Role of the Ca-pectates on the accumulation of heavy metals in the root apoplasm

In order to better understand the processes that regulate the accumulation in the apoplasm of heavy metals and their mobilization by the plant metabolites it is essential to study the mechanisms that regulate the interactions between metal ions and pectins. In such a context, the sorption of Cd(II), Zn(II), Cu(II) and Pb(II) from single and multi-metal solutions, by a Ca-polygalacturonate gel with a degree of esterification of 18.0 (PGAM1) and 65.5% (PGAM2) was studied in the 3.0–6.0 pH range in the presence of CaCl2 2.5 mM. The sorption of Cr(III) from single metal solution was also considered. The results show that the amount of each metal ion sorbed increases with increasing the initial metal ion concentration and pH. The data from the single metal solution tests show that at pH 6.0 the affinity of the metal ions towards the PGAM1 matrix follows the order: Cr(III) > Cu(II) ≅ Pb(II) ≫ Zn(II) ≅ Cd(II). The simultaneous sorption of the bivalent metal ions by the PGAM1 gels indicates that Pb(II) is selectively sorbed. The FT-IR spectra show that the carboxylate groups are mainly responsible for the metal ion coordination. The ability of PGAM2 to accumulate Cr(III), Cu(II), and Pb(II) was lower than that found in the PGAM1 systems whereas the sorption of Zn(II) and Cd(II) was negligible.

Research highlights
► Low esterified Ca-pectate has a high affinity towards Pb(II) and Cu(II).
► Pb(II) is selectively sorbed by Ca-polygalacturonate.
► Cd(II) and Zn(II) show a low affinity towards Ca-pectate.