Transport, fate and ecosystem impact of toxicants is intimately connected with how they partition or associate with solid matter in and below the water body. Unlike conventional pollutants (biodegradable organic matters, nutrients), toxic substance analysis must distinguish dissolved and particulate forms as certain mechanisms differentially impact the two forms. For example, volatilization (loss of contaminant from the water to the atmosphere) acts only on the dissolved fraction and settling acts solely on the particulate-associated fraction.
Following equations describe solid-liquid partitioning of total concentration of pollutants:
C = Cd + Cp; Cd = Fd C; Cp = Fp C Fd = 1/(1+Kd.m); Fp = (Kd.m )/(1+Kd.m)
where C = total contaminant concentration (µg m-3), Cd and Cp = dissolved and particulate components (µg m-3), Fd and Fp = dissolved and particulate fractions of the
Precipitated
Free Complexed
Adsorbed
total contaminant (Fd + Fp = 1), Kd = a partition coefficient (m3 g-1) and m = suspended solids concentration (g m-3).
For weakly sorbing contaminants (low Kd) in systems with low suspended solids (low m), the contaminant will be predominantly in the dissolved form. For strong sorbers in turbid systems, the contaminant will be strongly associated with suspended solids (Chapra, 1997).
Clay minerals [e.g., Kaolinite, Al2Si2O5(OH)] in the sediments possess large surface areas and can adsorb cations on their surfaces. Often the metal ions displace H+ ions from the OH groups on the surface of the clay particles (Brown et al., 1991):
M+ (aq) + H-O-Clay H+ + M-O-Clay
This situation gives rise to pH-dependent equilibria. Higher the concentration of H+(aq), the more the equilibrium is shifted to the left. Water of the Brahmaputra River is often basic and more metals should get transported through sediments by adsorption. Mandal (2005) found the average pH of the Brahmaputra River near the sampling sites of present study to exceed 8.0 most of the times in a study extending one year. According to Viers et al. (2009), most of the elements (except Ca, Mg, K and Na) in a river are mainly transported by the solid phase.
The surface charge of a particle is a function of a number of different processes as represented by the following equation (Eby, 2004):
NC = FC + NP + ISC + OSC
where NC is the net charge, FC is the fixed charge due to substitutions in the crystal structure, NP is the net proton charge due to binding or release of hydrogen ions from the surface, ISC is the charge due to the presence of inner-sphere complexes (complex formed by direct attachment of cations to oxygen ions exposed at the particle surface),
and OSC is the charge due to the presence of outer-sphere complexes (complex formed by attachment of water molecules of solvated cations with exposed surface functional groups). If the net charge is not zero, it is balanced by the ions in the diffuse double layer adjacent to the surface. As NP, ISC and OSC vary as a function of pH, surface will have no charge (point of zero charge) at some pH. A point of zero net proton charge (PZNPC) may be defined, which occurs when the charge due to binding and release of protons (NP) is zero. At pH values below PZNPC, particles will have positively charged surface;
at pH values above PZNPC, particles will have negatively charged surface. Thus, adsorptive characteristics of particles depend on the relationship between pH and surface charge.
Table 1.6: Point of Zero Net Proton Charge (Kehew, 2001)
Material pHpznpc Material pHpznpc
α-Al(OH)3 5.0 δ-MnO2 2.8
γ-AlOOH 8.2 SiO2 2.0
Fe3O4 6.5 Feldspars 2.0-2.4 α-FeOOH 7.8 Kaolinite 4.6
α-Fe2O3 8.5 Montmorillonite 2.5 Fe(OH)3 (am) 8.5 Albite 2.0
Metals are readily adsorbed by the surface over a relatively narrow pH range after the surface becomes negatively charged (Figure 1.6). The degree of adsorption increases with increasing pH. Ions with higher selectivity are adsorbed at lower pH. Selectivity series (order in which the ions are preferentially adsorbed, from most strongly adsorbed to least strongly adsorbed) for a subset of transition metals are as follows: Cu2+ > Ni2+ >
Co2+ > Fe2+ > Mn2+.
Surface of particle in aqueous solution acquires charge due to ionization of chemical groups on it. Biological surfaces usually have proteins as part of the surface
Figure 1.6: Adsorption of metal cations as a function of pH (Source: Cited by Eby, 2004 from Stumm and Morgan, 1996)
acidic and basic groups, respectively, which ionize in the manner shown in (Figure 1.7 A). At low pH, carboxylic groups are not dissociated and hence uncharged, whereas the amine groups are protonated and have a positive charge. At high pH the carboxylic groups dissociate to give a negative charge and the amine groups lose their proton and are uncharged. So such surfaces are positively charged at low pH and negatively charged at high pH. There is a characteristic pH value called point of zero charge (PZC) at which the number of negatively charged surface groups just balances the number of positive groups.
(A) (B)
Figure 1.7: Ionization of (A) surface carboxylic and amine groups, (B) metal hydroxide (MOH) groups at an oxide surface (Gregory, 2006)
For many biological surfaces (e.g., of bacteria and algae), the PZC is in the region of pH 4-5, so that in most natural waters such particles are negatively charged. Many inorganic particles (e.g., sediments) in the aquatic environment are negatively charged by virtue of an adsorbed layer of natural organic matter. Metal oxide (Al2O3, Fe2O3, TiO2, etc.) surfaces form amphoteric hydroxides (e.g., AlOH), which can ionize to give either positive or negative charge (Figure 1.7 B). Again, the surface is positively charged at low pH and negatively charged at high pH. For oxides, PZC values (SiO2 – 2; TiO2 – 6; Fe2O3
– 8; Al2O3 – 9; MgO – 12) depend on the acid-base properties of the metal and vary over a wide range (Gregory, 2006).