Proton andCopperAdsorption toMaizeandSoybeanRoot Cell Walls1

A surface complexation modelwhichhasbeenusedtodescribe inner-sphere complexation onmetaloxidesurfaces was applied totheadsorption ofCubyisolated cell walls of4-day and28-day-old maize(ZeamaysL.cvWF9xMo17)and21-dayoldsoybean (Glycine max[L.] Merr. cvDare) roots. Concentration dependence ofthetitration dataprevented thedetermination of unique pKandcapacitance values forthe4-day maizecell walls, though meanvalues obtained fortheintrinsic pKofthetitratable carboxyl groups were3.0(4-day maize), 3.6(28-day maize), and 3.0(21-day soybean) asdetermined bypotentiometric titration witheither NaOHorHCIin20millimolar NaCI. Theconstant capacitance modelwasapplied toCusorption datafromrapid batch equilibrium experiments inanionic mediumof20millimolar NaCIO4. Speciation calculations indicated that theformation ofa bidentate surface complex wassufficient todescribe theexperimental dataforallthree typesofplant material, withonlyone value forthepKandcapacitance density. Theintrinsic constants ofCucomplexation byaneutral site are:logK= -0.3±0.1, -0.2 ± 0.3, and0.9± 0.1for4-dayand28-day maize, and21-day soybean, respectively. Theintegral capacitance density parameter, whichdescribes therelationship between surface charge density andelectrical potential, isseveral timeslarger thanfor crystalline mineral surfaces. Thisresult indicates that thesurface electrical potential remains lowevenwhenthesurface charge density ishigh. Suchbehavior ischaracteristic ofgels andporous oxides.