Characterization Techniques as Supporting Tools for the Interpretation of Biochar Adsorption Efficiency in Water Treatment: A Critical Review.

Over the past decade, biochar (BC) has received significant attention in many environmental applications, including water purification, since it is available as a low-cost by-product of the energetic valorisation of biomass. Biochar has many intrinsic characteristics, including its porous structure, which is similar to that of activated carbon (AC), which is the most widely used sorbent in water treatment. The physicochemical and performance characteristics of BCs are usually non-homogenously investigated, with several studies only evaluating limited parameters, depending on the individual perspective of the author. Within this review, we have taken an innovative approach to critically survey the methodologies that are generally used to characterize BCs and ACs to propose a comprehensive and ready-to-use database of protocols. Discussion about the parameters of chars that are usually correlated with adsorption performance in water purification is proposed, and we will also consider the physicochemical properties of pollutants (i.e., Kow). Uniquely, an adsorption efficiency index BC/AC is presented and discussed, which is accompanied by an economic perspective. According to our survey, non-homogeneous characterization approaches limit the understanding of the correlations between the pollutants to be removed and the physicochemical features of BCs. Moreover, the investigations of BC as an adsorption medium necessitate dedicated parallel studies to compare BC characteristics and performances with those of ACs.

Microwave-assisted extraction and gas chromatographic determination of thirty priority micropollutants in biowaste fraction derived from municipal solid waste for material recovery in the circular-economy approach.

European and national waste directives prioritize recycling of wastes, as well as material and energy recovery from wastes themselves. Bio-waste fraction can be converted into new resources whose quality is strictly dependent upon that of waste feedstock. Methods to evaluate the contamination from organic micropollutants in bio-waste are rarely investigated. The aim of this work was to develop an innovative analytical method for the extraction and quantification of 16 polycyclic aromatic hydrocarbons (PAHs) and 14 polychlorinated biphenyls (PCBs, including dioxin-like compounds) in bio-waste. Through a full-factorial experimental design, a microwave-assisted extraction technique was optimized to extract the thirty targeted micropollutants, studying the effect of cyclohexane and dichloromethane as extraction solvents with or without acetone, and of extraction temperature. Purification of the extract was obtained by a silica-based solid-phase extraction cartridge, followed by a sulfuric acid treatment. The analysis was carried out by gas chromatography coupled with mass spectrometry. The optimized method, validated directly in the bio-waste matrix fortified with isotopically marked surrogates, is characterized by good extraction recoveries, included within 47 and 106% (relative standard deviations <10%), by satisfactory intra-day (<1.1%) and inter-day (<9.3%) precision, and by low matrix effect (<17%), despite the complexity of the matrix. The optimized procedure, applied to the analysis of PAHs and PCBs in a bio-waste sample collected from a local anaerobic digestion and composting plant, showed a total PAHs content of 562 µg/kg. As regards PCBs, the dioxin-like congener PCB 118 was the only compound quantified (25 ± 6 µg kg-1).

Biochars intended for water filtration: A comparative study with activated carbons of their physicochemical properties and removal efficiency towards neutral and anionic organic pollutants.

Seven biochars (BCs) obtained from pyrolysis or gasification of different vegetal feedstocks were thoroughly characterized in comparison with three commercial activated carbons (ACs) routinely used in drinking water treatment plants. BCs and ACs characterization included the determinations of ash, iodine and methylene blue adsorption indexes, and the release of metals and polycyclic aromatic hydrocarbons, which were performed according to international standards applied for adsorption media to be used in drinking waters. Total specific surface area, micropore and mesopore specific surface area, pH of the point of zero charge, and the release of polychlorinated biphenyls were also determined in all chars. Principal component analysis and cluster analysis were performed in order to summarize the complex set of information deriving from the aforementioned characterizations, highlighting the BC most similar (BC6 from high temperature gasification of woody biomass) and most different (BC7 from low-temperature pyrolysis of corn cob) from ACs. These BCs were studied for their adsorption in ultrapure water towards diiodoacetic acid (an emergent disinfection by-product), benzene, and 1.2-dichlorobenzene, in comparison with ACs, and results obtained were fitted by linearized Freundlich equation. Overall, BC6 showed higher sorption performances compared to BC7, even though both BCs were less performing sorbents than ACs. However, the sorption properties of BCs were maintained also in real water samples collected from drinking water treatment plants.