Maurizio Pettine

The reduction of chromium (VI) by iron (II) in aqueous solutions

Abstract The rates of the reduction of Cr(VI) with Fe(II) were measured in NaCl, NaClO 4 , and natural seawater as a function of pH (1.5–8.7), temperature (5–40°C) and ionic strength (I = 0.01–2 M). The pseudo first-order rate constant (log k 1 ) showed a parabolic dependence on pH decreasing from 1.5 to 4.5 and increasing from 5.5 to 8.7. The kinetics of the reaction in these two regions of pH also showed different influences of temperature, ionic strength, and reductant concentration. The rate of Cr(VI) reduction is described by the general expression −d[Cr(VI)]/dt = k [Cr(VI)] [Fe(II)] where k (M −1 min −1 ) can be determined from the log k=6.74−1.01 pH −188.5/T for the pH range 1.5–4.5 (σ = 0.2) and log k=11.93+0.95 pH −4260.1/T−1.06 I 0.5 for the pH range 5–8.7 (σ = 0.2) from 5 to 40°C and 0.01 to 2 M ionic strength. The effect of pH, temperature, and ionic strength on the reaction indicates that the reactions at low pH are due to H 2 CrO 4 + Fe 2+ → k H 2 A-Fe products While the reactions at high pH are due to HCrO 4 − + FeOH + → k HA-FeOH products HCrO 4 − + Fe(OH) 2 → k HA-Fe(OH) 2 products The overall rate expression over the entire pH range can be determined from (H 2 A = H 2 CrO 4 ) k=k H 2 A-Fe α( H 2 A)α( Fe 2+ )+k HA-FeOH α( HA − )α( FeOH + )+k HA-Fe(OH) 2 α( HA − )α( Fe(OH) 2 ) where k H2A−Fe = 5 x 10 6 , k HA−FeOH = 1 x 10 6 , k HA−Fe (OH)2 = 5 x 10 11 . In oxic aqueous systems Cr(VI) competes with O 2 in the oxidation of Fe(II) and an extension of the rate law for Cr(VI) reduction with Fe(II) in oxygenated solutions is proposed. The application of this extended rate law to environmental conditions suggests that this reaction influences the distribution of oxidized and reduced species of chromium in oxic and anoxic waters.

Arsenite oxidation by H2O2 in aqueous solutions

Abstract The rates of the oxidation of As( iii ) with H2O2 were measured in NaCl solutions as a function of pH (7.5–10.3), temperature (10–50C) and ionic strength (I = 0.01–4). The rate of the oxidation of As( iii ) with H2O2 can be described by the general expression: d[As( iii )]/dt = k[As( iii )] [H2O2] where k (mol/L−1 min−1) can be determined from (σ = ±0.12) log k=5.29+1.41 pH−0.57I+1.40 I0.5−4898/T. The effect of pH on the rates indicates that the reaction is due to AsO ( OH ) 2 − + H 2 O 2 k 1 → products AsO 2 ( OH ) 2− + H 2 O 2 k 2 → products , AsO 3 3− + H 2 O 2 k 3 → products where k = k1 αAsO(OH)2− + k2 αAsO2(OH)2− + k3 αAsO33− and αi are the molar fraction of species i. The values of k1 = 42 ± 20, k2 = (8 ± 1) × 104, and k3 = (72 ± 18) × 106 mol/L−1 min−1 were found at 25C and I = 0.01 mol/L. The undissociated As(OH)3 does not react with H2O2. The effect of ionic strength on the rate constants has been attributed to the effect of ionic strength on the speciation of As( iii ). The rate expression has been shown to be valid for NaClO4 solutions, northern Adriatic sea waters, and Tiber River waters. The cations Fe2+ and Cu2+ were found to exert a catalytic effect on the rates. Cu2+ plays a role at concentration levels (>0.1 μmol/L) which are typical of polluted aquatic systems, while Fe2+ is important at levels which may be found in lacustrine environments (>5–10 μmol/L). The reaction of As( iii ) with H2O2 may play a role in marine and lacustrine surface waters limiting the accumulation of As( iii ) resulting from biologically mediated reduction processes of As( v ).

