Granular activated charcoal from peanut (Arachis hypogea) shell as a new candidate for stabilization of arsenic in soil

Abstract

A sandy soil of pH = 5.0 artificially contaminated with 15 mg/kg As in form of arsenate ions [iAs(V)] corresponding to the permissible value in force in Hungary was incubated for 4 weeks after amendment with different peanut shell-derived activated charcoal (AC) materials at an application rate of 2 wt%. Composite formation with silicate materials and adsorption mechanism for As were corroborated by attenuated total-reflection Fourier-transform infrared spectroscopy. Moreover, 0.01 M oxalic acid (OA) was also applied to investigate further enhancement through protonation of iAs(V) and/or AC. Assessment of As stabilization was achieved by a fit-to-purpose BCR sequential soil extraction procedure (extraction of the water-soluble and carbonatic fraction with 0.11 M acetic acid solution and the easily reducible one by 0.5 M hydroxylamine hydrochloride). Arsenic phytoavailability was estimated by leaching with 0.02 M EDTA solution (pH = 4.65). Samples were analyzed by inductively coupled plasma sector field mass spectrometer applying high resolution mode. Total-reflection X-ray fluorescence spectrometer was used to check data accuracy. The largest As stabilization rate (approximately 30%) compared to the reference soil treatment was obtained when granular Florisil®-AC was applied. Stabilization efficiency did not improve by using OA. A 5 wt% AC application rate for the Florisil® composite resulted in a similar As distribution to the 2 wt% one also at a 30 mg/kg iAs(V) dosage. The amounts of immobilized As increased almost proportionally with the higher AC application rate by doubling the iAs(V) dose. Similarly, studies conducted on solutions proved that As adsorption onto ACs was slightly exponential.