Ion Exchange Properties of a Western Kentucky Low-Rank Coal

Abstract

The ion exchange properties of a low-rank coal were evaluated to explore its viability as an inexpensive material for removing radionuclides and heavy metals from solution. Cobalt was used as a model metal to investigate the kinetics and thermodynamics of the exchange process and the effect that solution pH has on the exchange capacity. Maximum metal adsorption was found to occur at pH ≥3.5. The exchange was found to follow first-order kinetics and to proceed rapidly; over the temperature range investigated (−15 to 50 °C) equilibrium was reached within 10−20 min. The exchange was well described by the Langmuir adsorption isotherm with a Gibbs free energy of −8.85 ± 0.24 kJ/mol. Additional studies with environmentally significant metals and radionuclides demonstrated that cations with higher charge density were preferentially adsorbed.

Immobilization of Heavy Metal Ions on Coals and Carbons

“…Its value can be determined experimentally (together with equlibrium adsorption capacity a e ) from the slope and intercept of plot t/a t versus t (Li et al, 2009;Shibi & Anirudhan, 2006). As confirmed by the authors that applied several kinetic models to analyse experimental data, the pseudo-second order kinetics usually gives the tightest courses with the adsorption data to be measured (Erenturk & Malkoc, 2007;Li et al, 2009 (Adamson & Gast, 1997), and they are close to 17.1 kJ/mol obtained by Kuhr et al (1997) for cobalt (II) adsorption on lignite. They are also quite comparable with activation energy 12.3 kJ/mol as was found by Li et al (2009) for lead(II) adsorption on modified spent grain; however, their interpretation that "positive value of E suggests …the adsorption process is an endothermic in nature" is hardly acceptable.…”

“…Because of increased content of oxygen, the oxidative altered bituminous coal should be of increased ability in cation exchange. Thus, their potential to remove heavy metals from aqueous solutions is expected to be comparable with that of subbituminous coal SB, the effectiveness of low rank coals for heavy metals adsorption having already been reported (Kuhr et al, 1997). Basic analyses and properties of the coal are summarised in Table 1.…”

“…In this respect, the Irving-Williams series is often referred to, showing that the adsorption selectivity of ions follows the stability order of metal -ligand complex formation (Murakami et al, 2001;Kuhr et al 1997). Guo et al (2010) confirmed the adsorption of metal ions on carbons to proceed exclusively through surface complexation regarding the importance of acidic functional groups in the complexation reactions.…”

“…To express the adsorption kinetics quantitatively, three kinetic models are mainly used: i. A simple first-order reaction kinetics (El-Shafey et al, 2002;Kuhr et al, 1997), which can be expressed generally as:…”

“…Guo et al, 2010;Qiu et al, 2008;Giraldo-Gutierrez & MorenoPirajan, 2008). Simultaneously, natural coals are investigated as economically accessible and efficient adsorbents to remove heavy metals (Kuhr et al, 1997;Zeledon-Toruno et al, 2005;Mohan & Chander, 2006). Radovic et al (2001) published a principal comprehensive review of the adsorption from aqueous solutions on carbons with incredible 777 references.…”