Chemistry, mineralogy & environmental impacts of arsenic

Chemistry, mineralogy & environmental impacts of arsenic

Introduction

Arsenic is a very famous poisonous element. Today, when human is more intelligent and sophisticated, he tries to eliminate all the factors that may affect the life of human beings or try to eliminate all the factors that serve as a source that may affect environment of human being negatively. The reason of doing such efforts is only to increase the life expectancy and quality of living of peoples especially living in areas that are located near industrial zones. Industrial zones are considered as a considerable source that contaminates the world with this poisonous element (O'Day, 2006, P 77-83). This essay deals with the issue of contamination of world with this element Arsenic and also explains the method of mineralogy and other available method to reduce its effect on human life.



Discussion

Common forms of Arsenic

There is only stable isotope of Arsenic is discovered and present in the worlds which is 75As. Unfortunately, Arsenic is 47th most abundant element among the 88 elements which are naturally occurred in the world. The average crustal abundance of arsenic is 1.5 mg kg -1 (1.5 ppm). Shales, slates and Mudstones have the highest concentrations among the common rocks, although extremely high concentrations can be found in some coals (Hopenhayn, 2006, P 103-107). The concentration of Arsenic is generally very low in natural water of rain and sea as compared to the water of river that has sometimes very high quantity of Arsenic. Another important source of Arsenic is the industrial waste.

Arsenic Oxides

Arsenic occurs in the environment in four oxidation states: As (III), As (0), As (III), and As (V). Arsenic common species found in most of the natural waters are the trivalent arsenite, inorganic oxyanions and pentavalent arsenate. Trivalent arsenite includes H3AsO3, H2AsO3-, HAsO32-, while, pentavalent arsenate includes H2AsO4, HAsO4 and H3AsO4. In addition, there are a variety of organic forms of arsenic such as methanearsonic acid and dimethylarsenic acid. As (III) is the more toxic form of the species. The main abiotic factors that control the speciation of arsenic are redox potential (Eh) and pH (Hopenhayn, 2006, P 103-107).

Phases of Arsenic

Solution Phase

Two oxidation states of arsenic, As (V) and As (III), predominate in surface and near surface environments. In solution, arsenic exists primarily as oxyanionic acids; arsenate (As (V) as H3AsO4) has pKa's of 2.2, 6.9 and 11.5 while arsenite [As (III) as H3AsO3] has pKa's of 9.2,

12.1 And 13.4. Thus, at circumneutral pH, HAsO4 H2AsO4 and H3AsO3 species dominate. Plant and microbial activity may methylate As (V) or As (III), forming, for example, DMAA (dimethylarsenic acid) and MMAA (monomethylarsonous acid).

Precipitation of Arsenic Phases

Both As (V) and As (III) may precipitate within sediments and soils, but the fundamental parts inducing the precipitation involve in this dramatically varies. Just like phosphate, Arsenic, tends to precipitate with multivalent and hard cations which includes ferric iron and aluminum under acidic conditions of magnesium and calcium under the conditions of ...