Lanthanide pore waters

The pore water composition of lanthanides is poorly characterized. Low concentrations and small volumes of pore water samples (typically 10-50 ml) combine to make detection difficult. Only the lanthanide-enriched pore waters of anoxic coastal and estuarine sediments have been measured at a few sites (sect. 6). Currently, the pore water chemistry of lanthanides in non-coastal marine sediments is unknown. The field of lanthanide pore water chemistry remains an important challenge. 

The application of lanthanides as indicators of paleoredox conditions must take into consideration their diagenetic chemistry (German and Elderfield 1990). More sensitive analytical methods are required before the lanthanide pore water chemistry of deep ocean sediments can be studied. This research subject is challenging and would provide answers to the transport rate of lanthanides across the sediment/water interface in pelagic enviromnents. 

Weltje L, Heideneeich H, Zhu W, Wolteebeek HT, Koehammee S, De Goeij JJM and Maekeet B (2002) Lanthanide concentrations in freshwater plants and mollusks, related to those in surface water, pore water and sediment. A case study in The Netherlands. Sci Total Environ 286 191-214. 

Inputs of lanthanides to seawater rivers and pore waters 583... 

This handbook article combines an up-to-date tabulation of the lanthanide composition of the ocean with a description of lanthanide distributions in the context of physical, chemical and biogeochemical processes controlling these distributions. The focus of this chapter is water column biogeochemistry. While pore waters and hydrothermal waters will be considered in this article, the extensive literature on the lanthanide geochemistry of minerals and marine sediments will not be discussed. 

Cerium, the only lanthanide with redox transformations at ambient oceanic conditions, has been exploited by geochemists to learn more about redox-controlled reactions in seawater. Background on the redox chemistry of Ce is presented in sect. 2. To emphasize a previous point, the Ce anomaly allows one to focus on the redox-controlled reactions of Ce by comparing its concentration to those of near neighbors (La and Nd in most cases cited next) which undergo no redox transformations. This section will focus on cerium and the Ce anomaly in open ocean seawater. The dynamic nature of the Ce redox chemistry in anoxic basins and pore waters will be discussed in the next section. 

Lanthanide cycling in anoxic marine basins, pore waters and hydrothermal waters... 

The objective of this section is to describe the major features of lanthanides in the water column of anoxic basins and briefly discuss the mechanisms responsible for spatial and temporal variations. Two examples will be used, the Black Sea and Chesapeake Bay. The next subsection on lanthanides in pore waters follows naturally in that many of the processes operating in anoxic water columns occur in marine sediments. 

A time-series study of Chesapeake Bay demonstrated that the dissolved lanthanides have a large seasonal cycle in both the water column and pore water in response to the development of anoxia in the spring and reoxygenation in the fall (Sholkovitz et al. 1992). Short-term variations in fractionation accompany the release and removal of lanthanides from the water column under seasonally-varying redox conditions. 

Large scale fractionation accompanies the seasonal cycle of the dissolved lanthanide concentrations in Chesapeake Bay. This is illustrated in fig. 37 which presents the time series of the Nd/Yb ratio for bottom waters (and upper pore waters). This light to heavy lanthanide ratio increases in the spring and is followed by an equally large decrease in the fall. At its maximum in the summer, the Nd/Yb ratio is three times greater than its winter baseline ratio. Hence, the lighter lanthanides are preferentially released in the spring and summer and then preferentially removed in the fall. 

What are pore waters and why are their compositions important with respect to the marine geochemistry of the lanthanides The term pore waters (or alternatively, interstitial waters) refers to the waters contained within the pores of sediments. Pore waters are extracted by different types of procedures. Centrifugation and squeezing of sediments are two common methods. The latter is accomplished by using hydraulic systems or gas pressure systems. In all methods the extracted waters are rapidly filtered (typically 0.22 or 0.45 [xm pore size) to yield dissolved matter for analysis. 

As described in sect. 3, small sample volumes (usually 10-50 ml) and low concentrations make it extremely difficult to measure the lanthanide concentrations of pore waters. Just as lanthanide concentrations in the water column increase several-fold under conditions of low or no oxygen, lanthanides in pore waters respond to redox conditions. There are a small number of studies of lanthanides in pore waters and they focus on sub-oxic and anoxic environments of estuaries and coastal regions. These include Chesapeake Bay (Sholkovitz and Elderfield 1988, Sholkovitz et al. 1992) Buzzards Bay (Elderfield and Sholkovitz 1987, Sholkovitz et al. 1989) and Saanich Inlet (German and Elderfield 1989). The first published paper on lanthanides in pore waters is less than a decade old (Elderfield and Sholkovitz 1987). All the above studies used ID-TIMS as the method of analysis. Pore water data are compiled in table A13. 

There is only one paper with data on the pore water composition of deep ocean sediments deposited under oxygenated water (Ridout and Pagett 1984). This paper mostly deals with extraction methods and provides poorly documented lanthanide data for a single sample. It is fair to state the lanthanide composition and chemistry of oxic marine sediments is unknown. 

Pore waters also exhibit large scale fractionation of lanthanides during release and removal processes. This is demonstrated by the time-series variations in Nd/Yb ratios (fig. 37). The summertime development of anoxic conditions results in the... 

In summary, the lanthanides undergo large scale diagenetic reactions under sub-oxic and anoxic conditions. Pore water concentrations increase greatly over those of oxic seawater. Large cerium anomalies develop and large scale fractionation occurs as the strictly-trivalent lanthanides are added to and removed from pore waters. These features develop in vertical profiles within sediments and in surface sediments exposed to seasonally varying redox conditions. 

Three-dimensional cationic lanthanide hydroxide frameworks structures have been prepared by using amino acids or other types of bridging ligands [56, 74]. These materials possess nanosized pores that are occupied by water and counterions from the original synthesis. These materials may find zeolitic applications such as their uses for anion exchange or occlusion and activation of electron-rich substrates. 

The anhydrous compound, whose chemical formula is Er(btc), has not been structurally characterized because of its lack of crystallinity and its physical properties have not been explored in details. This family is characteristic of the problem encoimtered when trying to constmct molecular based microporous materials, that is obstmction of the pores by interpenetration of networks or collapse of the stracture upon removal of the guest molecules intercalated in the cavities. This problem is, of course, also present for coordination polymers based on transition elements but it is much more crucial for lanthanide-based materials. Indeed, lanthanide ions present a high coordination number (generally ranging between 7 and 12) and their coordination sphere most often contains some solvent molecules. These coordination water molecules, when removed upon dehydration, lead to structural rearrangements and, sometimes, to a loss of the microporons character. 

---