Journal of Environmental Sciences 19(2007) 768–768 Comments on “Ion adsorption components in liquid/solid systems” LI Wei, PAN Gang * State Key Laboratory of Environmental Aquatic Chemistry, Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences, Beijing 100085, China. E-mail: leewee0720@yahoo.com.cn Recently, Wu et al. (J Environ Sci 18(2006) 1167– 1175) published a paper entitled as above. In the paper, the authors proposed a plotting method for describing adsorp- tion isotherms, where adsorption density (q e ) was plotted against the ratio of equilibrium concentration/particle con- centration (C e /W 0 ) rather than C e as traditionally defined. The authors claimed that this plot can eliminate the “parti- cle concentration effect” (i.e., adsorption isotherms decline with increasing particle concentration), which may oth- erwise be inevasible with traditionally defined adsorption isotherms. We think that their conclusion is conceptually flawed and the plot may cause substantial inconstancy problems in practice. Apparently, when adsorption density (q e ) is plotted against C e /W 0 , the “adsorption isotherm” obtained (Figs.1c and 2 c) for low particle concentration (low W 0 ) will be mathematically shifted to the left or lifted to a higher position than traditionally defined adsorption isotherm (a plot of q e vs. C e ). However, this treatment cannot shift all the isotherms under different W 0 (for a given adsorp- tion system) to the same position so that an inconstancy problem will be created. For example, it is well known that many adsorption systems do not have particle con- centration effect (O’Conor and Connolly, 1980; Pan and Liss, 1998a, b; Li et al., 2004), when this plot is applied to these systems the originally constant isotherms will be artificially separated into different isotherms (inconstancy problem), causing substantial confusion in data compari- son. The main reason why Wu et al. (2006) thought that adsorption isotherms should be correlated to C e /W 0 rather than C e is because the authors reached their deducted conclusion starting from the equation q e = (C 0 – C e )/W 0 (Section 2.2). This is only a mass balance equation, which does not necessarily describe the solid-liquid adsorption behavior. The adsorption experiments must satisfy the mass balance law, however, mass balance principle alone does not describe the thermodynamic relationships be- tween q e and C e for surface adsorption reactions. The latter is described by various isotherm equations (Giles et al., 1974). *Corresponding author. E-mail: gpan@rcees.ac.cn. References Giles C H, Smith D, Huitson, A, 1974. A General treatment and classification of the solute adsorption isotherm: I. Theoretical[J]. J Colloid Interface Sci, 47: 755–765. Li X, Pan G, Qin Y et al., 2004. EXAFS studies on adsorption- desorption reversibility at manganese oxide-water inter- faces: II. Reversible adsorption of zinc on δ-MnO 2 [J]. J Colloid Interface Sci, 271: 35–40. O’Conner D J, Connolly J P, 1980. The effect of concentration of adsorbing solids on partition coefficient[J]. Wat Res, 14: 1517–1523. Pan G, Liss P S, 1998a. Metastable-equilibrium adsorption theory: I. Theoretical[J]. J Colloid Interface Sci, 201: 71– 76. Pan G, Liss P S, 1998b. Metastable-equilibrium adsorption theory: II. Experimental[J]. J Colloid Interface Sci, 201: 77– 85. WuX, Hu Y, Zhao F et al., 2006. Ion adsorption components in liquid/solid systems[J]. J Environ Sci, 18(6): 1167–1175.