Heavy Metal Uptake by Lignin:  Comparison of Biotic Ligand Models with an Ion-Exchange Process

Abstract

Metal uptake by kraft lignin, hereafter referred to as lignin, occurs by displacement of protons or bound metals with equilibrium constants K(ex)H and K(ex), respectively. Values calculated for wide ranges of initial concentrations are reasonably constant, thereby demonstrating the validity of these displacement processes and proving that uptake in these systems is not simple adsorption. It was found that the stoichiometry for Sr and Cd uptake by Ca-loaded lignin is 1 mol of metal for 1 mol of Ca released. This observation for metals of very different binding strengths is difficult to rationalize with the biotic ligand model as generally applied but is in complete agreement with an ion-exchange process. Binding strengths to lignin, which contains only oxygen ligands, follow the order Pb > Cu > Zn > Cd > Ca (strongest to weakest). For proton displacement, only more tightly bound metals such as Pb, Cu, Zn, and Cd can compete with protons for anion-binding sites at low pH, but at high pH, uptake of Ca, Sr, and Li can occur. An observed logarithmic decrease of K(ex)H with pH can be explained by having only weaker acids available for proton displacement under more basic conditions. The advantages and disadvantages of using adsorption and biotic ligand models for an ion-exchange process are discussed.

Adsorption of metal ions on lignin

“…At higher pH precipitation may also have occurred. Crist et al observed that metal ion sorption was accompanied by stoichiometric release of protons and existing metals, and concluded that ion exchange mechanisms were responsible for the removal of metals by lignin [21][22][23]. In order to see if a similar process was occurring in the present study, release of the cations Na + , Mg 2+ , and Ca 2+ was recorded while the metal ion was adsorbed, by taking Cu as an example.…”

“…Intensive studies have therefore been carried out to develop more effective and inexpensive metal adsorbents. Candidate materials include industrial or agricultural waste products such as waste slurry [7,8], fly ash [9,10], lignite [11,12] pine bark [13], peat [14,15], and lignin [16][17][18][19][20][21][22][23][24][25]. Babel and Kurniawan [26] and Bailey et al [27] have reviewed the application of low-cost adsorbents for heavy metal removal from contaminated waters.…”

“…Moreover, the related mechanisms of metal sorption by lignin are still subject to debate. Some studies have found that ion-exchange mechanisms may be responsible for the sorption of metal ions on lignin [17,[21][22][23]. Srivastava et al [16] and Mohan et al [20] suggested that no single mechanism could explain the pro-cess of metal removal by lignin.…”

Copper (II) Removed from Aqueous Solutions by Aminated Bagasse Pith

“…The amino group as the functional groups for adsorption accompanied by stoichiometric release of protons with the pH changing. [22,23]. While pH of the solution approached 8, the colloidal suspension occurred.…”

Adsorption of Heavy Metal Ion from Aqueous Single Metal Solution by Aminated Epoxy-Lignin

“…At pH 4, about 50 mg/L Cu(II) and 45mg/L Pb(II) was adsorbed. Crist et al observed that metal ion sorption was accompanied by stoichiometric release of protons and existing metals, and concluded that ion exchange mechanisms were responsible for the removal of metals by lignin (Crist et al, 2002;Guo et al 2008). When the pH was higher than 7, the precipitation of the metal ions occurred, resulting in a colloidal suspension.…”