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Competitive Adsorption of Heavy Metals in Humic Substances by a Simple Ligand Model

^a Dep. of Renewable Resources, 317 Hamilton Hall, Univ. of Louisiana, Lafayette, LA 70504
^b School of Biosciences, Biology Building, Univ. of Nottingham, University Park, Nottingham NG7 2RD, UK

View larger version (14K): [in this window] [in a new window]   Fig. 1. Schematic diagram for the dialysis membrane tube.

View larger version (25K): [in this window] [in a new window]   Fig. 2. Plots of humic charge against pH as a function of ionic strength (NaNO3). The points are experimental values, and the lines are Model A fits. Model A was fitted to all six titrations as a single data set.

View larger version (16K): [in this window] [in a new window]   Fig. 3. Model A applied to metal binding data; the bound species considered included: monodentate Cd–carboxyl complex (CdC), bidentate Cd–carboxyl complex (CdCC), and bidentate Cd–(carboxyl + phenolic hydroxyl) complex (CdC). The background electrolyte was 0.1 M NaNO3. Data are presented as the natural log of the ratio of bound to free Cd concentrations ([Cd]bound/[Cd]free), modeled vs. measured. The concentration of Cdbound includes all Cd–humic acid complex forms and Cd held as a counter ion; Cdfree includes all free inorganic species. Different symbols show individual data subsets. The solid line indicates a 1:1 relationship. The values of the optimized Model A parameters were: –log intrinsic stability constant for metal complexation to a monodentate carboxyl binding site = 0.290, –log intrinsic stability constant for metal complexation to a monodentate phenolic hydroxyl binding site = 4.76, proportion of carboxyl groups capable of forming bidentate chelates = 0.051, proportion of monodentate binding carboxyl groups capable of forming bidentate chelates to a phenolic hydroxyl group = 0.950, and proportionality factor FW = 0.169; residual standard deviation = 0.519, n = 102.

View larger version (13K): [in this window] [in a new window]   Fig. 4. Model A applied to metal binding data; the bound species considered included: monodentate Zn–carboxyl complex (ZnC), bidentate Zn–carboxyl complex (ZnCC), and bidentate Zn–(carboxyl + phenolic hydroxyl) complex (ZnC). The background electrolyte was 0.1 M NaNO3. Data are presented as the natural log of the ration of bound to free Zn concentrations ([Zn]bound/[Zn]free), modeled vs. measured. The concentration of Znbound includes all Zn–humic acid complex forms and Zn held as a counter ion; Znfree includes all free inorganic species. Different symbols show individual data subsets. The solid line indicates a 1:1 relationship. The values of the optimized Model A parameters were: –log intrinsic stability constant for metal complexation to a monodentate carboxyl binding site = 0.968, –log intrinsic stability constant for metal complexation to a monodentate phenolic hydroxyl binding site = 4.52, proportion of carboxyl groups capable of forming bidentate chelates = 0.171, proportion of monodentate binding carboxyl groups capable of forming bidentate chelates to a phenolic hydroxyl group = 0.883, and proportionality factor FW = 0.259; residual standard deviation = 0.326, n = 42.

View larger version (13K): [in this window] [in a new window]   Fig. 5. Model A applied to metal binding data; the bound species considered included: monodentate Cu–carboxyl complex (CuC), bidentate Cu–carboxyl complex (CuCC), and bidentate Cu–(carboxyl + phenolic hydroxyl) complex (CuC). The background electrolyte was 0.1 M NaNO3. Data are presented as the natural log of the ratio of bound to free Cu concentrations ([Cu]bound/[Cu]free), modeled vs. measured. The concentration of Cubound includes all Cu–humic acid complex forms and Cu held as a counter ion; Cufree includes all free inorganic species. Different symbols show individual data subsets. The solid line indicates a 1:1 relationship. The values of the optimized Model A parameters were: –log intrinsic stability constant for metal complexation to a monodentate carboxyl binding site = 0.042, –log intrinsic stability constant for metal complexation to a monodentate phenolic hydroxyl binding site = 4.04, proportion of carboxyl groups capable of forming bidentate chelates = 0.126, proportion of monodentate binding carboxyl groups capable of forming bidentate chelates to a phenolic hydroxyl group = 0.585, and proportionality factor FW = 0.397; residual standard deviation = 0.219, n = 26.

