Influence of humic acid on adsorption of Hg(II) by vermiculite.

Geochemical mobility of Hg(II) species is strongly affected by the interactions of these compounds with naturally occurring adsorbents such as humic acids, clay minerals, oxides, etc. Interactions among these sorbents affect their affinity for Hg(II) and a full understanding of these processes is still lacking. The present work describes the influence of a humic acid (HA) sample on the adsorption of Hg(II) by vermiculite (VT). Adsorption isotherms were constructed to evaluate the affinity of Hg(II) by VT, HA, VT modified with humic acid (VT-HA), and VT-HA in presence of soluble humic acid (VT-HA + HA). All experiments were made at pH 6.0 ± 0.1 in 0.02 M NaNO3 and 25.0 ± 0.5 °C for initial Hg(II) concentrations from 1.0 to 100 µM. Determinations of Hg(II) were made by square wave voltammetry automated by sequential injection analysis, an approach that enables the determination of the free plus labile fractions of Hg(II) in HA suspensions without the need for laborious separation steps. The adsorption isotherms were fitted to Langmuir and Freundlich equations, showing that HA was the material with the higher adsorption capacity (537 ± 30 µmol g(-1)) in comparison with VT and VT-HA (44 ± 3 and 51 ± 11 µmol g(-1), respectively). Adsorption order was HA >> VT-HA + HA > VT = VT-HA. At pH 6.0 the interaction of HA with VT is weak and only 14% of C initially added to the suspension was effectively retained by the mineral. Desorption of Hg(II) in acidic medium (0.05 M HCl) was higher in binary (VT-HA) and ternary (VT-HA + HA) systems in comparison with that of VT and HA alone, suggesting that interactions between VT and HA are facilitated in acidic medium, weakening the binding to Hg(II).

Complexation of Hg(II) by humic acid studied by square wave stripping voltammetry at screen-printed gold electrodes.

This paper reports the development of a sequential injection (SI) method to study the complexation of Hg(II) by humic acid (HA) using square wave anodic stripping voltammetry at a screen-printed gold electrode (SPGE). The SI system injected samples (in 0.020 mol L(-1) NaNO(3) and pH 6.0) to the flow cell during the deposition step and exchanged the medium to 0.050 mol L(-1) HCl to perform the stripping step under stopped flow conditions. For sample volume=750 µL, flow rate=15 µL s(-1) (deposition step), and square wave frequency=100 Hz, the detection limit was 20 nmol L(-1) and the sampling throughput was 17 analyses per hour. Complexation of Hg(II) was studied using a 25.0 mg L(-1) suspension of HA (72.5 µmol L(-1) of ionizable sites), and total Hg(II) concentrations from 1 to 20 µmol L(-1). The diffusion coefficient of the complex was (1.3±0.2)×10(-7) cm(2) s(-1). A complexing capacity of 537 µmol g(-1) was found. The log of the differential stability constant (logK(DEF)) decreased from 7.0 to 5.3 as the log of the degree of site occupation (logθ) increased from -1.6 to -0.5. The values of logK(DEF) were consistent with complexation of Hg(II) by carboxylic and phenolic groups.