The kinetics and mechanism of removal of selected metals from water by adsorption on amino functionalized carbon nanomaterials

Abstract

<p>Predmet istraživanja ove doktorske disertacije bio je ispitivanje adsorpcionog pona&scaron;anja As(III) i odabranih metala (Cu(II), Cd(II), Pb(II), Cr(VI)) na amino- funkcionalizovanim vi&scaron;eslojnim ugljeničnim nanocevima, MWCNT-NH₂, na četiri odabrane pH vrednosti (3; 4,5; 6 i 11) i pri različitim sastavima vodenog rastvora (prisustvo hlorida, nitrata, sulfata i fosfata), a u cilju ispitivanja mogućnosti primene MWCNT-NH₂ za uklanjanje arsena i jona metala iz vodenih rastvora u oblasti relativno niskih koncentracija (0,01-3 mg L ^-1 ). Fokus ove doktorske disertacije<br />je bio na utvrđivanju mehanizma procesa adsorpcije u ispitivanim uslovima.&nbsp;&nbsp; Adsorpciju arsena i metala na MWCNT-NH₂ najbolje je opisao reakcioni model pseudo-drugog reda &scaron;to&nbsp; znači da adsorpcija može biti pripisana uspostavljanju hemijskih interakcija između adsorbata i vezivnih grupa na povr&scaron;ini adsorbenta. Modelovanje brzine Weber Morris-ovim difuzionim modelom ukazalo je&nbsp; na to da unutarčestična difuzija, iako sporiji korak&nbsp; od eksterne difuzije, nije jedini ograničavajući korak u procesu adsorpcije, već su za to odgovorne i interakcije ispitivanih jona sa vezivnim mestima na povr&scaron;ini adsorbenta.&nbsp; U ravnotežnim uslovima, na različitim pH vrednostima, adsorpcija se mogla opisati&nbsp; Langmuir-ovim i Freundlich-ovim adsorpcionim modelima. Kapacitet i afinitet adsorbenta, kao i sam mehanizam adsorpcije, značajno su bili uslovljeni pH vredno&scaron;ću rastvora, kako zbog karakteristika povr&scaron;ine ispitivanog adsorbenta uslovljenih pH vredno&scaron;ću, tako i zbog uticaja pH vrednosti rastvora na pojavne hemijske oblike u kojima se arsen i metali nalaze. Ispitivanje uticaja anjona pokazalo je da je prisustvo hlorida povećalo afinitet povr&scaron;ine MWCNT-NH₂ka jonima arsena i svih metala u oblasti niskih<br />koncentracija, dok je prisustvo hlorida, nitrata, sulfata i fosfata pozitivno uticalo na afinitet povr&scaron;ine (od 2 do 8 puta veći u odnosu na dejonizovanu vodu) MWCNT-NH₂<br />ka Cu(II) u oblasti vi&scaron;ih koncentracija. Najmanji uticaj anjona na afinitet povr&scaron;ine ka vezivanju metala primećen je kod hroma. Pretpostavljeni&nbsp; mehanizmi uklanjanja arsena i odabranih metala u prisustvu anjona ukljuĉuju reakcije građenja različitih vrsta spolja&scaron;nje-&nbsp; i unutra&scaron;nje-sfernih kompleksa, jonske izmene, kao i&nbsp; elektrostatičke interakcije koje se ostvaruju između pozitivno naelektrisane povr&scaron;ine MWCNT-NH_{<span style="font-size: 8.33333px;">2</span>}&nbsp;i anjona, na pH vrednostima manjim od 6,4, a koje menjaju afinitet<br />povr&scaron;ine ka jonima arsena i metala. Rezultati ove doktorske disertacije pokazuju da se ispitivani adsorbent, MWCNT-NH₂,&nbsp; može koristiti za uklanjanje arsena i te&scaron;kih metala iz vode u prisustvu različitih anjona i da, svojim adsorpcionim karakteristikama, može konkurisati komercijalnim i&nbsp; ekonomski pristupačnim adsorbentima.</p>

<p>The subject of this doctoral thesis was the examination of the adsorption behaviour of As(III), and selected metals (Cu(II), Cd(II), Pb(II), Cr(VI)) on the amino-functionalized multiwalled carbon nanotubes, MWCNT-NH₂, at four pH values (3, 4.5, 6, and 11) with different compositions of aqueous solutions (in the presence of chlorides, nitrates, sulphates and phosphates), in order to examine the possibilities of applying MWCNT-NH₂ in removal of arsenic and metal ions from aqueous solutions in relatively low concentration (0,01- 3mg L^-1). The focus of this doctoral thesis was to establish the mechanism of adsorption process in given conditions. Arsenic and metal adsorption on MWCNT-NH₂ were best described with the pseudo-second order kinetic model, which assumes that adsorption can be attributed to the establishment of chemical reactions between the adsorbate, and the binding groups at the surface of adsorbent. The Weber - Morris diffusion model indicated that intercellular diffusion, although being a slower step than external diffusion, is not the only limiting step in the adsorption process, but also that the interactions of ions with the adsorbent surface binding sites, are responsible. In equilibrium, at different pH values, the adsorption could be described by the Langmuir and Freundlich adsorption models. The capacity and affinity of the adsorbent, and adsorption mechanism, were considerably determined by the pH value of the solution, either because of the characteristics of the surface of the investigated adsorbent depending on pH value, or the effect of the solution&rsquo;s pH value on the chemical forms of arsenic and other metals. Investigation of the influence of the anion showed that the presence of chloride increased affinity of the surface of MWCNT-NH₂ to arsenic ions and to all metals in low concentration ranges, while presence of chlorides, nitrates, sulphates and phosphates had a positive impact on the affinity of the surface (2 or 8 times greater than for deionized water) MWCNT-NH₂ for Cu(II) in high concentration ranges. The smallest impact of anions to affinity of the surface to bond metals was noticed for chromium. The assumed mechanisms of removing arsenic and selected metals in the presence of anions include reactions of forming various kinds of outer- and inner-spheric complexes, ionic exchange, as well as electrostatic interaction which occur between positively charged surface of MWCNT-NH₂ and anions, for pH &lt; 6.4, and which change affinity of the surface towards arsenic and metal ions. Results of this doctoral thesis show that the investigated adsorbent, MWCNT-NH₂, can be used for the removal of arsenic and heavy metals from water, in the presence of different anions, and that with its adsorbent characteristic it can compete with commercially, and economically more affordable adsorbents.</p>