Adsorption behavior of selected heavy metals on oxidized multiwalled carbon nanotubes

Svođenje sadržaja teških metala na zakonski propisane nivoe, bez obzira da li je reč o otpadnim ili pak vodama već prisutnim u okolini, a namenjenim ljudskoj upotrebi, zadatak je od prioritetnog značaja, kako sa ekološkog, tako i sa stanovišta bezbednosti javnog zdravlja. Premda su za potrebe usklađivanja sa prethodno pomenutim, za očekivati je sve strožijim normama, do danas predlagana brojna rešenja, među kojima po većini relevantnih kriterijuma rednjače ona zasnovana na procesu adsorpcije, nesporno je da prostora za napredak na datom polju još uvek postoji, i to prevashodno kroz razvoj i sagledavanje delotvornosti sofisticiranih materijala, u poređenju sa konvencionalnim konkurenti ma, potencijalno superiornijih adsorpcionih karakteristika. Oslanjajući se na upravo iznetu konstataciju, ustanovljeni su i ciljevi ove doktorske disertacije, usredsređeni na sledeće: pronalaženje ad sorpcione učinkovitosti dva tipa oksidovanih višeslojnih ugljeničnih nanocevi (oxMWCNT), formiranih izvođenjem odgovarajućeg postupka modifikacije tokom 3 (oxMWCNT3h), odnosno 6 h (oxMWCNT6h), pri obradi vodenih medijuma obogaćenih relativno niskim koncentracijama odabranih teških metala (Cd(II), Cu(II), Ni(II), Pb(II) ili Cr(VI); ≤ 5 mg L-1), praćenje učešća i mere eventualne uslov ljenosti adsorpcione efikasnosti oxMWCNT od trajanja realizovanog koraka oksidacije i predlaganje mehanizama odgovornih za ustanov ljeno ponašanje sučeljavanih parova od interesa. U svrhu postizanja triju definisanih zadataka, najpre je pristupljeno određivanju morfoloških, teksturalnih i termalnih osobina, kao i hemijskih svojstava površina oxMWCNT3h i oxMWCNT6h, posredstvom narednih tehnika: SEM-EDS, TEM, FT-IR, TGA-MS, Boehm-ove titracije, pH drift pro cedure i strukturne analize njihovih pora i specifičnih površina. Ispiti vani predstavnici oxMWCNT zatim su primenjivani u okviru različito koncipiranih adsorpcionih eksperimenata, postavljanih zarad identify kacije obrazaca, ali i uzroka evidentiranih odgovora deset posmatranih sistema na promene kontaktnog perioda konfrontiranih faza, početne koncentracije adsorbata, temperature, pH vrednosti i jonske jačine tečnih disperzija. Podaci proistekli iz implementiranih metoda karakterizacije sagledava nih oxMWCNT ukazuju da je reč o dvama adekvatno prečišćenim, uspešno funkcionalizovanim i generalno veoma sličnim nanomaterija lima, od kojih po ukupnom sadržaju kiselih kiseoničnih grupa ipak donekle prednjače oxMWCNT6h. Adsorpciona ravnoteža u svim obuhvaćenim M(II) slučajevima nastupa izrazito brzo, praktično već po okončanju prvih 5 min, što je u potpunoj suprotnosti sa Cr(VI) izvedbama, odlikovanim pojavom ovakvog stanja tek nakon 72, tj. 96 h, i to u zavisnosti da li ulogu adsorbenta igra oxMWCNT3h ili oxMWCNT6h. Širina upravo istaknutih vremenskih intervala ograničila je mogućnost smislenog modelovanja adsorpcione kinetike isključivo na oxMWCNT/Cr(VI) probe, u datom smislu najprikladnije opisane funkcijom Elovich-evog oblika. Rast početne koncentracije obrađivanih teških metala pozitivno se odražava na ukupne nivoe ukla njanja istih, a odgovarajuće adsorpcione izoterme ispoljavaju visoku saglasnost sa izrazima Freundlich-ovog, Langmuir-ovog, Dubinin Radushkevich-evog i Temkin-ovog tipa. Adsorpcioni kapaciteti i afiniteti primenjivanih nanocevi ka članovima praćenog M(II) skupa grade sledeće, identične trendove: Pb(II) > Cu(II) > Cd(II) > Ni(II) I oxMWCNT6h > oxMWCNT3h, dok se u odnosu na Cr(VI) pozicioniraju tako da se značajnija razlika opaža samo po pitanju poslednje od prethodno navedenih veličina, i to u korist oxMWCNT6h. Povećanje temperature, ukoliko se zanemari par izuzetaka, ne narušava gore iznete obrasce, unutar kojih adsorpcioni kapaciteti ispitivanih oxMWCNT obično bivaju poboljšani. Pad kiselosti sredine ima za posledicu inten zivizaciju stepena usvajanja M(II) predstavnika, i obrnuto, već pri pre koračenju pH ∼3,00 rezultuje smanjenjem obima izdvajanja Cr(VI). Rast jonske jačine, u slučaju svih analiziranih sistema, nepovoljno utiče na delotvornost upotrebljivanih oxMWCNT. Sudeći na osnovu ovde predočenih, ali i konstatacija izvedenih iz vrednosti parametara pomenutih kinetičkih i ravnotežnih matematičkih modela, posmatrani adsorpcioni procesi mogu se opisati kao favorizabilni, egzotermni i prevashodno indukovani pojavom elektrostatičkog, tj. spoljno-sfernog kompleksiranja, ostvarivanog, uz redukcionu Cr(VI)/Cr(III) konverziju, na relaciji kompatibilnih formi adsorbovanih vrsta i kiseoničnih funkcionalnih grupa, smeštenih na energetski heterogenim površinama sagledavanih oxMWCNT. Kako rezultati obuhvaćeni ovom doktorskom disertacijom pokazuju, oxMWCNT3h i oxMWCNT6h poseduju visok potencijal za obradu vodenih medijuma onečišćenih relativno niskim koncentracijama Cd(II), Cu(II), Ni(II), Pb(II) ili Cr(VI), a da li će, i eventualno u kojoj meri, isti biti ispunjen sada prvenstveno zavisi od nekih drugih aspekata njihove proizvodnje i primene.Reducing the content of heavy metals to legally prescribed levels, regardless of whether it is in waste water or water already