Root-zone modeling of heavy metal uptake and leaching in the presence of organic ligands

Abstract

We present a mechanistic model which describes root uptake and leaching of heavy metals in the plant root zone, accounting for solution-and surface-complexation, (kinetic) mineral dissolution, heavy metal diffusion towards the root, root uptake, root exudation, ligand degradation and convective-dispersive transport of the soluble species. The model was used to simulate the influence of EDTA addition on Cu transport and plant uptake and the effect of oxalate exudation by roots on Cu transport and bioavailability using parameter values from the literature. In the simulations we assumed that free Cu 2+ is the bioavailable form. Under slightly acidic conditions (pH 6) the model predicted that EDTA stabilizes Cu while at a slightly alkaline pH (pH 7.5), EDTA mobilizes Cu. The addition of EDTA approximately halved the cumulative Cu uptake after 360 days at pH 4.5, and reduced the uptake by a factor of 100 and 1000 at pH 6 and 7.5, respectively. Although the total dissolved concentration was increased, plant uptake was reduced by the formation of bio-inavailable complexes. The exudation of oxalate resulted in a decrease of the Cu concentration breaking through below the root zone, due to sorption of Cu-oxalate. In the presence of dissolved organic carbon (DOC), the exudation of oxalate increased Cu leaching considerably at pH 6 and 7.5. In the absence of DOC, the exudation of oxalate reduced Cu uptake due to the formation and adsorption of Cu-oxalate on goethite surface sites. Exudation of oxalate in the presence of DOC resulted in a further decrease of Cu uptake. Oxalate gradually takes over from DOC in binding Cu due to simultaneous production of oxalate and leaching of DOC. The simulations show that addition or exudation of ligands does not necessarily increase the solubility, transport and bioavailability of metals. Depending on the conditions (mainly the pH), also reduced transport and uptake can be observed, either by formation of ternary surface complexes or reduction of free metal concentration. The model can be easily extended to include further processes.Abbreviations: DOC -dissolved organic carbon; EDTA -ethylenediaminetetraacetate; NTA -nitrilotriacetic acid.

A Review on Heavy Metals (As, Pb, and Hg) Uptake by Plants through Phytoremediation

“…In chelate-assisted phytoremediation, synthetic chelating agents such as NTA and EDTA are added to enhance the phytoextraction of soil-polluting heavy metals. The presence of a ligand affects the biouptake of heavy metals through the formation of metal-ligand complexes and changes the potential to leach metals below the root zone [48].…”

Deposition, persistence and turnover of pollutants: First results from the EU project AquaTerra for selected river basins and aquifers

“…Within the first 2 years of the project, integrated work among the various partners has led to the collection of more than 1700 soil, sediment, water and biological samples. Apart from increasing availability of specialist results, project activities also included various meetings, the publishing of internal reports and national as well as peer-reviewed manuscripts (Barth and Grathwohl, 2005;Barth et al, 2006;Beinhorn et al, 2005;Bernhard et al, 2005;Bertagnoli et al, 2006;Bertoldi et al, 2006;Botter et al, 2005Botter et al, , 2006Brouyère, 2006;Chen et al, 2005;Eljarrat and Barcelo, 2006;Eljarrat et al, 2005aEljarrat et al, ,b, 2004Hermand and Holland, 2005;Hermand et al, 2006;Kalbacher et al, 2005;Kolditz and Bauer, 2004;Kolditz et al, 2005;Lacorte et al, 2006;Lair et al, 2006;Meyer and Hermand, 2005;Meyer et al, 2006;Ratola et al, 2006;Rigon et al, 2006;Rinaldo et al, 2006a,b;Roulier et al, 2006;Seuntjens et al, 2004;Van Beusekom et al, 2006;Zanotti et al, 2004). Further information about the project is available on the AquaTerra Website (http://www.eu-aquaterra.de/).…”

The HPx software for multicomponent reactive transport during variably-saturated flow: Recent developments and applications

“…Examples of PHREEQC implementations of these types of processes were given by Seuntjens et al (2004) and Nowack et al (2006). Compared to the HYDRUS implementations, also available in HPx, some additional features can be taken into account when the rate equations R kin m,k in Eq.…”