Investigation of mass transfer phenomenna during sorption of Cu (II)-ions onto inorganic sorbents

Predmet rada ove doktorske disertacije je bio ispitivanje sorpcije Cu (II)- jona sintetičkim HAP-om i prirodnim sorbentima zeolitom, glinom i dijatomitom, u cilju definisanja ravnoteže i kinetike procesa pri različitim uslovima.Fizičko-hemijska karakterizacija je pokazala da se sintetički HAP sastoji od čistog nestehiometrijskog, kalcijum deficitarnog, kalcijum-hidroksiapatita, zeolit od klinoptilolita, dok je glina kompozit ilita, kaolinita i montmorilonita. Glavna faza u dijatomitu je amorfni silicijum-dioksid. Takođe, HAP i zeolit su okarakterisani kao mezoporozni sorbenti specifičnih površina 58 m2/g i 23 m2/g, redom, dok su se glina i dijatomit pokazali neporozni, nižih specifičnih površina od 8 m2/g i 5 m2/g. Definisana je pHPZC koja iznosi 6,6 za HAP, glinu i dijatomit, dok je kod zeolita nešto viša i iznosi 7,5. Dobijene vrednosti pHPZC HAP-a, gline i zeolita su u dobroj saglasnosti sa literaturom, dok pHPZC dijatomita do sada nije objavljena.Ispitivanjem uticaja inicijalne pH vrednosti rastvora na ravnotežu, pokazano je da se finalne pH vrednosti razlikuju u zavisnosti od ispitivanog sorbenta. Generalno, ravnotežne pH vrednosti najviše rastu sa porastom inicijalnih pH rastvora u oblasti 2 - 4. Oblast platoa, u kome su finalne pH vrednosti nezavisne od inicijalnih i koja ukazuje na puferska svojstva sorbenata, zavisi od vrste sorbenta i koncentracije jona metala u rastvoru. Dalje povećanje polaznih pH prouzrokuje porast i finalnih. Krive zavisnosti sorbovane količine od inicijalnih pH su istog oblika kao i krive pH finalno-pH inicijalno, što se objašnjava povećanom rastvorljivoću sorbenata ili njihovih komponenti i kompeticijom Cu(II) i H+ jona u jako kiselim rastvorima. Porast finalnih pH iznad 6 dovodi do precipitacije bakar – hidroksida što rezultira 100% uklanjanjem jona metala iz rastvora.Maksimalni sorpcioni kapaciteti prema Cu(II)- jonima, na sobnoj temperaturi, opadaju u nizu: HAP (0,585 mmol/g) > zeolit (0,128 mmol/g) > glina (0,098 mmol/g) > dijatomit (0,047 mmol/g). U datim eksperimentima je utvrđen pad pH vrednosti rastvora u odnosu na početnu, što ukazuje na postojanje mehanizma specifične sorpcije, koji je najdominantniji tokom sorpcije na dijatomitu. Za vezivanje jona metala ispitivanim materijalima, odgovoran je i mehanizam jonske izmene, gde se Cu(II)- joni izmenjuju sa Ca2+ jonima sa površine HAP-a, odnosno sa jonima Na+, K+, Ca2+, Mg2+ sa površineprirodnih sorbenata.Stabilnost sistema sorbent/jon metala je ispitana u kiselom rastvoru za izluživanje i zavisi od vrste sorbenta, kao i prethodno sorbovane količine Cu(II)- jona. Sa porastom sorbovane količine na uzorcima HAP-a i dijatomejske zemlje, raste i procenat desorbovane količine, dok desorpcija sa zeolita i gline pokazuje suprotan trend - u kiseloj sredini su stabilniji uzorci sa većom sorbovanom količinom jona metala. Ako posmatramo desorbovanu količinu izraženu u mmol/g ili mg/g, ona je veća na uzorcima koji su više zasićeni, nezavisno od vrste sorbenta.Kinetika procesa je ispitana u različitim šaržnim sistemima. Prvi sistem su pojedinačne šarže sa mešanjem na horizontalnom šejkeru, gde je proces sorpcije ispitan u funkciji od početne koncentracije Cu(II)- jona na svim ispitanim sorbentima. Drugi šaržni sistem je sud sa mešalicom u kom je ispitana kinetika procesa sorpcije HAP-om i zeolitom u funkciji od koncentracije sorbata, mase sorbenta, brzine mešanja, a za zeolit i od granulacije sorbenta. Pokazano je da je sorpcija glinom i dijatomitom, kao neporoznim sorbentima, limitirana otporom u filmu fluida. Nasuprot tome, difuzija u filmu fluida tokom sorpcije na šejkeru HAP-om i zeolitom je značajna na početku procesa, tokom prvih 15 minuta. Vrednosti zapreminskog koeficijenta prenosa mase su izračunate Boyd-ovim modelom. Dobijene vrednosti kf·a su nezavisne od početne koncentracije Cu(II)- jona u rastvoru i variraju oko srednje vrednosti 0,046 1/min za