present in the environment and intended for human use, is a task of priority importance, both from an ecological and public health safety point of view. Although for the purposes of harmonizing with the previously mentioned, and expected to be increasingly strict norms, numerous solutions have been proposed to date, among which, the leading ones according to most relevant criteria, are those based on the adsorption process, it is indisputable that there is still room for progress in this field, primarily through the development and assessment of the effecttiveness of sophisticated materials, with potentially superior adsorption characteristics, compared to conventional competitors. Relying on the statement just made, the aims of this doctoral dissertation were set, focused on the following: investigating the adsorption efficiency of two types of oxidized multi-walled carbon nanotubes (oxMWCNT), produced by carrying out the appropriate modification procedure during 3 (oxMWCNT3h) or 6 h (oxMWCNT6h), when processing aqueous media enriched with relatively low concentrations of selected heavy metals (Cd(II), Cu(II), Ni(II), Pb(II) or Cr(VI); ≤ 5 mg L-1), examining the existence and magnitude of the possible conditioning of the oxMWCNTs adsorption efficiency by the duration of the realized oxidation step and proposing the mechanisms responsible for the observed behavior of the interfacing adsorption pairs. In order to achieve the three defined tasks, the first step was to determine the morphological, textural and thermal properties, as well as the chemical characteristics of the oxMWCNT3h and oxMWCNT6h surfaces, using the following techniques: SEM-EDS, TEM, FT-IR, TGA-MS, Boehm titration, pH drift procedure and structural analysis of their pores and specific surface areas. The tested oxMWCNTs were then subjected to differently designed adsorption experiments, set up for the purpose of identifying patterns, but also the causes of the recorded responses of the ten considered systems to changes in the contact time of confronted phases, initial adsorbate concentration, temperature, pH value and ionic strength of liquid dispersions. The data derived from the implemented characterization methods of the analyzed oxMWCNTs indicate that these are two suitably purified, successfully functionalized and generally very similar nanomaterials, of which oxMWCNT6h has a certain advantage in terms of the total acidic oxygen groups content. The adsorption equilibrium in all in cluded M(II) cases develops extremely quickly, almost immediately after the end of the first 5 min, which is in complete contrast to Cr(VI) systems, characterized by the occurrence of the above-specified state only after 72, i.e. 96 h, depending on whether oxMWCNT3h or oxMWCNT6h was used as the adsorbent. The duration of these time intervals restricted the possibility of meaningful application of adsorption kinetics modeling exclusively to oxMWCNT/Cr(VI) variants, in the given sense most appropriately described by the Elovich equation. The increase in the initial concentration of selected heavy metals has a positive influence on their overall removal levels, and the corresponding adsorption isotherms show a high agreement with Freundlich, Langmuir, Dubinin-Radushkevich and Temkin type expressions. The adsorption capacities and affinities of the applied nanotubes towards the tested M(II) set form the following, identical trends: Pb(II) > Cu(II) > Cd(II) > Ni(II) and oxMWCNT6h > oxMWCNT3h, while in relation to Cr(VI) they are positioned in such a way that a more significant difference is observed only in regard to the last of the noted quantities, and that in favor of oxMWCNT6h. The increase in temperature, with only several exceptions, does not interfere with the patterns presented above, within which the adsorption capacities of the examined oxMWCNTs are mostly improved. A decrease in the liquid phase acid ity results in an intensification of the M(II) adsorption degree, and vice versa, immediately after exceeding pH ∼3.00, it causes a weakening of Cr(VI) removal. Increasing the ionic strength, in the case of all analyzed systems, adversely affects the effectiveness of the investigated oxMWCNTs. Judging on the basis of the statements presented here, as well as the findings derived from the parameter values of the previously mentioned kinetic and equilibrium mathematical models, the observed adsorption processes can be described as favorable, exothermic and dominantly driven by the occurrence of electrostatic, that is outer sphere complexation, taking place, along with reductive Cr(VI)/Cr(III) conversion, between compatible forms of selected heavy metals and oxygen-rich functional groups, positioned on the energetically heterogeneous surfaces of the tested oxMWCNTs. As the results included in his doctoral dissertation show, oxMWCNT3h and oxMWCNT6h have a high potential for treating water media polluted with relatively low concentrations of Cd(II), Cu(II), Ni(II), Pb(II) or Cr(VI), and whether, and eventually to what extent, it will be fulfilled now primarily depends on some other aspects of their production and application