Development and validation of heavy metal retention index (HMRI) for the evaluation of heavy metal mobility in artificial infiltration facilities

In this study, the most important parameters governing metal mobility were first identified based on laboratory batch sequential extraction and isotope dilution experiments with three types of samples, i.e., surface soil, underlying soil (depth > 1.0 m) and soakaway sediment collected at Tokyo (n = 3). The selected parameters were first brought to normalization step, where each parameter was transformed into a 0–100 % scale, followed by assigning relative weight factors reflecting the importance of each parameter for metal retention property. Heavy Metal Retention Index (HMRI) then combined the different parameters in a unique number (index) to facilitate the comparison. In this way, hierarchical levels of normalized factors were obtained in the form of HMRI to make the decision. Properties that were evaluated in this study were of three types: i) classical physicochemical properties, i.e., the content of clay, organic, Fe-Mn oxide, silicates and CEC and thus varied with a particular sample; ii) specific variables for each metal like isotopic exchangeability (% E-value), sorption coefficient (K_d) and exchangeable fraction which varies with each metal type viz. Cu, Zn and Pb; and iii) integrated properties (both sample-specific and metal-specific properties) using which HMRI were developed. Finally, all these results were compared with column results obtained after 10 days of continuous leaching with artificial road runoff (ARR), prepared in the laboratory using highway road dust and deionized water (liquid/solid ratio = 25 L/kg).