HAP, 0,023 1/min za zeolit, 0,017 1/min za glinu i 0,031 1/min za dijatomit. Iz prethodno navedenog proizilazi da je kf·a kod neporoznih sorbenata funkcija veličine čestica, tj. najveći je za najsitnije čestice jer je i specifična površina veća. Primenom jednačina za difuziju u porama, pokazano je da efektivne difuzivnosti blago opadaju sa porastom c0 za sorpciju HAP-om, dok za sorpciju zeolitom Deff nije funkcija c0. Zavisnost Deff od c0 i nezavisnost kfa od c0 kod HAP-a može se objasniti suprotnostrujnom difuzijom odlazećih katjona sa površine sorbenta, kao i elektrostatičkim odbijanjem istoimenih naelektrisanja koji su zanemarljivi u masi fluida, ali se ne mogu zanemariti u unutrašnjosti pora.Analizom rezultata sorpcije HAP-om u sudu sa mešanjem, dobijeno je da kf·a za prvih 10 minuta procesa ne zavisi od mase sorbenta i brzine mešanja, a blago opada sa porastom početne koncentracije metala u rastvoru, što je najverovatnije uslovljeno postojanjem otpora difuzije u porama. Nasuprot tome, kf·a izračunat za zeolit ne zavisi od mase sorbenta, kao ni početne koncentracije, ali raste sa porastom brzine mešanja i padom veličine čestica sorbenta.Efektivne difuzivnosti i kod HAP-a i kod zeolita ne zavise od mase sorbenta, kao ni brzine mešanja. Deff opada sa porastom c0 usled postojanja suprotnostrujne difuzije u porama, a raste sa porastom veličine čestica.Kao krajnji zaključak, može se izvesti da je za procese sorpcije HAP-om i zeolitom na šejkeru u prvih 15 minuta limitirajući otpor u filmu fluida, a zatim difuzija u porama sorbenta, pri čemu se dešava sorpcija u mezoporama. Takođe, šejker ne obezbeđuje dovoljno efikasno mešanje, te su vrednosti kfa i Deff značajno niže od vrednosti dobijenih za sud sa mešanjem. Sorpcija HAP-om u sudu sa mešanjem je limitirana samo difuzijom u porama jer se difuzija u filmu fluida i zasićenje površine dešava skoro trenutno. Tokom sorpcije zeolitom u sudu sa mešanjem, prvih 10 minuta procesa značajan je otpor u filmu, a nakon toga otpor u porama sorbenta.The subject of the work presented in this dissertation was to investigate the sorption of Cu (II)- ions onto synthetic hydroxyapatite (HAP)-and natural sorbents: zeolite, clay and diatomite, in order to define process equilibrium and kinetics under different conditions.Physico-chemical characterization showed that synthetic HAP was consisted of pure non-stoichiometric, calcium deficient, calcium hydroxyapatite, zeolite of clinoptilolite, while clay was the composite of illite, kaolinite and montmorillonite. The main phase of diatomite was amorphous silicon dioxide. Also, HAP and zeolite were characterized as a mesoporous sorbents with specific surface areas of 58 m2 / g and 23 m2 / g, respectively, while the clay and diatomite were non-porous, with low specific surface areas of 8 m2/g and 5 m2 / g. pHPZC of 6.6 was defined for HAP, clay and diatomite, while higher value of 7.5 was obtained for zeolite. Obtained results for pHPZC for HAP, clay and zeolite are in good agrement with literature, while pHPZC of diatomite has not been published.The investigation of influence of initial solution pH in the range of 2 - 10 on equilibrium showed that the final pH values varied depending on the tested sorbent. In general, final pH values increased the most in pH range 2 – 4. Areas of the plateau, where the final pH values were independent on the initial pH indicating the buffering properties of sorbents, were dependent on the sorbent and the concentration of metal ions in solution. Further increase of initial pH caused increase of final pH values. The plots of sorbed amounts vs. initial pH were of the same shape as plots pH final vs. pH initial, which can be explained with higher solubility of sorbents and their components and competition between Cu(II) and H+ ions in the strong acidic media. The final pH increase above 6 caused copper hydroxide precipitation and 100 % removal of metal ions from solution.The maximal sorption capacities at room temperature decreased