A new spectrofluorometric method for the determination of total arsenic in sediments and its application to kinetic speciation

Abstract

A simple and sensitive spectrofluorometric method has been developed for the determination of arsenic (As) using rhodamine-B as a fluorescent agent. This method is based on the reaction of As(III) with potassium iodate (KIO₃) in acid medium to liberate iodine, which decreases the fluorescence intensity of rhodamine-B. This decrease in intensity was used to quantify As(III). A linear decrease in the response was observed with the increasing As(III) concentrations. An R² value of 0.995 was obtained. As(III) in the concentration range of 0.4 µg  mL⁻¹ to 12.5 µg mL⁻¹ showed linearity and reproducibility by this method. The limit of quantitation was found to be 0.4 µg mL⁻¹ of As(III) was the limit of quantitation. This method was successfully used to determine the total concentration of As in coastal and estuarine sediments. This study suggests that the estuarine sediments were more contaminated with As than the coastal sediments and the probable source of high As content in estuarine sediment is agricultural sewage in the study area. The concentrations of dynamic (non-residual) As species in sediments which can be subsequently released to the overlying water column as a result of either physical or biogeochemical disturbances was determined by using this method. It was observed that total As content does not correlate with the dynamic fraction of As in the sediments. Total organic carbon present in the sediments played a crucial role in controlling its bioavailability. Dissociation rate constants of As-sediment complexes were successfully determined by using this newly developed spectrofluorometric method.

Keywords:

• arsenic determination
• rhodamine-B
• spectrofluorometry
• non-residual arsenic
• coastal sediment
• kinetic speciation

Acknowledgements

We are thankful to the Director, NIO, Goa for his encouragement and support. We would also like to thank Dr Dileep Kumar, NIO, Goa for his constructive feedback. We also acknowledge Mr N.P.C. Reddy, NIO, Visakhapatnam for providing sediment standards. This work is a part of the Council of Scientific and Industrial Research (CSIR) supported Supra Institutional Project (SIP 1308). P. Chakraborty dedicates this work to the memory of his beloved PhD supervisor, the late Prof C.L. Chakrabarti from Carleton University, Canada, who passed away on 1 January 2010. This article bears NIO contribution No. 4774.