Behaviour of redox-sensitive elements (Ti, As, V and Fe) in clay pit samples with and without Al normalization

Natural clay minerals, because of their physical and chemical characteristics are good adsorbents of metals. Among metals, heavy metals are often investigated, but redox-sensitive metals are increasingly getting attention. Redox-sensitive elements were measured in clay pit samples from a clay mine in Bosnia and Herzegovina. For the purpose of analysis, a 23 m deep clay pit core sample was taken, and it was randomly subsampled to give 45 clay pit samples from different depths. The pH, electrical conductivity (EC), oxidation-reduction potential (ORP) and redox-sensitive elements contents were measured. Amounts of redox-sensitive metals and aluminium were measured with an inductively coupled plasma optical emission spectrophotometry instrument. For statistical evaluation, maximum, minimum, average, median, Pearson's prior correlation, correlation applying Al normalization and centred log-ratio (clr) transformation were calculated. When justified, principal component analysis (PCA) was also performed. The results show acidic pH and oxidative ORP. EC varied significantly through the sediment. The element concentrations lie in the sequence: Fe > Ti > V > As. There is a significant change in correlations with and without Al normalization. The highest difference was found for Ti and V (coefficient of correlation (r) value ranging from -0.553 to 0.835). Correlations and PCA after clr transformation showed mainly strong negative correlations, meaning that redox-sensitive elements behave differently in clay samples.