in the order: HAP (0,585 mmol/g) > zeolite (0,128 mmol/g) > clay (0,098 mmol/g) > diatomite (0,047 mmol/g). In these experiments pH drop was observed, signifying the existence of specific cation sorption mechanism which was the most dominant during the sorption onto diatomite. Also, ion-exchange mechanism between Cu(II)- ions and Ca2+ ions fromHAP surface was responsible for metal bonding, as well as ion-exchange with Na+, K+, Ca2+, Mg2+ from surfaces of natural mineral sorbents.The stability of system sorbent/ metal was investigated in acidic leaching solution and it was dependant on sorbent type, as well as on the previously loaded amounts. With increase of loaded metal amounts onto HAP and diatomite, the percentages of desorption also increased, while the opposite was observed for desorption from zeolite and clay- the samples with higher loadings were more stable in the acidic media. The desorbed amounts expressed in mmol/g or mg/g were higher for the more saturated samples, independent of sorbent type.The process kinetics was investigated as a function of initial metal concentration in batch system, on horizontal shaker. It was shown that the sorption onto non-porous sorbents, clay and diatomite was limited with diffusion film resistance. Contrary, the film diffusion was significant only during the first 15 minutes of process onto HAP and zeolite. The values of volumetric mass transfer coefficient kf·a were calculated using Boyd model. The values of kf·a were independent on initial concentration, and varied around averaged value of: 0,046 1/min for HAP, 0,023 1/min for zeolite, 0,017 1/min for clay and 0,031 1/min for diatomite. According to this it can be concluded that kf·a was a function of particle size for nonporous sorbents, i.e. it was highest for the smallest particles because specific surface area was higher. The obtained values for kf·a decreased in the order: HAP>diatomite>zeolite>clay, but probably this order would be different if the zeolite particles were smaller, where value for kf·a onto zeolite would be higher. Using pore diffusion models, it was shown that the effective diffusivities slightly decreased with c0 increase, while for sorption onto zeolite Deff was independent on c0. Dependence of Deff on c0 and independence of kfa on c0 can be explained by counter diffusion of cations released from sorbent surface, as well as with electrostatic repulsion of positive charged particles. These phenomena can be neglected in the fluid mass, but not inside the pores.From the analysis of results obtained for sorption onto HAP in the agitated vessel, it was observed that kf·a calculated for the first 10 minutes of the process was independent on sorbent mass and agitation speed, but slightly decreased with initial metal concentration increase, which was probably caused by the existence of pore diffusion resistance. Contrary, kf·a calculated for zeolite was independent on metal concentration and sorbent mass, but increased with agitation speed increase and particle size decrease.Effective diffusivities for sorption onto HAP and zeolite were independent on sorbent mass and agitation speed. Deff decreased with c0 increase because of counter diffusion inside the pores, but increased with particle size increase.Finally, it can be concluded that for the first 15 minutes of the sorption onto HAP and zeolite using shaker, the limiting step was film diffusion, and after that the pore diffusion, where sorption occurred inside the mesopores. Also, the mixing using shaker was not good enough, thus the obtained values for kfa and Deff were significantly lower than the values obtained for the sorption in the agitated vessel. The sorption onto HAP in the agitated vessel was limited only by pore diffusion because the film diffusion and surface saturation occurred almost instantaneously. The sorption onto zeolite in agitated vessel was governed by film diffusion during the first 10 minutes, whereas after that by diffusion inside the pores