Influence Of Chloride Salts Research Articles

Gold in sulfide fluids revisited

Gold solubility was measured at temperatures of 350, 400, 450, and 490 °C and pressures of 500 and 1000 bar in an ’oxidized sulfide’ system, as a function of pHT (2 – 10) and sulfur concentration (m(Stotal) = 0.03 – 1.2 [mol·(kg H2O)-1]). In this system, sulfur primarily exists as H2S, H2SO3, H2SO4, their dissociation products, and radical species such as S2- and S3-. The complexes Au(HS)2-, Au2S22-, AuHS(aq), AuHS(H2S)3(aq), and AuOH(aq) were identified as the primary gold species in the experimental fluids, varying with pH and m(Stotal). The solubility constants for Au(HS)2-, a critical (hydro)sulfide complex, align excellently with literature for ’reduced sulfide’ fluids, where sulfur predominantly exists in the 2- oxidation state. New experimental data from the ’oxidized sulfide’ system were regressed along with reliable literature data from ’reduced sulfide’ systems to calculate standard thermodynamic properties and parameters of the Helgeson-Kirkham-Flowers (HKF) model. The solubility constants for charged complexes, Au(HS)2- and Au2S22-, increase sharply with temperature, whereas those for neutral species, AuHS(aq) and AuHS(H2S)3(aq), show a pronounced peak near 300 °C. These (hydro)sulfide complexes account for gold solubility ranging from a few tens of ppb to a few tens of ppm in natural sulfide fluids, depending on the fluid pH. Thermodynamic calculations also indicate that, in addition to (hydro)sulfide species and the hydroxide complex, AuCl2- significantly contributes to Au mobility in high-temperature acidic chloride fluids. Based on new experimental data and prior studies using solubility and X-ray absorption spectroscopy methods, other gold complexes including mixed Au-HS-Cl, Au-HS-S3- species, and complexes with alkali metal cations, are deemed redundant. Above 250 °C, the influence of chloride salts on gold solubility can be accurately modeled using a simple extended Debye-Hückel equation with the term bγ·I = 0. The Setchenov coefficient bn = 0 suffices for calculating the activity coefficients of neutral species. This streamlined thermodynamic model aligns closely with earlier experimental work by Terry Seward and his team, effectively describing the state of Au in all types of natural fluids where Au exists in the 1+ oxidation state, under any set of P-T-f(O2)-f(S2)-compositional parameters.

Influence of chloride salts on hydrogen generation via hydrolysis of MgH2 prepared by hydriding combustion synthesis and mechanical milling

The effects of chloride salts (NaCl, MgCl2 and NH4Cl) on the hydrolysis kinetics of MgH2 prepared by hydriding combustion synthesis and mechanical milling (HCS+MM) were discussed. X-ray diffraction (XRD) analyses show that high-purity MgH2 was successfully prepared by HCS. Hydrolysis performance test results indicate that the chloride salt added during the milling process is favorable to the initial reaction rate and hydrogen generation yield within 60 min. A MgH2–10% NH4Cl composite exhibits the best performance with the hydrogen generation yield of 1311 mL/g and a conversion rate of 85.69% in 60 min at room temperature. It is suggested that the chloride salts not only play as grinding aids in the milling process, but also create fresh surface of reactive materials, favoring the hydrolysis reaction.

The Role of Chloride Salts in Chemically Enhanced Phytoextraction of Heavy Metals From a Contaminated Agricultural Soil

Chemically enhanced phytoextraction of heavy metals has proved in several cases to be a perspective and costeffective method of soil remediation. The main drawback of this method continues to be the low mobility (and thus the bioavailability) of some heavy metals (e.g., lead [Pb]). The use of synthetic chelating agents, such as ethylenediaminetetraacetic acid (EDTA), significantly increased the mobility of heavy metals in soils and their translocation from the roots to the shoots of several plants (Blaylock et al., 1997; Huang et al., 1997). However, because of their high solubility and persistence in soils, chelates can possibly leach down the soil profile. Thus, they pose an important environmental risk for groundwater quality (Romkens et al., 2002). In addition, free EDTA and EDTAheavy metal complexes are potentially toxic to plants (Vassil et al., 1998). For these reasons, several alternatives to EDTA have been proposed. Among many, (S, S)-N, N’-ethylenediaminedisuccinic acid (EDDS), a biodegradable chelating agent, was introduced to minimize unwanted negative effects associated with the use of EDTA during phytoextraction experiments (Kos and Lestan, 2003). Xiong and Feng (2001) introduced chloride salts as another alternative for enhancing Pb accumulation in Brassica pekinensis. Highly soluble chloride salts dissociate in soil solution and thus provide (1) cations capable of exchanging the adsorbed heavy metals and (2) ligands needed to form watersoluble mobile complexes such as CdCl, CdCl3 , or CdCl4 . Other studies (Li et al., 1994; Smolders et al., 1998) also have proved the positive influence of chloride salts on heavy metal (especially Cd) accumulation in plants. The authors add that the use of chloride salts is preferred over the use of EDTA because of the lower costs and shorter dwell time of the formed soluble complexes in soils. Furthermore, the application of sodium chloride (NaCl) did not negatively influence plant biomass production to the extent that EDTA did (Xiong and Feng, 2001). However, many authors have focused on soils artificially spiked with a single metal in a form that does not correspond to natural conditions (e.g., Pb[NO3]2 or Cd[NO3]2) (e.g., Begonia et al., 2003, 2004; Hovsepyan and Greipsson 2005; Xiong and Feng, 2001). In addition, the application of mobilizing agents to soils contaminated with multiple metals can lead to a reduction in plant biomass yields and the total amount of heavy metals phytoextracted due to the toxicity of other metals present at high concentrations in the soil solution (Chen and Cutright, 2001). The main objectives of this study were (1) to determine the effect of different chloride salt (NH4Cl) concentrations on the mobility of cadmium (Cd), zinc (Zn), and Pb in a contaminated agricultural soil originating from the contaminated mining and smelting area of Přibram, Czech Republic, (2) to evaluate the phytoextraction efficiency of a hybrid poplar (Populus nigra · Populus maximoviczii) and maize (Zea mays) after the application of different NH4Cl concentrations, (3) and to compare the effectiveness of NH4Cl and EDTA in the phytoextraction process. M. Komarek (&) P. Tlustos J. Szakova Department of Agrochemistry and Plant Nutrition, Czech University of Agriculture in Prague, Kamýcka 129, 165 21 Prague 6, Czech Republic e-mail: komarek@af.czu.cz