Reduction of chromium (VI) with hydrogen sulfide in NaCl media

The rates of reduction of chromium (VI) to chromium (III) with H2S in NaCl solutions have been measured as a function of temperature (15–50°C). pH (7.5–10.5) and ionic strength (0–2 M). The overall rate constant (k, M−1 min–1) in: d[Cr(VI)]dt=−k[Cr(VI)][H2S]T can be determined from (SD=0.07 in log k): logk = 16019−1.06pH − 2300.9T where the ionic strength term has been overlooked because it does not significantly affect the kinetics. The energy of activation is 43.6 ± 1.5 kJmol−1. The rates of reduction in real seawater samples appeared to be 2–3 times faster than in NaCl at the same ionic strength. Based on these kinetic measurements, half times of Cr(VI) range from few hours (4–6 h) to many days (160–250 d), respectively, at 1 mM or 1μM or H2ST in anoxic waters.

Seasonal variations of dissolved organic matter in the northern Adriatic Sea

Abstract Concentrations and seasonal variability of dissolved organic carbon (DOC), total dissolved carbohydrates (TDCHO), and dissolved free amino acids (DFAA), were measured in two frontal regions of the northern Adriatic Sea during two surveys (June, 1996 and February, 1997). Colloidal (>1 kDa) and truly dissolved (

Nutrients, standing crop and primary production in western coastal waters of the adriatic sea

Nutrients and standing crop over a 2-year period and primary production rates over a 1-year period were measured at three stations located 1·5, 6 and 15 nautical miles (NM) from the coast, 150 km south of the Po river mouth. Both nutrients and standing crop (the latter measured as chlorophyll a concentration and phytoplankton cell density) showed a concentration gradient from west to east with marked changes of average values from 6 to 15 NM off the coast. Nutrients and standing crop appeared to be positively correlated and both showed an inverse dependence on salinity which was mainly influenced by freshwater inputs from the major Italian river, the Po. Primary production rates showed values of about 260 and 210 g C m−2 year−1 at the 1·5 and 6 NM stations respectively, indicating the presence of a narrow coastal belt with productivity levels significantly higher than those previously reported for other Adriatic areas affected by the Po. The offshore station at 15 NM gave an average annual production of about 120 g C m−2 year−1 which lies in the range reported for lagoons and coastal waters in the Po delta area. The photosynthetic efficiency was similar at the 1·5 and 6 NM stations (respectively 4·0 ± 2·3 and 3·5 ± 2·7) but significantly lower at the 15 NM station (1·9 ± 1·7). This difference reflects either different nutrient levels in inshore and offshore stations or different structures in phytoplankton community: diatoms and phytoflagellates prevailed at 1·5 and 15 NM, respectively. Photosynthetic efficiency increased with increasing concentration of nutrients. The behaviour was found to be linear as a function of total dissolved phosphorus, and hyperbolic as a function of total dissolved nitrogen. This different dependence suggests a surplus of nitrogen with respect to phosphorus for photosynthetic processes in the study area, which is consistent with the assumption of phosphorus as limiting element often made for the northern Adriatic.

Soluble and particulate metals in the Po River : factors affecting concentrations and partitioning

Fourteen metals (Ca, Mg, Al, Fe, Mn, Ba, As, Cd, Co, Cr, Cu, Ni, Pb and Zn) were monitored over a 2-year period in the waters of the lower Po River. Concentrations in the dissolved and particulate phases were measured, thus constructing a large database on metal variability. The influence of flow and solid transport on dissolved, particulate and total metal concentrations is discussed. In addition to flow rate and solid content, biological processes seem to be one of the main factors affecting concentrations in the dissolved phase, while the inorganic components of solids seem to control metal concentration in suspended matter. Data on partitioning of metals between the dissolved and particulate phases are presented, together with related information on the affinity sequence of metals for the particulate phase, and the influence of solid load on the modes of metal transport.

