Characterisation of Suspended Particles Collected from an Estuary in an Urban and Industrialised Centre using Magnets onboard an Autonomous Underwater Vehicle Paulo A. de Souza Jr., Andrew Davie, G. Timms Tasmanian ICT Centre, CSIRO GPO Box 1538 Hobart, TAS 7001 Australia Edward C. V. Butler Wealth from Oceans Flagship, Marine Laboratories CSIRO, Hobart 7000 TAS, Australia Vijayendra Kumar Garg Instituto de Fisica Universidade de Brasilia Brasilia, DF 70919-970 Brazil Abstract- An Autonomous Underwater Vehicle (AUV) equipped with rare-earth (Sm-Co) magnets was used to sample suspended particles during its transects of the Derwent estuary in Tasmania. Mineral characterization was then performed in the laboratory using 57 Fe Mössbauer spectroscopy (MB) and X-ray diffractometry (XRD). Results indicate the sources of the suspended particles to be both natural and industrial. I. INTRODUCTION Traditional survey methods for estuarine and inshore marine suspended particles and sediments, involving sediment grabs or even diver-deployed probes, are proving inadequate for current environmental management needs [1]. They can also prove costly in both time and logistics, often demanding prolonged use of small boats with several field staff, can potentially place these staff in high-risk environments. In addition to these aspects, sampling is usually restricted to limited sites and as a result poor in spatial resolution. AUVs are being used as a new frontier technology for benthic mapping and water quality data recording [2]. Here, we propose the use of AUVs to sample suspended particles for later characterization in the laboratory. In a typical mission, distances between 4 and 20 km can be navigated. Provenance of sampled particles can be determined, since engineering data (e.g., position, depth) are recorded, and for the Derwent region, outputs from hydrodynamic and sedimentary resuspension models are available. The characterisation of suspended particles allows us to determine the origin of the material; and therefore, this information is in demand for environmental management of industrialized urban estuaries. II. EXPERIMENTAL A. Magnetic Sampling Samples are collected during each deployment of the AUV using a pair of rare-earth magnets (Sm-Co) placed beside the autonomous underwater vehicle (Figure 1). Each magnet consists of a disk of 2 cm in diameter and 1 cm wide. The missions typically last for 4-6 hours and traverse 5-20 km. Figure 1. The AUV developed at CSIRO used to sample suspended particles in the Derwent Estuary. This AUV is 1.2m long and the magnets are placed where the red circles are indicated. B. 57 Fe Mössbauer spectroscopy Typical MB spectra recorded for a suspended particle sample are shown in Figure 2. The Mössbauer experiments, both at room temperature (298 K) and liquid nitrogen temperature (78 K), were performed in the standard transmission geometry with a 57 Co/Rh source of 100mCi of activity. The least-square fitted MB parameters were calculated using a linear combination of Lorentzians lines optimised using genetic algorithms [3]. The Mössbauer analysis is non-destructive, so all samples could be kept for other experiments. Relative concentrations of each Fe-bearing mineral phase is shown in Table I. C. X-Ray Diffractometry The powder XRD analysis was performed with a Siemens D500 powder diffractometer equipped with a CuK α radiation source.