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Significance of Anion Exchange in Pentachlorophenol Sorption by Variable-Charge Soils

^a Dep. of Agronomy, Purdue Univ., West Lafayette, IN 47907-2054
^b School of Civil Engineering, Purdue Univ., West Lafayette, IN 47907-2051

View larger version (29K): [in a new window]   Fig. 1. Pentachlorophenol (PCP) sorption isotherms measured in soils suspended in 0.005 M CaCl2 or Ca(H2PO4)2 near the natural soil pH. The term fa is the fraction of PCP existing as an anion at the noted pH.

View larger version (16K): [in a new window]   Fig. 2. Correlation between the decrease in the pentachlorophenol (PCP) Kd value measured in Ca(H2PO4)2 relative to that measured in CaCl2 versus the absolute value of the ratio of anion exchange capacity [AEC, cmol(+)/kg] to the absolute value of the net surface charge. The line is a linear regression fit with R2 = 0.86 for the correlation coefficient.

View larger version (24K): [in a new window]   Fig. 3. Anion exchange capacity (AEC) and cation exchange capacity (CEC) as a function of pH on selected soils. The value in the x axis represents the absolute values of AEC and CEC.

View larger version (36K): [in a new window]   Fig. 4. Linear sorption coefficient (Kd) as a function of pH for pentachlorophenol (PCP) sorption from 0.005 M CaCl2, Ca(H2PO4)2, and CaSO4. Linear distribution coefficients (Kd, L/kg) were estimated from either a zero-intercept linear regression or from a single-point concentration where Cw was between 1 and 5 µmol/L. Fitted curves are arbitrary lines to visualize the difference of Kd as a function from 0.005 M CaCl2, Ca(H2PO4)2, and CaSO4 matrices. The terms Cl, P, and S noted in the legend refer to 0.005 M CaCl2, Ca(H2PO4)2, and CaSO4 matrices, respectively.

View larger version (21K): [in a new window]   Fig. 5. Pentachlorophenol (PCP) sorption–desorption isotherms from 0.005 M CaCl2 and 0.005 M Ca(H2PO4)2 matrices noted as Cl and P, respectively. Solid and dotted lines are linear regression fits to the sorption isotherms measured in Cl and P, respectively, reported in Fig. 1. Solid circles are the data from three sequential desorption steps with Cl and the open circles are data from the final desorption step with P.

View larger version (19K): [in a new window]   Fig. 6. Dependence of pH on Kd for pentachlorophenol (PCP) sorption by A1 soil from KCl-saturated, CaCl2–saturated, and nonhomoionic 0.005 M CaCl2 systems.