Dissolved and particulate transport of arsenic and chromium in the Po River (Italy)

The modes of transport of arsenic and chromium have been investigated, as part of a larger study, on the most important Italian river, the Po. Dissolved, particulate and total concentrations are reported and compared with reference background values. The main factors affecting dissolved concentrations appear to be two interrelated variables, flow and suspended matter concentration, and biological activity. The predominant dissolved species were found to be As(V) and Cr(VI). The ratio between oxidized and reduced species appears in the case of arsenic to be significantly influenced by iron and manganese oxides (Fep and Mnp); the ratio for chromium shows no significant dependence on particulate manganese concentration (Mnp), possibly because its effect is masked by other variables (H2O2 and Fe2+) connected with photochemical reactions. Particulate transport was found to be more important for chromium than arsenic. As and particularly Cr concentrations in suspended matter are strongly influenced by major particulate components including Fep, Alp and Mgp. It is suggested that adsorption is probably not the only mechanism responsible for Crp levels; the presence of mixed CrFe or CrMg hydroxides may better explain this dependence. Previously published laboratory work supports the existence and role of these hydroxides in natural waters. The dissolved fraction of total transport was found to be strongly dependent on suspended matter concentration for As and Cr and equations describing this dependence are proposed.

Chromium (III) interactions in seawater through its oxidation kinetics

The rates of oxidation of chromium(III) to chromium(VI) with H2O2 have been measured in NaCl and the major sea salts at pH 8 and 25°C. The effect of aging the chromium(III) solutions before the addition of H2O2 has also been investigated for the same solutions. Borate was the only major component of seawater found to affect the oxidation in seawater. Like OH−, an increase in the B(OH)4− concentration causes the rates of oxidation to increase. The rate-determining steps in seawater involve two chromium (III) species Cr(H2O)5OH2++H2O2→KOH products Cr(H2O)4(OH) [B(OH)4] + + H2O2→KB products and the rate equation is given by dCr(III)/dt = −k3[H2O2] [OH−] [B(OH)4−]0.3 where k3=k1/[H2O2][OH−][B(OH)4−]0.3 (k1 is the pseudo-first-order rate constant). Aging of the solutions causes the rates of oxidation to decrease and is affected by the concentration of borate, carbonate and magnesium ions. Borate causes a reduction of the aging effect, while carbonate and magnesium produce an increase of the aging effect. The Mg2+ influence is much stronger than CO32− when these ions are present at their seawater levels in individual Na-Mg-Cl and Na-Cl-CO3 solutions. When CO32− and Mg2+ are present at the same time, the combined aging effect is less than that found in simple Na-Mg-Cl solutions. Since the aging effect is related to the precipitation of chromium(III), magnesium and carbonate ions will probably affect the solubility of chromium in natural waters.

C, N and Their Stable Isotopes in Suspended and Sedimented Matter from the Po Estuary (Italy)

C and N content, C/N (atomic) ratio, and C and N isotopic composition (Δ13C and δ15N) were determined on suspended particulate matter and sediment samples obtained from riverine, estuarine and marine environments in two cruises (September 1995 and March 1996) in the Po estuary (Italy). Isotopic tracers of C and N, reported for the first time for this environment, gave information on sources of organic matter and their distributions. An end-member mixing model based on δ13C values was applied to estimate the relative importance of riverine and marine sources of organic matter in suspended particulate matter and sediments.

Distribution of As, Cr and V species in the Po-Adriatic mixing area, (Italy)

Abstract The distribution of dissolved and participate concentrations of As, Cr and V was investigated in the mixing area of the Po river in five surveys over the period March 1992-March 1995. The data obtained indicate noticeable contamination of coastal waters of the northern Adriatic with Cr but not As or V. Total dissolved concentrations for As and V were comparable with oceanic levels but were higher by a factor of 4 for Cr. These results suggest either low anthropogenic inputs of As and V, from terrigenous and atmospheric sources, or efficient removal mechanisms. The investigated metals had low reactivity during freshwater-seawater mixing and the distribution of concentrations in the dissolved and solid phases suggests that the dilution process is the main factor controlling the transport of these metals to the sea in low-to-medium solid transport conditions. Further investigations are required to elucidate the behavior of these metals during high solid transport conditions. Significant changes in the speciation of As occurred during the transition between the two end-member matrices. In the coastal waters of the northern Adriatic, the arsenite fraction in winter, and the arsenite and dimethylarsinate fractions in summer increased with respect to riverine waters. Changes in the speciation of chromium were minor, but also showed a small increase in reduced species in marine waters.