## Materials and Methods
### Analytical Procedures  
The soil samples were air-dried, gently crushed to pass through a 2-mm sieve, and mixed thoroughly. Soil texture was determined by particle-size distribution analysis using a laser diffraction analyzer ([PERSON], 2000, Malvern Panalytical, Malvern, Worcestershire, UK). Soil reaction (pH), redox potential (Eh) and electrical conductivity (EC) were determined by stirring soil in deionized water at the ratio of 1:2.5 (g mL\({}^{-1}\)). All measurements were performed in triplicate and averaged. Aliquots of the sieved material (<2 mm) were ground in an agate mortar until getting a fine powder (<63 \(\upmu\)m) for analyses. Major crystalline phases were identified by X-ray powder diffraction (XRPD) using a BRUKER AXS D8-Advance diffractometer (Bruker Corp., Karlsruhe, Germany), with monochromatic CuK\(\alpha\) radiation operated at 40 kV and 30 mA. Scans of randomly oriented bulk powders were run from 3 to 70\({}^{\circ}\) 20 with a step size of 0.02\({}^{\circ}\) and a counting time of 0.6 s per step. The XRPD raw data were processed with the software MacDiff 4.2.6 (from [PERSON], Goethe-Universitat Frankfurt, Frankfurt, Germany) to precisely determine the position, intensity, and area of the diffraction peaks. Semi-quantitative estimates of mineral abundance were made by empirical intensity factors weighting the integrated peak area values of diagnostic reflections [34], with absolute errors averaging less than 3% [35]. Samples were examined by environmental scanning electron microscopy (ESEM) on a JEOL JSM-IT500 instrument (JEOL, Tokyo, Japan) coupled with energy dispersive X-ray spectroscopy (EDS). Back-scattered electron (BSE) images and EDS spectra were acquired at 20 kV accelerating voltage to assist in the identification of accessory minerals of environmental concern. The soil samples were subjected to multi-acid digestion (HF-HCl\({}_{4}\)-HNO\({}_{3}\)-HCl) followed by inductively coupled plasma-optical emission spectroscopy (ICP-OES) analysis to determine total concentrations of As, Cd, Cr, Cu, Ni, Pb, Sb, Tl, and Zn, all which are of environmental significance. The analyses were carried out at Activation Laboratories Ltd. (Ancaster, ON, Canada), which is compliant to the ISO/IEC 17/025 standard. Reliability of the analytical procedure was evaluated by the use of reagent blanks, certified reference materials (GRX-4, GRX-6, SDC-1, DNC-1a, OREAS-45d), and replicates to check accuracy and precision of the data. The average relative standard deviation (RSD) of the analyses was between 1.6 and 11.7%. Chemical partitioning of PHE in the topsoil of both the amended and the control soil was determined using an improved Community Bureau of Reference (BCR) sequential extraction method [36]. This protocol fractionates PHE into three operationally defined pools: (F1) exchangeable and weak acid soluble fraction (extraction with 20 mL 0.11 mol L\({}^{-1}\) acetic acid, CH\({}_{3}\)COOH); (F2) reducible fraction (extraction with 20 mL 0.5 mol L\({}^{-1}\) hydroxylammonium chloride, NH\({}_{2}\)OH:HCl); (F3) oxidizable fraction (digestion with 5 mL 8.8 mol L\({}^{-1}\) hydrogen peroxide, H\({}_{2}\)O\({}_{2}\), and extraction with 25 mL 1 mol L\({}^{-1}\) ammonium acetate, CH\({}_{3}\)COOH\({}_{4}\)); and (F4) residual fraction (open vessel, hot plate acid digestion with 10 mL HF, 3.8 mL HNO\({}_{3}\), 5 mL HClO\({}_{4}\), and 1.2 mL HCl). Following each extraction step, the extract was centrifuged at 5000 rpm for 10 min, then the supernatant was filtered through a 0.45-\(\upmu\)m membrane filter, and stored at 4\({}^{\circ}\) C until analysis. The PHE concentrations in the filtered soil extract were chemically analyzed by ICP-MS using an Agilent 7900 ICP-MS instrument (Agilent Technologies, Santa Clara, CA, USA), with detection and quantification limits of 0.01 \(\upmu\)g L\({}^{-1}\) and 0.03 \(\upmu\)g L\({}^{-1}\), respectively, for all the determinations. The sequential extraction procedure was repeated at least twice to ensure repeatability of results.