OR/14/067 Introduction

Lapworth, D J, Surridge, B, Williams, P J, Heaton, T H E, and Gooddy, D C. 2014. Method for analysing phosphate 18O/16O ratios for waters with high C:P ratios. British Geological Survey Internal Report, OR/14/067.

There has been growing interest in recent years in the application of phosphate δ¹⁸O for investigating sources and processing of P in the aquatic environment (e.g. McLaughlin et al., 2004; Elsbury et al., 2009; Chang and Blake 2014). It is now widely acknowledged that these early methods first used for ocean and estuary studies are not ideal for samples with high C:P ratios, such as groundwater, river water and treated sewerage waters, due to the potential for co-precipitation of organics with Ag₃PO₄. For such samples, the McLaughlin (2004) method has been found to not fully remove organics with resulting C contents >1% possible in samples used for analysis. This can potentially lead to δ¹⁸O values that are not specific to PO₄, but rather are a mixture of organic oxygen and PO₄ oxygen, leading to erroneous values if the two pools have different δ¹⁸O signatures. This report brings together work undertaken at the BGS Wallingford laboratory to develop and use a method for extracting phosphate (PO₄) from waters, with high C:P ratios, for phosphate δ¹⁸O characterisation.

Anion exchange resins have been used for several decades (e.g. Hoering 1957; Qian et al., 1992) to isolate nutrients for elemental and isotope analysis including phosphate δ¹⁸O analysis of rocks (Crowson et al., 1991), and more recently for waters (e.g. Gross et al., 2013). DAX-8 resins have been used for removal of organics prior to phosphate δ¹⁸O analysis for soil extractable P (Tamburini et al., 2010) and waters (Gross et al., 2013). Li et al (2011) presented a modified McLaughlin et al (2004) method which involved recycling steps within the original method to remove organics and obtain a more pure Ag₃PO₄ precipitate. However, this is a very time consuming alternative to that presented here, and independent tests carried out at the BGS laboratories suggest that it may not be a fully effective method and reduced P recoveries are also inevitable. This method includes the use of a front-end two-step column system first employed by Dr Ben Surridge (co-author from Lancaster University) which combines the use of column resins in series to (i) remove the majority of dissolved organic carbon and (ii) isolate/pre-concentrate PO₄. This is followed by the use of a modified McLaughlin et al (2004) method for PO₄ precipitaiton, with an additional final hydrogen peroxide clean-up step to remove residual organic matter following precipitation of Ag₃PO₄. The reagents required for this method are listed and then a step-by-step account of the process is outlined. Importantly, we hope that it contains adequate detail to be used by other researchers in this field or modified to suit their particular research objectives.