Ex situ treatment of sediments with granular activated carbon : a novel remediation technology

Over the last decades, industrial and urban development and emisions of many hazardous organic compounds have threatened the ecological quality of marine and freshwater sediments. Sediments accumulate hydrophobic organic compounds (HOCs) such as polycyclic aromatic hydrocarbons (PAHs), polychlorinated biphenyls (PCBs) and thus may pose serious risks to ecosystems and human health. Over the past years sediment treatment by sorbent addition such as activated carbon (AC) to achieve sequestration of HOCs <em>in situ</em> has been proposed as an alternative approach to traditional remediation technologies such as dredging and disposal. The present research was meant to explore <em>ex situ</em> extraction of sediment by granular AC (GAC) (‘active stripping’) as a novel approach in comparison to traditional <em>in situ</em> AC sediment remediation technologies using amendments of powdered AC (PAC) or GAC. <strong>Chapter</strong><strong>2</strong> discusses the current state of the art in AC amendment technology as a method for sediment remediation. In this chapter, major knowledge gaps are revealed on sediment-AC-HOC interactions controlling the effectiveness of HOC binding such as AC type, particle size, dosage, sediment and sorbate characteristics,and efficiency of AC to reduce bioaccumulation in benthic invertebrates.In addition, the review discusses potential negative effects of AC on aquatic life. Finally, we discuss whether the effects of AC addition can be predicted using fate and transport models. <strong>Chapter 3</strong>explores the potential of GAC in the context of <em>ex situ</em> sediment remediation technology. Since the added GAC would compete for the sorption of HOCs with natural sediment phases, its effectiveness would strongly depend on its dosage. Consequently, in this chapter we investigate the distribution coefficients for short-term sorption processes, and the optimal dosage level of GAC to be used in intensive sediment remediation. A suite of candidate GAC materials is screened for maximum efficiency in extracting PAHs from sediment with very high PAH and oil pollution levels within 24 h. The effectiveness of GAC is compared to a single-step solid phase extraction (SPE) with Tenax beads, Sorption data are interpreted in terms of aqueous phase concentration reduction ratios and distribution coefficients. Despite the considerable fouling of GAC by organic matter and oil, 50-90% of the most available PAH was extracted by the GAC during 28-d contact time, at a dose as low as 4%. A prerequisite for the application of active stripping with GAC in contaminatedsediment remediation is effective transport of pollutantsfrom the sediment to the GAC during the relatively short mixing stage. Therefore, in <strong>Chapter 4</strong>kinetics of PAH transfer from sediment using GAC at a relatively low dose as a solid extraction phase kinetic parameters are obtained by modeling experimental sediment-GAC exchange kinetic data following a two-stage model calibration approach. Rate constants (<em>k_GAC</em>) for PAH uptake by GAC range from 0.44 to 0.0005 d^-1, whereas GAC sorption coefficients (<em>K</em>_GAC) range from 10^5.57to 10^8.57L kg^-1. These results show that <em>ex situ</em> extraction with GAC is sufficiently fast and effective to reduce the risks of the most available PAHs among those studied, such as fluorene, phenanthrene and anthracene. It is unclear how the GAC/sediment mixing step affects desorption kinetics of HOCs for instance by changing the sediment particle size distributions, and whether these factors may influence the effectiveness of <em>ex situ</em> GAC extraction technology. <strong>Chapter 5</strong> presents the results of investigations on the effect of mixing intensity on the extraction rate of PAHs from contaminated sediment. Desorption data are interpreted using a radial diffusion model. Mixing caused the 161 µm particles originally present at a stirring rate of 200 rpm to decrease in size to 9 µm at a rate of 600 rpm. Desorption rate constants decreased with increasing PAH hydrophobicity but increased with the intensity of mixing. The results demonstrate that desorption of PAHs is significantly accelerated by a reduction of particle aggregate size caused by shear forces induced by mixing. So far, the remediation effectiveness and ecological side effects of AC application have been studied in the short term, and mainly in laboratory studies. However, it is still not clear to what extent these reduced pore water concentrations change over longer times and how they differ for chemicals and for different AC remediation scenarios under field conditions. <strong>Chapter 6</strong> presents (pseudo-)equilibrium as well as kinetic parameters for <em>in situ </em>sorption of a series of PAHs and PCBs to powdered and granular activated carbons (AC) after three different sediment treatments: sediment mixed with powdered AC (PAC), sediment mixed with granular AC (GAC), and addition of GAC followed by 2 d mixing and subsequent removal (‘sediment stripping’) in the field. Remediation efficiency is assessed by quantifying fluxes towards SPME passive samplers inserted in the sediment top layer, which shows that efficiency decrease in the order of PAC > GAC stripping > GAC addition. Sorption was very strong to PAC, with log <em>K</em>_AC (L/kg) values up to 10.5. Log <em>K</em>_AC values for GAC ranged from 6.3 - 7.1 and 4.8 - 6.2 for PAHs and PCBs, respectively. Log <em>K</em>_AC values for GAC in the stripped sediment were 7.4 - 8.6 and 5.8 - 7.7 for PAH and PCB. Apparent first order adsorption rate constants for GAC (<em>k</em>_GAC) in the stripping scenario were calculated with a first-order kinetic model and ranged from 1.6×10^-2 (PHE) to 1.7×10^-5 d^-1(InP). This study showed that sediment treatment with PAC is most effective and less prone to organic matter fouling and ongoing natural processes in the field. The effectiveness of GAC is higher in the 48 h sediment stripping scenario than in the GAC amendment approach. In <strong>Chapter 7</strong> the effects of three different AC treatments (see above) on HOC concentrations in pore water, benthic invertebrates, zooplankton and fish (<em>Leuciscus idus melanotus</em>) are tested. The AC treatments result in a significant decrease in HOC concentrations in pore water, benthic invertebrates, zooplankton, macrophytes and fish. In 6 months, PAC treatment caused a reduction of accumulation of PCBs in fish by a factor of 20 bringing pollutant levels below toxic thresholds. All AC treatments supported growth of fish, but growth was inhibited in the PAC treatment, which is likely to be explained from reduced nutrient concentrations, resulting in lower zooplankton (i.e., food) densities for the fish. During the course of the field study, sediment stripping as well as sediment treatment with GAC turned out to be slower in reducing PCB bioaccumulation in biota, but the treatments were not harmful to any of the biota either. In the final chapter (<strong>Chapter 8</strong>), overarching answers to the main research questions (see above) are formulated and an outlook regarding the actual use of <em>ex situ</em> GAC is provided.