Barite Recrystallization to Witherite in the Presence of Carbonate, and the Impact on Radium Retention&#160;
Authors: Alzaydan Mohammed , Roth Thomas , Heberling Frank , Polly Robert , Schild Dieter , Metz Volker 
Published: March 2022

&lt;p&gt;In deep geological repositories for nuclear waste, groundwater may saturate the (geo-) technical barriers, and waste container corrosion may be occurring. This may finally lead to radionuclides being released from the waste [1]. The sorption capacity of potential host rocks, such as crystalline rocks and clay rocks, and of potential secondary phases is an essential aspect in the Safety Case. Barite (BaSO&lt;sub&gt;4&lt;/sub&gt;) can be present in crystalline rocks as a fracture filling material. In clay rocks, sulfate minerals ((Ba,Sr)SO&lt;sub&gt;4&lt;/sub&gt;) are found as accessories. Barium (Ba) is present in the waste as a fission product and as sulfate containing groundwater contacts the waste, barite may form. Ra uptake by barite occurs when dissolved Ra reacts with barite, leading to Ra retention by (Ba,Ra)SO&lt;sub&gt;4&lt;/sub&gt; solid-solution formation [2]. Barite reaction with aqueous carbonate at high pH, e.g. due to cementitious material alteration inside a repository, may cause barite to convert into witherite (BaCO&lt;sub&gt;3&lt;/sub&gt;) [3]. The presence of carbonate likely alters the chemical behaviour of barite surfaces, via surface mixing or by witherite layer formation through dissolution-precipitation [4]. Aim of this study is to investigate the effect of carbonate on Ra retention by barite.&lt;/p&gt;&lt;p&gt;The presented results highlight barite transformation into witherite in the absence and presence of Ra. Barite suspensions were contacted with carbonate solutions (1-100 mM, pH 7-11). X-ray diffraction (XRD) showed shifts of reflection peaks in batches with pH 9-11 and carbonate concentrations of 10-100 mM, indicating a witherite-barite solid-solution formation [3], though this is not energetically favorable according to Density Function Theory calculations. Reaction rates depend on the type of barite. Generally, reactions slow down after about 7 days, likely due to surface passivation. Cubes of natural barite were reacted with 100 mM carbonate solutions at pH 11. As a result, a porous witherite layer with (271&amp;#177;32) &amp;#956;m formed, giving insight into recrystallization process progress and mechanism. Atomic Force Microscopy measurements were conducted for witherite&lt;sub&gt;&lt;/sub&gt;growth. Barite was equilibrated at 35&amp;#176;C and contacted with a carbonate solution (0.01M, pH 10). Orientated particles with different shapes grew. More measurements will be conducted with different conditions to obtain a distinct growth. For Ra uptake, witherite batches&lt;sub&gt;&lt;/sub&gt;(S/L 0.01, pH 8.7) were prepared for Ra to be spiked subsequently. &amp;#160;&lt;/p&gt;&lt;p&gt;We acknowledge KACST funding and the German federal ministry for education and research for funding the travel through the collaborative project KRIMI, grant agreement 02NUK056A.&lt;/p&gt;&lt;p&gt;&amp;#160;&lt;/p&gt;&lt;p&gt;[1]&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160; Curti, E., et al. (2010) Geochim. Cosmochim. Acta 74.12, 3553-3570.&lt;/p&gt;&lt;p&gt;[2]&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160; Heberling et al. (2018) Geochim. Cosmochim. Acta 232, 124-139.&lt;/p&gt;&lt;p&gt;[3]&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160; Rend&amp;#243;n-Angeles et al. (2008) J. Materials Science 43, 2189-2197.&lt;/p&gt;&lt;p&gt;[4]&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160; Putnis and Putnis (2007) J. Solid State Chemistry 180, 1783-1786.&lt;/p&gt;&lt;p&gt;&amp;#160;&lt;/p&gt;