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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.

Since phosphate is strongly related to eutrophication of environmental waters, several research groups quest for materials that can efficiently remove phosphate from wastewaters before it contaminates lakes and reservoirs. In the present work, a commercial clay mineral (K10 montmorillonite) modified with Fe 3þ polyhydroxy cations was investigated as an adsorbent for phosphate. The incorporation of the polycations did not alter the main conformational characteristics of the montmorillonite, as verified by specific surface area measurements, X-ray diffractometry, FTIR, electron microscopy, and zeta potential titrations. On the other hand, the materials supporting Fe 3þ polyhydroxy cations exhibited a significant enhancement of adsorption capacity, as determined by Langmuir-Freundlich isotherms, from 39 AE 2 to 104 AE 15 μmol g À1 . The different ratios of OH À to Fe 3þ did not affect the adsorption capacities. The adsorption kinetics was best described by the pseudo 2 nd order model, approaching the equilibrium after 120 min of contact time. A variation of pH between 4.6 and 8.5 did not affect the adsorption percentages. The adsorption capacities increased with the increase of the ionic strength, thus suggesting that the formation of inner-sphere complexes prevails over electrostatic interactions as the adsorption mechanism. The materials removed phosphate from three polluted water samples having phosphate concentrations between 0.0919 and 1.211 mg L À1 . The remaining phosphate concentration was below the limit of quantification of the analytical method (0.063 mg L À1 in P, or 2.0 μmol L À1 ). The presence of 10 mg L À1 humic of fulvic acid did not affect the performance of the materials. In conclusion, the modification of clay minerals with Fe 3þ polyhydroxy cations is useful in producing low-cost adsorbents for phosphate.

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