In situ investigation of heavy metals at trace concentrations in greenhouse soils via portable X-ray fluorescence spectroscopy

Soil pollution by heavy metals (HMs) has rapidly become a major threat to vegetable security. Nearly all cultivated soils are at risk of metal accumulation, and greenhouse soils are among the most heavily impacted soils. In this study, a rapid assessment of HMs at trace concentrations was conducted via portable X-ray fluorescence (PXRF) spectroscopy in Shouguang, China. Measurements were made via PXRF under in situ, ex situ and sieved conditions and by inductively coupled plasma mass spectrometry (ICP-MS) after acid digestion. The performance of each PXRF measure relative to the ICP-MS method was assessed by linear regression. Redundancy analysis was performed to quantify the proportion of explained variability between the PXRF and ICP-MS data. Evaluation of the possible sources of HMs and their potential risks was then conducted by multivariate analysis. The results showed that the PXRF data were closely correlated with ICP-MS quantification for Cu, Mn and Zn, whereas no significant correlations were found for As, Ni and Pb. The uncertainties of PXRF measurement derived from soil heterogeneity accounted for 20.02% of total variability and those from moisture and particle size accounted for 20.15%. The geo-accumulation index (Igeo) indicated that the greenhouse soils were potentially contaminated by Cu and Zn (Igeo > 0), which can be attributed to anthropogenic activities. Overall, PXRF spectroscopy is promising as a rapid and nondestructive in situ technique for assessing the potential risks of HMs at trace concentrations in greenhouse soils.