Sorption of PAHs and PCBs to activated carbon: Coal versus biomass-based quality
Highlights
► We studied differences between biomass and coal-based Active Carbon (AC). ► Biomass AC showed a narrower pore size than coal-based AC. ► Clean biomass AC showed a stronger HOC sorption than coal-based AC. ► In the presence of sediment, this trend was reversed. ► The observations be explained by a pore clogging mechanism.
Introduction
Amendment with activated carbon (AC) is a promising in situ technique for the remediation of sediments contaminated by hydrophobic organic compounds (HOCs) such as polyaromatic hydrocarbons (PAHs) and polychlorinated biphenyls (PCBs). The principle of the technique is that HOCs are sorbed so strongly by the AC that the equilibrium porewater HOC concentration is decreased, leading to reduced bioaccumulation in benthic organisms and sediment-to-water fluxes (Cho et al., 2009, Beckingham and Ghosh, 2011, Cornelissen et al., 2011, Ghosh et al., 2011). During the past few years the AC amendment technique has reached the stage of field pilot testing in the USA and Europe (Ghosh et al., 2011). An important remaining question is which type of AC is most suitable in which situation, both with regard to pollutant sequestration and the overall impact of the remediation.
AC is generated by chemical or steam activation of charred residues. These charred residues can be made from several starting materials that can be divided into two groups; fossil (bituminous coal or anthracite) or biomass sources (for example coconut shells or peat). A recent complete life-cycle analysis of sediment remediation methods, where natural recovery was compared to thin capping with clay, limestone, biomass-based AC and coal-based AC, revealed that biomass-related AC is more sustainable than coal-based material (Sparrevik et al., 2011), due to the carbon sequestration effect that accompanies the placement of biomass-based AC on the seafloor, similar to soil amendment with biochar (Lehmann, 2007).
Another interesting observation is that AC blended in sediment often sorbs around one order of magnitude less extensively than clean AC (Cornelissen et al., 2006, Werner et al., 2006, Hale et al., 2009), probably due to pore throat clogging by natural organic matter (NOM) and/or oil, as well as competition among various HOCs for sorption sites (Kwon and Pignatello, 2005). There are few studies that compared surface area and porosity of AC made from various feedstocks. Two studies found that biomass-based AC possesses narrower pores than coal-based AC (McDougall, 1991, Azargohar and Dalai, 2011). Other studies describe either coal-based AC only (Ahmadpour and Do, 1998) or mention the narrow pore system and high surface area of biomass-based AC (Jagtoyen and Derbyshire, 1993, Cao et al., 2006, Yunos et al., 2011). Branton and Bradley (2011) found higher rates of adsorption for AC qualities with wider pores. The question is whether differences in porosity and surface area between AC qualities have implications for their sorption behavior magnitude in the presence and absence of natural matrices the AC is amended to.
This is the first study that aims to compare the pollutant sequestration ability of biomass-based and coal-based AC in a geosorbent system. To this end, we measured sorption isotherms for one probe PAH and one PCB in both types of AC. In order to compare the attenuation effect for the two AC qualities, sorption to the same ACs was also determined in the presence of a contaminated harbor sediment. Both AC types were characterized with regard to pore size distribution and surface area.
Section snippets
Materials
Chemicals were of analytical-grade purity or higher and purchased from Merck or Fluka (Oslo, Norway). PAH and PCB standards were purchased from Fluka (Oslo, Norway). Solvents for gas chromatography (GC) application and the Silica gel 100 for chromatography (particle size: 0.063–0.200 mm) were purchased from Merck (Oslo, Norway). Two AC qualities from Jacobi Carbon (Kalmar, Sweden) were used: one coal-based material fabricated from anthracite (BP2 fine powder) and one biomass-based material
Pore size distribution of the activated carbons
The surface areas and pore size distributions were different for the two AC qualities used in the present study (Table 2, Fig. 1). The anthracite-based AC (BP2) provides a rather even distribution of the surface area over various pore sizes (Fig. 1), including a large portion of pores >15 Å (541 m2 g−1). In contrast, a clear dominance of pores in the size range of 3.5–15 Å was observed for the biomass-based AC (CP1; Fig. 1), and almost 10 times lower pore surface area >15 Å (61 m2 g−1) than the BP2
Acknowledgements
Sarah Hale (NGI) is thanked for constructive comments and linguistic edits on the manuscript. The Research Council of Norway, project “Opticap”, number 182720, funded the study.
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