Rapid evaluation of leaching potential of heavy metal elements from tunnel excavated rocks using handheld X-ray fluorescence spectrometer

Abstract

The effective assessment of elution of toxic heavy metal elements such as arsenic, selenium and lead from excavated rocks is one of the challenges in the safety evaluation in the tunnel construction in the volcanic field. We measured whole rock compositions of multi elements including As using Si-PIN type handheld-XRF instrument on 230 m long boring core from a tunnel construction where the elution of heavy metal elements was a concern. The measurement conditions are determined by pellet standards as 30 seconds for trace element of Cu, Zn, As, Se, Sb, Pb, Rb, Sr and Ba and 15 seconds for major elements of S, K, Ca, Ti, Fe and Mn. Under these conditions, the handheld-XRF measurement was carried out about 500 points at intervals of about 50 cm. The accuracy and detection limit were roughly about 10-20 mg/kg and 20% for trace elements and a few thousands mg/kg and 3-30 % for major elements. The result shows that changes in lithological description about color, weathering and hydrothermal alteration are represented by changes in covariation of heavy metal elements with other elements, such as between As and S, Fe and Mn. The horizontal variation of As composition by handheld-XRF of the core is consistent with the high As leaching (>0.3 mg/L) zone detected in a leaching test using a mixture of five samples taken at 10 m intervals from the core. This indicates that the handheld-XRF method on boring core sample is effective as a higher-resolution and rapid method for estimating or screening the leaching amount and trend of heavy metals from hydrothermally altered excavated rocks in volcanic field.