Methodological Approach to the Study of the Formation and Physicochemical Properties of Phosphate−Metal−Humic Complexes in Solution

Journal of Hazardous Materials

Cai²

2013

Arsenic (As) can form complexes with dissolved organic matter (DOM), which affects the fate of arsenic in waste sites and natural environments. It remains a challenge to analyze DOM-bound As, in particular by using a direct chromatographic separation method. Size exclusion chromatography (SEC) hyphenated with UV spectrophotometer and inductively coupled plasma mass spectrometry (ICP-MS) was developed to characterize the complexation of arsenite (AsIII) with DOM. This SEC-UV-ICP-MS method is able to differentiate AsIII-DOM complexes from free As species and has the advantage of direct determination of both free and DOM-bound AsIII through mild separation. The suitability of this method for studying AsIII-DOM complexation was demonstrated by its application, in combination with the Scatchard plot and nonlinear regression of ligand binding model, for characterizing AsIII complexation with humic acid (HA) in the absence or presence of natural sand. The results suggest that, consistent with polyelectrolytic nature of HA, the AsIII-HA complexation should be accounted for by multiple classes of binding sites. By loosely classifying the binding sites into strong (S1) and weak (S2) sites, the apparent stability constants (Ks) of the resulting As-DOM complexes were calculated as log Ks1 = 6.5–7.1 while log Ks2 = 4.7–5.0.

Section: Methodsmentioning

confidence: 99%

Studying arsenite–humic acid complexation using size exclusion chromatography–inductively coupled plasma mass spectrometry

Journal of Hazardous Materials

Cai²

2013

“…A Fe-DOM stock solution was prepared by adding Fe III into HA solution at pH 8–9, according to a previously reported procedure (27). In this solution, Fe was predominantly (>95%) present in DOM-complexed/colloidal form, whereas free Fe is nominal (see Results and Discussion below).…”

Section: Methodsmentioning

confidence: 99%

Complexation of Arsenite with Humic Acid in the Presence of Ferric Iron

Maza

Cai

2011

Environ. Sci. Technol.

154

In the presence of iron (Fe), dissolved organic matter (DOM) may bind considerable amounts of arsenic (As), through formation of Fe-bridged As-Fe-DOM complexes and surface complexation of As on DOM-stabilized Fe-colloids (collectively referred to as As-Fe-DOM complexation). However, direct (e.g., chromatographic and spectroscopic) evidence and fundamental kinetic and stability constants have been rarely reported for this As-Fe-DOM complexation. Using a size exclusion chromatography (SEC)-UV-inductively coupled plasma mass spectrometry (ICP-MS) technique, arsenite (AsIII)-Fe-DOM complexation was investigated after adding AsIII into the priorly prepared Fe-DOM. A series of evidence, including coelution of As, Fe, and DOM from the SEC column and coretention of As, Fe, and DOM by 3 kDa MWCO centrifugal filtration membrane, demonstrated the occurrence of AsIII-Fe-DOM complexation. The kinetic data of AsIII-Fe-DOM complexation were well described by a pseudo-first order rate equation (R2 = 0.95), with the rate constant (k′) being 0.17±0.04 1/h. Stability of AsIII-Fe-DOM complexation was characterized by apparent stability constant (Ks) derived from two-site ligand binding model, with log Ks ranging from 4.4±0.2 to 5.6±0.4. Considering the kinetics (within hours) and stability (similar to typical metal-humates) of AsIII-Fe-DOM complexation, this complexation needs to be included when evaluating As mobility in Fe and DOM rich environments.

“…The Scatchard plot (also called Rosenthal plot) and ligand binding model were used here to determine K s . The Scatchard plot, widely used in protein chemistry, has been applied to the determination of metal humate stability constants (Stevenson, 1994; Yates and Von Wandruszka, 1999; Guardado et al, 2005). Based on the Scatchard equation,

C_{b}^{italicAs} / C_{f}^{italicAs}

was plotted against

C_{b}^{italicAs}

to check the homogeneity of binding sites present in DOM

\frac{C_{b}^{italicAs}}{C_{f}^{italicAs}} = - K_{s} \times C_{b}^{italicAs} + K_{s} \times B_{max}

where B max is the maximum binding capability of DOM toward As III in mol L −1 .…”

Section: Methodsmentioning

confidence: 99%

Complexation of arsenite with dissolved organic matter: Conditional distribution coefficients and apparent stability constants

Cai

2010

Chemosphere

The complexation of arsenic (As) with dissolved organic matter (DOM), although playing an important role in regulating As mobility and transformation, is poorly characterized, as evidenced by scarce reporting of fundamental parameters of As-DOM complexes. The complexation of arsenite (As III ) with Aldrich humic acid (HA) at different pHs was characterized using a recently developed analytical technique to measure both free and DOM-bound As. Conditional distribution coefficient (K D ), describing capacity of DOM in binding As III from the mass perspective, and apparent stability constant (K s ), describing stability of resulting As III -DOM complexes, were calculated to characterize As III -DOM complexation. Log K D of As III ranged from 3.7 to 2.2 (decreasing with increase of As/DOM ratio) at pH 5.2, from 3.6 to 2.6 at pH 7, and from 4.3 to 3.2 at pH = 9.3, respectively. Two-site ligand binding models can capture the heterogeneity of binding sites and be used to calculate K s by classifying the binding sites into strong (S1) and weak (S2) groups. Log K s for S1 sites are 7.0, 6.5, and 5.9 for pH 5.2, 7, and 9.3, respectively, which are approximately 1-2 orders of magnitude higher than for weak S2 sites. The results suggest that As III complexation with DOM increases with pH, as evidenced by significant spikes in concentrations of DOM-bound As III and in K D values at pH 9.3. In contrary to K D , log K s decreased with pH, in particular for S1 sites, probably due to the presence of negatively charged H 2 AsO 3 − and the involvement of metal-bridged As III -DOM complexation at pH 9.3.