Per- and polyfluoroalkyl substances in soil and sediments: Occurrence, fate, remediation and future outlook.

Perfluoroalkyl and polyfluoroalkyl substances (PFASs) are contaminants of great concern due to their wide-spread occurrence and persistence in the environments (i.e., in water, soil and sediment) and potential toxicology even at very low concentration. The main focus of this review is on the PFASs in soil and sediments. More specifically, this review systematically examines the occurrence and toxicological effects with associated risks, fate (i.e., PFASs adsorption by soil and sediment, transportation and transformation, and bioaccumulation), and remediation practices of PFASs in soil and sediment. Various models and equations such as fugacity-based multimedia fate and hydrodynamic models are used to study the fate, transport, and transformation of PFASs. Among different remediation practices, sorption is the dominant process for the removal of PFASs from soil and sediments. Results also indicate that PFASs adsorption onto activated carbon decrease with the increase of carbon chain length in the PFASs. The longer-chain PFASs have larger partition coefficient values than shorter-chained PFASs. Sorption of PFASs to soil and sediments are mainly governed by different electrostatic interactions, hydrogen bonds formation, hydrophobic interactions, organic content in soil and sediments, and ligand exchange. Other technology such as thermal treatment might be potential in the removal of PAFSs, but need further study to elucidate a conclusion. Finally, the associated challenges and future outlook have been included.

A comprehensive review on the framework to valorise lignocellulosic biomass as biorefinery feedstocks.

An effective pretreatment is the first step to enhance the digestibility of lignocellulosic biomass - a source of renewable, eco-friendly and energy-dense materials - for biofuel and biochemical productions. This review aims to provide a comprehensive assessment on the advantages and disadvantages of lignocellulosic pretreatment techniques, which have been studied at the lab-, pilot- and full-scale levels. Biological pretreatment is environmentally friendly but time consuming (i.e. 15-40 days). Chemical pretreatment is effective in breaking down lignocellulose and increasing sugar yield (e.g. 4 to 10-fold improvement) but entails chemical cost and expensive reactors. Whereas the combination of physical and chemical (i.e. physicochemical) pretreatment is energy intensive (e.g. energy production can only compensate 80% of the input energy) despite offering good process efficiency (i.e. > 100% increase in product yield). Demonstrations of pretreatment techniques (e.g. acid, alkaline, and hydrothermal) in pilot-scale have reported 50-80% hemicellulose solubilisation and enhanced sugar yields. The feasibility of these pilot and full-scale plants has been supported by government subsidies to encourage biofuel consumption (e.g. tax credits and mandates). Due to the variability in their mechanisms and characteristics, no superior pretreatment has been identified. The main challenge lies in the capability to achieve a positive energy balance and great economic viability with minimal environmental impacts i.e. the energy or product output significantly surpasses the energy and monetary input. Enhancement of the current pretreatment techno-economic efficiency (e.g. higher product yield, chemical recycling, and by-products conversion to increase environmental sustainability) and the integration of pretreatment methods to effectively treat a range of biomass will be the steppingstone for commercial lignocellulosic biorefineries.

Laser-induced fluorescence logging as a high-resolution characterisation tool to assess LNAPL mobility.

High-resolution characterisation tools such as Laser-Induced Fluorescence (LIF) logging represent a step forward towards a more effective management of sites contaminated by light non-aqueous phase liquid (LNAPL) petroleum hydrocarbons. In this paper, the applicability of LIF response as an indicator of LNAPL mobility at one site with an unconsolidated aquifer was investigated. LIF profiles were logged adjacent to twin coring locations and wells with LNAPL transmissivity (Tn) measurements in a heterogeneous gasoline contaminated site in Western Australia. LIF response was correlated to Tn to a greater extent than LNAPL saturation (Sn) measurements from coring. In particular, LIF signal maxima were a better indicator of Tn than the integral LIF signal. Furthermore, LIF allowed rapid identification of areas with long-term near-immobile LNAPL (entrapped and residual) because of the multi-wavelength waveforms associated with distinct subsurface characteristics. It was also demonstrated that the delineation of presumably less-mobile intervals could be enhanced by using the relative LIF response in the 350 nm wavelength channel. Thus, this work gave evidence that LIF logging provides valuable information about LNAPL distribution and mobility in commonly found subsurface settings, despite generally poor correlations with Sn measurements. LIF probes can be successfully used to guide the installation and application of more costly conventional methods in addition to the development of existing site models.

