Inorganic content of rock dust waste from northwest of Rio de Janeiro, Brazil: do environmental risks incur from its use as natural fertilizer?

The ornamental rock exploration and coating industry has led to significant environmental impacts due to the inadequate manner in which these residues are released into the environment. This impact may be reduced by using these residues for soil enrichment, as they contain high levels of inorganic nutrients. However, some elements may be potential contaminants, thus requiring a thorough previous research before employing these substances in agriculture. In this context, this study aims to determine potentially toxic elements present in rock dust residues, As, Cd, Cr, Cu, Pb, and V, through Inductively Coupled Plasma Optical Emission Spectroscopy (ICP OES) after wet decomposition. Cd concentrations in residual rock dust waste surpassed the maximum limit established by the Brazilian Ministry of Agriculture, Livestock and Supply stipulated in normative instruction No. 5/2016 for remineralizers and substrates for plants intended for agriculture. In addition, the official digestion method adopted by the Brazilian Agricultural Research Corporation Embrapa (used for soil analysis) was used for a rock dust waste analysis and compared to two different block digester-based procedures. Accuracy was confirmed by using the Certified Reference Material (CRM) NIST SRM 1646 (Estuarine Sediments) and addition/recovery tests. The limits of detection for the aforementioned elements, after aqua-regia-induced digestion, were of 0.07 mg kg for As, 0.06 mg kg for Cd, 0.01 mg kg for Cr, 0.2 mg kg for Cu, 0.03 mg kg for Pb, and 0.09 mg kg for V.