi lib rium of the sys tem (e.g., Topolska et al., 2016). There fore, the res i due was sam pled and ana lysed us - ing SEM. As pre sented in Fig ure 5A and B, vanadinite was not pres ent in the sus pen sions at pH = 2.0 or 2.5: it seemed that the con ver sion of the min eral into chervetite oc curred rel a - tively fast and was com plete at this point. There fore, these ex - per i men tal con di tions were ex cluded as meet ing the re quire - ments for fur ther ther mo dy namic cal cu la tions. How ever, the
amount of cherve tite in the sus pen sions with dis solv ing vana - dinite de creased with in creas ing pH. As in di cated in Fig ure 5D and E, at pH 5.0 and 6.0, the pre cip i tate was lack ing vis i ble ad - mix tures of chervetite and con sisted to tally of pure vanadinite crys tals (within the de tec tion lim its of the meth ods used). This sug gested the po ten tially con gru ent na ture of vanadinite dis - so lu tion at these con di tions. How ever, the geo chem i cal mod - el ling us ing PHREEQC (Park hurst, 1995) did not con firm this in ter pre ta tion (data not shown). The re sults of the so lu tion and sed i ment anal y sis ob tained for the ex per i ments at pH = 3.0 and 3.5 in di cated a sta ble equi lib rium state in which both vanadinite and chervetite were pres ent. Hence, these ex per i - ments were se lected as po ten tial for es ti mat ing the sol u bil ity prod uct of vanadinite.
DISSOLUTION OF VANADINITE: IMPLICATION OF RESULTS OF 9-YEAR RESEARCH
The ex per i men tal sus pen sions de scribed above were kept for a pe riod of 9 years. The re ac tors were sealed, oc ca sion ally mixed, and the tem per a ture was re duced from 25°C to room tem per a ture ~22 ±2°C. This en sured a qual ity of the long-term ex per i ment, so that the ef fect of po ten tial evap o ra tion af ter 9 years was neg li gi ble. The last points on the graph pre sented in Fig ure 4 show the con cen tra tion of Cl in the sam ples col lected af ter 9 years. The el e vated amounts of Cl in the so lu tions with pH 2.0 and 2.5 in di cate that, con trary to the as sump tions, the recrystallization of vanadinite into chervetite un der these con - di tions had not been com pleted af ter 8 months. Sim i larly, the so lu tion at pH = 3.0 had not been in equi lib rium, but rather in a metastable state. It is also worth men tion ing that, af ter 9 years, Cl ap peared as well in the so lu tions with an ini tial pH of 4.0–6.0, for which it was be low the de tec tion limit be fore. Af ter com plet ing the re search (9 years), the con cen tra tions of Cl in these so lu tions were (in mM) 0.031 ±0.004, 0.021 ±0.01 and 0.013 ±0.006 for pH = 4.0, 5.0 and 6.0, re spec tively. Fur ther - more, con trary to the state that had oc curred in the 8th month of the ex per i ments, af ter 9 years of stud ies, the solid res i due at pH = 5.0 and 6.0 (pre sented in Fig ure 6F and G, re spec tively) did not con sist of pure vanadinite, but some ev i dent ad mix ture of chervetite ap peared (tab u lar chervetite crys tals are par tic u - larly ev i dent in Fig ure 6F’ and G’ which show mi cro pho to - graphs taken at higher mag ni fi ca tion). These ob ser va tions de - nied the pre vi ous con clu sions that vanadinite had dis solved con gru ently un der these con di tions; the recrystallization was in deed on go ing, but the re ac tion rate was re duced by or ders of mag ni tude with in creas ing pH, which led to mis lead ing con clu - sions af ter 8 months of ex per i ment. In terms of sta bil ity, among all tested dis so lu tion con di tions, the ex cep tion was at pH 3.5, for which a Stu dent’s t-test showed that the Cl con cen - tra tions sam pled at hours 1,200, 1,300 and 83,000 of the ex - per i ment were sta tis ti cally iden ti cal (at a = 0.05). The long- term ex per i ments have dem on strated un am big u ously that, de spite ob served Cl fluc tu a tions in this so lu tion, the re ac tions can per form as the ba sis for some ther mo dy namic es ti mates.
CONCLUSIONS
The pre sented re search shows that the ag ing time for the Pb-ap a tite pre cip i tates which formed dur ing dropwise syn the - sis de pends not only on the ex per i men tal con di tions, but pri - mar ily on the chem i cal com po si tion of the syn the sized phase.
Fig. 4. The evo lu tion of Cl con cen tra tion in the dis so lu tion ex per i ment of vanadinite
Data points rep re sent the av er age of trip li cate ex per i ments, er ror bars are two stan dard de vi a tions
Fig. 5. SEM mi cro pho to graphs of the re sid ual sol ids from the dis so lu tion ex per i ments of vanadinite af ter
8 months of re ac tion time
A – ex per i ment at pH = 2.0; B – ex per i ment at pH = 3.0; C – ex per i ment at pH = 4.0; D – ex per i ment at pH = 5.0; E – ex - per i ment at pH = 6.0; hex ag o nal stubby crys tals were iden ti - fied as vanadinite, while tab u lar ones as chervetite
Het er o ge ne ity of a solid af fects its dis so lu tion in terms of re - peat abil ity of the re sults and equilibrating rate. There fore, spe - cial at ten tion to the Ostwald rip en ing time should be paid for each syn the sized phase in di vid u ally. The unique, 9-year-long ex per i ments on vanadinite dis so lu tion re vealed that the rate of trans for ma tion and recrystallization within this group of min er - als may be so slow that, with faulty as sump tions on the du ra - tion of ex per i ment, the metastable state can be eval u ated as equi lib rium. The ob tained re sults in di cate that, due to the recrystallization of vanadinite into chervetite, among all tested dis so lu tion con di tions only the re ac tions oc cur ring at the pH = 3.5 can be re li ably used as the ba sis for some fur ther ther mo - dy namic es ti ma tions. Since it is not al ways pos si ble to con -
duct long-term re search, some other means en sur ing re li able de ter mi na tion of equi lib rium in the sys tem should be pro vided by the ex per i men tal setup. One of the ways to avoid mis takes is to run par al lel pre cip i ta tion and dis so lu tion ex per i ments for the same con di tions, as pre sented in one of our pre vi ous pa - pers (Flis et al., 2011).
Ac knowl edge ment. This work was funded by the AGH Uni ver sity of Sci ence and Tech nol ogy stat u tory pro ject No.
11.11.140.158. We would like to thank the re view ers: Dr.
O. Borkiewicz and Prof. J. Janeczek for their con tri bu tion to the fi nal shape of the manu script.
Fig. 6. SEM mi cro pho to graphs of the re sid ual sol ids from the dis so lu tion ex per i ments of vanadinite
af ter 9 years of re ac tion time
A, A’ – ex per i ment at pH = 2.0; B, B’ – ex per i ment at pH = 2.5; C, C’
– ex per i ment at pH = 3.0; D, D’ – ex per i ment at pH = 3.5; E, E’ – ex - per i ment at pH = 4.0; F, F’ – ex per i ment at pH = 5.0; G, G’ – ex per i - ment at pH = 6.0; mag ni fi ca tions of se lected places in the main pho to graphs were marked with apos tro phes fol low ing the let ters;
hex ag o nal stubby crys tals were iden ti fied as vanadinite, while tab u - lar ones as chervetite
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