LNAPL transmissivity as a remediation metric in complex sites under water table fluctuations.

Water table fluctuations affect the recoverability of light non-aqueous phase liquid (LNAPL) petroleum hydrocarbons. LNAPL transmissivity (Tn) is being applied as an improved metric for LNAPL recoverability. In this paper, the applicability of Tn as a lagging and leading metric in unconsolidated aquifers under variable water table conditions was investigated. Tn values obtained through baildown testing and recovery data-based methods (skimming) were compared in three areas of a heterogeneous gasoline contaminated site in Western Australia. High-resolution characterisation methods were applied to account for differences in the stratigraphic profile and LNAPL distribution. The results showed a range of Tn from 0 m2/day to 2.13 m2/day, exhibiting a strong spatial and temporal variability. Additionally, observations indicated that Tn reductions may be more affected by the potentiometric surface elevation (Zaw) than by the application of mass recovery technologies. These observations reflected limitations of Tn as a lagging metric and a Remedial Endpoint. On the other hand, the consistency and accuracy of Tn as a leading metric was affected by the subsurface conditions. For instance, the area with a larger vertical LNAPL distribution and higher LNAPL saturations found Tn to be less sensitive to changes in Zaw than the other two areas during the skimming trials. Tn values from baildown and skimming tests were generally in a close agreement (less than a factor of 2 difference), although higher discrepancies (by a factor up to 7.3) were found, probably linked to a preferential migration pathway and Zaw. Under stable Zaw, Tn was found to be a relatively reliable metric. However, variable water table conditions affected Tn and caution should be exercised in such scenarios. Consequently, remediation practitioners, researchers and regulators should account for the nexus between Tn, LNAPL distribution, geological setting and temporal effects for a more efficient and sustainable management of complex contaminated sites.

Non-equilibrium zinc uptake onto compost particles from synthetic stormwater.

Zinc uptake onto different particle size compost was evaluated in batch and column studies using a synthetic stormwater to quantify sorption capacity and kinetics. The results showed that the pseudo equilibrium time for uptake increased from 2h to greater than 120h as the particle size of compost increased from 75µm to 6.75mm. This was due to intra-particle diffusion becoming a rate limiting process as the particle size increased. Column effluent data with 1.18mm particles could be fitted by Freundlich isotherm while that from the 4.75mm particles and a mixed particle size columns showed rate limited sorption with tailing and could not be adequately fitted using an equilibrium based isotherm. The results have established rate-limited sorption in amended filtration media due to larger particles under these flow conditions. This needs to be accounted for in the design of these filtration media and during performance modelling.

Simple models for the release kinetics of dissolved organic carbon from woody filtration media.

The mechanisms and kinetics of dissolved organic carbon (DOC) release from woody materials (pine, hardwood and compost) under non-equilibrium leaching conditions was examined through batch and column tests. Mechanistically based kinetic models (first and second order) had a low predictive power for DOC release compared to those based solely on regression (Elovitch, power law). The DOC release data showed a bi-phasic response, with an early period of rapid release (<24h) controlled by film diffusion followed by a slower rate controlled by intra-particle diffusion. After flow interruption, DOC release was primarily controlled by intra-particle diffusion; however, the specific rate parameters generally varied with each flow cycle and between different wood types.

Effect of media type and particle size on dissolved organic carbon release from woody filtration media.

Sequential batch leaching tests were used to evaluate the mass of DOC released from composted garden organics (yard waste), pine and hardwood under pseudo-equilibrium conditions. All media showed an initial rapid decline in DOC values followed by a slower rate during later time periods. Greater than 50% of the DOC leached occurred within the initial time period (<24 h). The mass of DOC leached varied significantly between the materials and to a lesser degree between different particle size ranges. The pine had the lowest leached DOC fraction (2.8-4.8 mg/g), while the hardwood (21-27 mg/g) and compost (13.6-32.7 mg/g) were significantly greater. The type and processing of the woody material incorporated into these systems can have a significant impact on the treated stormwater.