View larger version (13K): [in this window] [in a new window]   Fig. 6. Model applied to Cd and Zn competition data; Cd and Zn were present in solution. The background electrolyte was 0.1 M NaNO3. Data are presented as the natural log of the ratio of bound to free metal ions ([M]bound/[M]free), modeled vs. measured. The bound species considered in Model A included: monodentate Cd–carboxyl complex (CdC), bidentate Cd–carboxyl complex (CdCC), bidentate Cd–(carboxyl + phenolic hydroxyl) complex (CdC), monodentate Zn–carboxyl complex (ZnC), bidentate Zn–carboxyl complex (ZnCC), and bidentate Zn–(carboxyl + phenolic hydroxyl) complex (ZnC). The values of constants and parameters used in Model A were: for Cd, –log intrinsic stability constant for metal complexationto a monodentate carboxyl binding site (pßintM,C)=0.290, –log intrinsic stability constant for metal complexation to a monodentate phenolic hydroxyl binding site (pßintM,)=4.76, proportion of carboxyl groups capable of forming bidentate chelates (PCC) = 0.051, proportion of monodentate binding carboxyl groups capable of forming bidentate chelates to a phenolic hydroxyl group (PC) = 0.951, and proportionality factor FW = 0.169; for Zn,pßintM,C=0.968, pßintM,=4.52, PCC = 0.171, PC = 0.883, and FW = 0.259.

View larger version (12K): [in this window] [in a new window]   Fig. 7. Model applied to Cd, Zn, and Cu competition; total concentration of each metal ion ([M]) was 2.50 x 10–4 M and the mole ratio of Cd/Zn/Cu was 1:1:1. The background electrolyte was 0.1 M NaNO3. The bound species considered included: monodentate Cd–carboxyl complex (CdC), bidentate Cd–carboxyl complex (CdCC), bidentate Cd–(carboxyl + phenolic hydroxyl) complex (CdC), monodentate Zn–carboxyl complex (ZnC), bidentate Zn–carboxyl complex (ZnCC), bidentate Zn–(carboxyl + phenolic hydroxyl) complex (ZnC), monodentate Cu–carboxyl complex (CuC), bidentate Cu–carboxyl complex (CuCC), and bidentate Cu–(carboxyl + phenolic hydroxyl) complex (CuC). The values of constants and parameters used were: for Cd, intrinsic stability constant for metal complexation to a monodentate carboxyl binding site (pßintM,C) = 0.290, intrinsic stability constant for metal complexation to a monodentate phenolic hydroxyl binding site (pßintM,)=4.76, proportion of carboxyl groups capable of forming bidentate chelates (PCC) = 0.051, proportion of monodentate binding carboxyl groups capable of forming bidentate chelates to a phenolic hydroxyl group (PC) = 0.951, and proportionality factor FW = 0.169; for Zn,pßintM,C=0.969,pßintM,=4.52, PCC = 0.172, PC = 0.887, and FW = 0.260; for Cu,pßintM,C=0.042, pßintM,=4.04, PCC = 0.126, PC = 0.585, and FW = 0.397.

View larger version (13K): [in this window] [in a new window]   Fig. 8. Model applied to Cd, Zn, and Cu competition; total metal concentrations ([M] = 2.00 x 10–4 M for Cd and Zn, 1.00 x 10–4 M for Cu) were in the ratio Cd/Zn/Cu = 2:2:1. The background electrolyte was 0.1 M NaNO3. The bound species considered included: monodentate Cd–carboxyl complex (CdC), bidentate Cd–carboxyl complex (CdCC), bidentate Cd–(carboxyl + phenolic hydroxyl) complex (CdC), monodentate Zn–carboxyl complex (ZnC), bidentate Zn–carboxyl complex (ZnCC), bidentate Zn–(carboxyl + phenolic hydroxyl) complex (ZnC), monodentate Cu–carboxyl complex (CuC), bidentate Cu–carboxyl complex (CuCC), and bidentate Cu–(carboxyl + phenolic hydroxyl) complex (CuC). The values of constants and parameters used were: for Cd, intrinsic stability constant for metal complexation to a monodentate carboxyl binding site (pßintM,C)=0.290, intrinsic stability constant for metal complexation to a monodentate phenolic hydroxyl binding site (pßintM,)=4.76, proportion of carboxyl groups capable of forming bidentate chelates (PCC) = 0.051, proportion of monodentate binding carboxyl groups capable of forming bidentate chelates to a phenolic hydroxyl group (PC) = 0.951, and proportionality factor FW = 0.169; for Zn,pßintM,C=0.969, pßintM,=4.52, PCC = 0.172, PC = 0.887, and FW = 0.260; for Cu,pßintM,C=0.042, pßintM,=4.04, PCC = 0.126, PC = 0.585, and FW = 0.397.

View larger version (13K): [in this window] [in a new window]   Fig. 9. Testing the sensitivity of Model A [through the distribution coefficient log10(Kd)] to the value of proportionality factor FW at various values of surface charge (Z). The free Cd ion concentration was 1.00 x 10–8 M; pH = 5.50; ionic strength was 0.01 M (NaNO3); total carboxyl group concentration (TC) was 3.13 molc kg–1. The values of constants and parameters used for Cd were: –log intrinsic stability constant for metal complexation to a monodentate carboxyl binding site = 0.290, –log intrinsic stability constant for metal complexation to a monodentate phenolic hydroxyl binding site = 4.76, proportion of carboxyl groups capable of forming bidentate chelates = 0.051, and proportion of monodentate binding carboxyl groups capable of forming bidentate chelates to a phenolic hydroxyl group = 0.951.