A total of 10.00 g of iron-coated quartz sand was taken from Columns I, III and IV, air-dried, and stored in a vacuum in the dark. The iron-coated quartz sand particles were selected and treated with gold coating, and the morphology of the coating layer on the surface of the quartz sand was observed using an ultra-high resolution field emission scanning BIO 1211 microscope (Hitachi, SU8010). The chemical composition and the element content of the target area were determined using X-ray energy dispersive spectroscopy (Ametek, EDAX APPOLLO XP). Several quartz sand particles with intact surfaces were selected and inlayed with epoxy resin and then polished. They were then subjected to an electron probe for microanalyzer (Japan Electronics Co., JXA-8230) equipped with an X-ray microanalysis system (Oxford Company, Inca X-Act) to analyze the microstructure and mineral composition of the iron coating. The accuracy and precision of SEM-EDS and EPMA-EDS for iron coating analysis were evaluated by repeated analysis of two laboratory reference standards Arsenopyrite and Chalcopyrite. Quantified values (wt.%) were within 10% of accepted values for major and minor elements, and reproducibility was better than 8% relative for all compounds analyzed. Additionally, 5.00 g of the sample particles was mixed with 5.00 g of pure solid KBr, ground to powder in an agate mortar and passed through a 100-mesh sieve. The forms of arsenic and iron bound to the mineral surface were analyzed using a Fourier transformation infrared spectrometer (Thermo-Fisher, Nicolet 6700) with the KBr powder as the background control. The preparation of KBr pellets relied on a standardized procedure to enhance reproducibility. IR spectra of powered samples were recorded from the 4000 cm− 1 to 400 cm− 1 and the IR spectra were obtained at a resolution of 0.6 cm− 1.