How can I claim that a particular As-bearing mineral is present in a soil sample by XRD analysis? How many and which peaks of the miranal should be matched with XRD pattern of the soil sample?
Dear Mr. Gerdelidani,
if you have a good match between the XRD pattern of your sample and a reference database the likelihood of an As-bearing mineral to be present in your sample will be high. For chemical support I suggest to take a look at the chemical composition derived from XRF analysis. In case of As being accommodated into the lattice of a mineral like scorodite or annabergite you will also find elevated As contents. In this case carefully check the preparatory work done prior to the XRF analysis because of the high volatility of arsenic compounds leading to reduced amounts of this element. The third important question is whether As can be present in the soil sample at all. Arsenic is not a widespread element in nature and always related to sulfides (such as pyrite) and/or arsenides. Metallogenetic processes be it naturally on top of gossans or be it man-made in technosols on waste and mining dumps should be carefully monitored. Also pay attention to all soils and substrates undergoing strong redox processes.
With kind regards
H.G.Dill
Dear Mr. Gerdelidani,
It is very hard to find an As-bearing mineral in soil, unless if it reside in sulphide (arsenopyrite) inclusions in a particular heavy mineral. Generally, oxidation of As-bearing sulphides led to the leaching of metals into groundwater. Redox reactions, complexation, and sorption reactions onto Fe, Al oxides, carbonates, organic matter, and clay minerals are the most significant process controlling the solubility and mobility of As(III) and As(V) in the environment. Organic matter may also serve as binding agents, thereby reducing As mobility. Plants and microbes are able to detoxify the immobile As(V) via root uptake, followed by intracellular reduction to the mobile As(III) and its efflux into the surrounding rhizosphere . Reductive co-dissolution from Fe oxy/hydroxides has been suggested as another possible mechanism of As(III) production in the rhizosphere.
So, look for the concentration of As using XRF or ICP-MS analyses and if the As is not elevated in the soil samples then it may be associated with Fe oxides , clays or organics. Also be aware of the XRD has a limitation of detecting amorphous, adsorped or minerals of low content in samples.
Best regards
Walid
Excellent suggestions given by Dr. Walid. In addition what Dr. Walid has suggested you may try to identify the As-bearing minerals under petrographic microscope. Handpick them and try for x-ray.
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