Mechanism of enhanced oxidation ability of dilute nitric acid and dissolution of pure gold in seawater with nitric acid

Abstract

It has been discovered that dilute nitric acid in reversed micelle systems can oxidize the Br ion to Br 2 and we have proposed that the nitryl (or nitronium) ion NO 2 + should be the active species in the oxidation process. Nitration of phenol in reversed micelle systems with dilute nitric acid, CHCl 3 /CTAC/H 2 O (2.0 mol dm-3 HNO 3 in the 1.0% (v/v) H 2 O phase), has been performed at 35 ºC to obtain 2-and 4-nitrophenols, where CTAC represents cetyltrimethylammonium chloride. In aqueous 2.0 mol dm-3 HNO 3 solution accompanied by 4.0 mol dm-3 LiCl (and a small amount of LiBr as the bromide resource), trans-1,4-dibromo-2-butene was successfully brominated to 1,2,3,4-tetrabromobutane. This result is good evidence that the Br ion can be oxidized to Br 2 in dilute nitric acid (2.0 mol dm-3) providing it contains concentrated salts. For chloride salts, the cation effects increased as Et 4 N + << Na + < Li + < Ca 2+ < Mg 2+. Even the evolution of Cl 2 has been demonstrated from < 2.0 mol dm-3 HNO 3 solution containing concentrated LiCl, MgCl 2 , and CaCl 2 as well as AlCl 3. The dissolution of precious metals (Au, Pt, and Pd), especially, of gold has been demonstrated in 0.1-2 mol dm-3 HNO 3 accompanied by alkali metal, alkaline earth metal, and aluminum chlorides. The complete dissolution time of pure gold plate (20±2 mg, 0.1 mm thickness) in 2.0 mol dm-3 HNO 3 accompanied by 1.0 mol dm-3 AlCl 3 has been shortened remarkably with temperature increase from 15 to 80 ºC. The dissolution rate constants, log (k /s-1), of a piece of gold wire (19.7±0.5 mg) in 20 mL of 2.0 mol dm-3 HNO 3 accompanied by the metal chlorides, in general, increase with increasing salt concentrations at 40 and 60 ºC. The gold can be dissolved in the solution of <1.0 mol dm-3 HNO 3 and <1.0 mol dm-3 HCl, i.e. a "dilute aqua regia." We have achieved a total dissolution of five pieces of the gold wire (totally 0.10 g) in 100 mL of the 1:1 mixture between seawater and 2.0 mol dm-3 HNO 3 at ca. 100 ºC.

Synthesis of Gold Nanoparticles from Gold Coatings Recovered from E-Waste Processors

“…The continuous formation of the gold complex drives the reaction and generates H 2 O and NO as byproducts, the latter combining with ambient oxygen to generate NO 2 . Some studies have suggested that HNO 3 may not be the only oxidizing agent in the reaction, as chlorine gas and nitrosyl chloride (produced during the reaction with aqua regia) can also oxidize small amounts of gold [ 23 , 24 , 25 ].…”