The interaction of sedimentary organic matter with metals and organic pollutants in landfill barriers

Abstract

This research examines the composition, sedimentary history and chemical properties of sedimentary organic matter [SOM] and its application in determining the content and source of heavy metals and hydrophobic organic contaminants [HOCs] and their fate in waste disposal sites.
Previous research has utilised Oxford Clay (OxC) from across the outcrop. In this study a continuous shallow core of OxC and adjoining Kellaways Sand (KS) from Brogborough, UK was used. This avoided the influence of different sedimentary conditions, thereby reducing the problem of sample variability. Relationships have been made between sedimentary and organic petrology and SOM abundance and metals concentration (V, Cr, Co, Ni, Cu, Zn and As). The co-association/binding of different metals to different types of SOM components from the OxC was shown to be dictated by the depositional environment.
The organic rich parts (Total Organic Carbon (TOC) > 5 %) of the OxC displayed positive correlations between the elements V, Co, As and total pyrite % (p<1.1E-3, ? >0.7); however, there was an apparent negative correlation with TOC % (p<4.3E-5, ?<-0.4). This may be because of: 1) the rapid accumulation of the OM reduced the time for development of the pyrite (Ave. dia. =5.3?m, SD=2.4); and/or 2) the insufficient concentration of Fe to allow pyrite formation. This could be due to the high sedimentation rate that accelerates the accumulation of OM, which prevents its decomposition in the water column, as well as reducing the time for pyrite that is entrapped in the marine snow to capture metals from seawater during its settlement.
Cu has a significant positive correlation with TOC (p=1.9E-2, ?=1.3), which could be the consequence of Cu being an essential element in OM.
The OxC with a lower OM content (TOC < 5 %) showed a different distribution of metals. V, Cr and Cu found in OM was found to have a significant positive correlation with TOC (p<2.9E-2, ?>0.05).
The Kellaways Sand core showed that the V, Co, Ni, Cu, Zn and As had a strong connection both with TOC (p<1.6E-3, ?=0.03~37.3) and pyrite (p<4.9E-3, ?=0.23~5.5), suggesting that both OM and pyrite were major metal sinks in Kellaways Sand. The more significant positive correlation between V, Cr and pyrite compared to V, Cr and TOC implies that pyrite contains more V and Cr than did TOC.
Other studies used the OxC from Bletchley, the Kimmeridge Clay (KC) from Kimmeridge Bay, Dorset, and Tertiary mud (Wittering Formation-WF) from Whitecliff, Isle of Wight due to their distinctive OM characteristics (AOM rich and/or phytoclast rich). Organic material was isolated for identification and analysis using a novel extraction method (heavy liquid extraction) and traditional methods involving HF digestion. These organic materials were then used to determine influences of extraction on HOCs (toluene and naphthalene) sorption and desorption. Organic petrology classification was applied to identify the various types of isolated OM. AOM from KC displayed a higher sorption capacity (Kd=6,481, 59,670; for toluene and naphthalene, respectively) compared to literature values. AOM rich sorbent extracts demonstrated a higher absorption capacity than the phytoclasts rich sorbents (e.g. WF, Kd=219, 20,661; for toluene and naphthalene, respectively). However, the phytoclast rich sorbent showed a higher sorption/desorption hysteresis capacity for toluene than AOM. Implications of results in landfill design/risk assessment and modelling are discussed.

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ORCID for John Marshall: orcid.org/0000-0002-9242-3646

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