Effects of long-term storage of phosphate sludges on their physicochemical properties, the distribution and mobility of heavy metals, and acid generation

Some phosphate sludges (Ps) were collected in 2009, 2014, and 2018. Results of their analyses by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM-SED), differential thermal and thermogravimetric analyses (TG-DTA) and X-ray fluorescence spectrometry (XRF) confirmed that the Ps are highly carbonated. Descriptive and exploratory statistical procedures for heavy metals (HM) concentrations and chemical parameters were performed and revealed the differences between the Ps of three different ages. The results indicate that there are three sources responsible for the behavior of Ps over years: common origin and accumulation of HIM and solar radiation exposure. The sequential extraction shows that the residual fractions of Cd, Ni, Zn, Pb, and Cr were predominant The mobility factor was quite low and did not change over the years, except for Cd. The net acid generation pH for the three samples was greater than 4.5 classifying the Ps as non-acid-generating.