A study on the dissolution rates of K-Cr(VI)-jarosites: kinetic analysis and implications.

Elia G. Palacios,Ister Mireles,Thangarasu Pandiyan,Emmanuel J. Gutiérrez,Iván A. Reyes,Francisco Patiño,Martín Reyes,Mizraim U. Flores

doi:10.1186/s12932-016-0035-7

Abstract

BackgroundThe presence of natural and industrial jarosite type-compounds in the environment could have important implications in the mobility of potentially toxic elements such as lead, mercury, arsenic, chromium, among others. Understanding the dissolution reactions of jarosite-type compounds is notably important for an environmental assessment (for water and soil), since some of these elements could either return to the environment or work as temporary deposits of these species, thus would reduce their immediate environmental impact.ResultsThis work reports the effects of temperature, pH, particle diameter and Cr(VI) content on the initial dissolution rates of K-Cr(VI)-jarosites (KFe3[(SO4)2 − X(CrO4)X](OH)6). Temperature (T) was the variable with the strongest effect, followed by pH in acid/alkaline medium (H3O+/OH−). It was found that the substitution of CrO42−in Y-site and the substitution of H3O+ in M-site do not modify the dissolution rates. The model that describes the dissolution process is the unreacted core kinetic model, with the chemical reaction on the unreacted core surface. The dissolution in acid medium was congruent, while in alkaline media was incongruent. In both reaction media, there is a release of K+, SO42− and CrO42− from the KFe3[(SO4)2 − X(CrO4)X](OH)6 structure, although the latter is rapidly absorbed by the solid residues of Fe(OH)3 in alkaline medium dissolutions. The dissolution of KFe3[(SO4)2 − X(CrO4)X](OH)6 exhibited good stability in a wide range of pH and T conditions corresponding to the calculated parameters of reaction order n, activation energy EA and dissolution rate constants for each kinetic stages of induction and progressive conversion.ConclusionsThe kinetic analysis related to the reaction orders and calculated activation energies confirmed that extreme pH and T conditions are necessary to obtain considerably high dissolution rates. Extreme pH conditions (acidic or alkaline) cause the preferential release of K+, SO42− and CrO42− from the KFe3[(SO4)2 − X(CrO4)X](OH)6 structure, although CrO42− is quickly adsorbed by Fe(OH)3 solid residues. The precipitation of phases such as KFe3[(SO4)2 − X(CrO4)X](OH)6, and the absorption of Cr(VI) after dissolution can play an important role as retention mechanisms of Cr(VI) in nature.

Highlights

The presence of natural and industrial jarosite type-compounds in the environment could have important implications in the mobility of potentially toxic elements such as lead, mercury, arsenic, chromium, among others
Extreme pH conditions cause the preferential release of K+, SO42− and CrO42− from the KFe3[(SO4)2 − X(CrO4)X](OH)6 structure, CrO42− is quickly adsorbed by Fe(OH)3 solid residues
In most of the reactions, the chemical reaction is the stage controlling the dissolution process, in the reactions at low [H3O+] (1.5 ≥ pH ≤ 4.5) and T ≤ 30 °C, the diffusion of H3O+ ions on the unreacted core can play an important role in the dissolution rate of KFe3[(SO4)2 − X(CrO4)X] (OH)6

Summary

Introduction

The presence of natural and industrial jarosite type-compounds in the environment could have important implications in the mobility of potentially toxic elements such as lead, mercury, arsenic, chromium, among others. These precipitates can limit the mobility of Cr(VI) and its bioavailability, like the Cr phase identified by Baron et al [2] in soil polluted by chromate solutions These phase was identified as KFe3(CrO4)(OH), which is the structural analog of jarosite KFe3(SO4)(OH). An extensive literature search related to the formation of solid solutions in the alunite/ jarosite group suggests the existence of the solid solution KFe3[(SO4)2 − X(CrO4)X](OH) between jarosite and its chromate analog [3, 4] The presence of these solid solutions could have important implications in the mobility of Cr(VI). Understanding the dissolution reactions of these solid solutions is notably important for an environmental assessment of the effects of chromium, because Cr(VI) frequently enters the environment [5]

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