Modelling the formation of granules: the influence of manganese ions on calcium pyrophosphate precipitates Ombretta Masala, Eric J.L. McInnes, Paul O’Brien * Manchester Materials Science Centre and Department of Chemistry, The University of Manchester, Oxford Road, M13 9PL Manchester, UK Received 26 November 2001; accepted 1 February 2002 Dedicated in honor of Professor Helmut Sigel Abstract A large number of invertebrates form granules of amorphous inorganic material. Such granules are principally composed of calcium pyrophosphate but also act to accumulate of a wide range of metals, such as copper, iron, manganese and zinc. The mechanism of the formation of these granules is not clear. In the present work some aspects of the chemistry associated with the formation of such materials in vitro have been studied. The synthesis of calcium pyrophosphate, doped with manganese(II) ions at different [Mn]:[Ca] molar ratios has been investigated at pH 7 and 25 8C. Speciation of the Ca  /P 2 O 7 and Ca  /Mn  /P 2 O 7 systems in the pH range 1  /14 has also been modelled. Considerable variations occur in the structure of calcium pyrophosphate formed under close to equilibrium growth conditions when manganese ions are present in solution. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Barnacle; Detoxification; Heavy metal; Pyrophosphate granule; Speciation 1. Introduction Invertebrates accumulate trace metals in their tissues whether or not these metals are essential to their metabolism [1  /3]. Different invertebrates accumulate metals to varying extents and accumulated concentra- tions vary greatly at tissue, organ and body level [4]. In most invertebrates a central role in the metabolism of metals has been attributed to the hepatopancreas [5,6]. Barnacles are mussels that differ from other inverte- brates as they lack a hepatopancreas. Possibly because of the absence of this organ, they accumulate trace metals mainly into granular deposits located in the body cells of the stratum perintestinale. These deposits are usually amorphous with spherical shapes and diameters ranging between 1 and 10 mm [7,8]. The granules frequently contain pyrophosphate as the main anion and calcium as the major cation. In addition, a variety of other metals, such as aluminium, iron, manganese and zinc [9  /11] accumulate in these materials. Barnacles have been shown to concentrate metals in amounts proportional to those of the surrounding marine envir- onment [12] and because of this peculiar property they have been used as biological monitors of pollution in many countries [13  /15]. It has been proposed [16] that the deposits of calcium pyrophosphate (Ca 2 P 2 O 7 ) act as traps for foreign metal ions that could be exchanged with calcium and then incorporated inside the granules. This ion exchange appears to be facilitated by the amorphous structure of the deposits since the random network structure enables the movement of ions from the granule into the solution and vice versa. It has been also suggested that the formation of these granules in intracellular membrane-bound compart- ments is responsible for their specific composition, morphology and amorphous nature [17,18]. However, little is known about the factors that control the accumulation of trace metals in the granules. An understanding of the role of cellular detoxificatory processes for trace metal ions into granules will need an understanding of their chemical properties. * Corresponding author. Tel.:  /44-161-275 4653; fax:  /44-161-275 4616 E-mail address: paul.obrien@man.ac.uk (P. O’Brien). Inorganica Chimica Acta 339 (2002) 366  /372 www.elsevier.com/locate/ica 0020-1693/02/$ - see front matter # 2002 Elsevier Science B.V. All rights reserved. PII:S0020-1693(02)00936-2