Environmental Pollution 230 (2017) 81e86
Contents lists available at ScienceDirect
Environmental Pollution journal homepage: www.elsevier.com/locate/envpol
Quantification and speciation of volatile fatty acids in the aqueous phase* Jechan Lee a, Jieun Kim a, Jeong-Ik Oh b, Sang-Ryong Lee c, **, Eilhann E. Kwon a, * a
Department of Environment and Energy, Sejong University, Seoul 05006, South Korea Advanced Technology Department, Land & Housing Institute, Daejeon 34047, South Korea c Animal Environment Division, National Institute of Animal Science, Jeollabuk-do 55365, South Korea b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 29 March 2017 Received in revised form 1 June 2017 Accepted 8 June 2017
This study lays great emphasis on establishing a reliable analytical platform to quantify and specify volatile fatty acids (VFAs) in the aqueous phase by derivatizing VFAs into their corresponding alkyl esters via thermally-induced rapid esterification (only 10 s reaction time). To this end, reaction conditions for the thermally-induced rapid esterification are optimized. A volumetric ratio of 0.5 at 400 C for VFA/ methanol is identified as the optimal reaction conditions to give ~90% volatile fatty acid methyl ester (VFAME) yield. To maintain a high yield of VFAMEs, this study suggests that dilution of the sample to an optimum concentration (~500 ppm for each VFA) is required. Derivatization of VFAs into VFAMEs via the thermally-induced rapid esterification is more reliable to quantify and specify VFAs in the aqueous phase than conventional colorimetric method. © 2017 Elsevier Ltd. All rights reserved.
Keywords: Anaerobic digestion Odour indicator Short-chain fatty acids Porous material Derivatization
1. Introduction Unprecedented environmental threats such as global warming, attributed to anthropogenic activities, have gained public awareness. In particular, anthropogenic carbon emissions owing to our heavy dependence on fossil fuels as a source of energy and chemical feedstocks are accountable for global warming, based on longstanding scientific research. Accordingly, significant scientific and political attention has been paid to developing carbon-free, or carbon-neutral, resources to replace conventional fuels (Monlau et al., 2015; Zhang et al., 2014). Moreover, the concept of energy recovery has been established as an effort to minimise energy input into the overall system by exchanging energy from one sub-system with that from another sub-system (Consonni et al., 2005; Klinghoffer and Castaldi, 2013). Waste-to-energy (WtE), a form of energy recovery, aims to transform urban/rural waste into energy via fuel processing technologies (Pan et al., 2015). Fuel processing technologies for WtE can be classified based on two possible
*
This paper has been recommended for acceptance by Charles Wong. * Corresponding author. ** Corresponding author. E-mail addresses:
[email protected] (S.-R. Lee),
[email protected] (E.E. Kwon). http://dx.doi.org/10.1016/j.envpol.2017.06.042 0269-7491/© 2017 Elsevier Ltd. All rights reserved.
routes: 1) thermo-chemical conversion (combustion, pyrolysis, and gasification) and 2) non-thermochemical conversion (aerobic and anaerobic digestion). The greatest merit of WtE is the coincidence of waste disposal and energy generation (Klinghoffer and Castaldi, 2013). Steady supply of waste as the feedstock can be another benefit of WtE (Rogoff and Screve, 2011) compared to production of other types of carbon-neutral energy such as bioenergy. Among WtE technologies, anaerobic digestion has been widely employed to produce biogas from organic waste (Batstone and Virdis, 2014; Singhania et al., 2013; Yasar et al., 2015). It involves a series of microbial reactions starting from the hydrolysis of complex organic matter, followed by acidogenesis, acetogenesis, and methanogenesis (Ostrem et al., 2004). However, to improve economic viability and ensure process efficiency of anaerobic digestion, it is crucial to optimise operating conditions and recognise any sudden perturbation (e.g., change in feedstock loading, composition of feedstock, and failure of process control) in a short time (Ahring et al., 1995; Labatut and Gooch, 2012). These commonly observed perturbations during anaerobic digestion can be a serious threat to the efficiency of the process (Siegert and Banks, 2005). Volatile fatty acids (VFAs) are monocarboxylic aliphatic acids converted from carbohydrates and amino acids by acidogenic and acetogenic bacteria (Franke-Whittle et al., 2014; Wang et al., 1999; Weiland, 2010). VFAs are also known as the main intermediates when producing biogas from organic matter by
82
J. Lee et al. / Environmental Pollution 230 (2017) 81e86
anaerobic digestion (Labatut and Gooch, 2012). VFAs are closely associated with the odour of animal manure; thus, they can act as an effective odour indicator in anaerobic digester systems (Page et al., 2015). In addition, when a certain amount of VFAs have been accumulated in an anaerobic digester, it not only indicates abnormal conditions within the digester, but also inhibits methane generation by acidifying the digesters (Babel et al., 2004; Li et al., 2014). In this regard, monitoring the amount of VFAs is a sensitive and reliable indicator, not only for odour, but also for any imbalance in the overall anaerobic digestion process. Therefore, the development of a highly reliable analytical technique to quantify VFAs is essential. In general, distillation and gas chromatography (GC) are considered representative techniques to quantify VFAs in water (Rice et al., 2012). Besides them, other analytical techniques such as colorimetric determination, ion chromatography, and highperformance liquid chromatography (HPLC) are also in use (Siedlecka et al., 2008; Ullah et al., 2014). Distillation methods suffer from insufficiently precise results and lack of VFA speciation ndez et al., 2016). Compared to other methods, GC analysis (Ferna could be a great alternative due to its simple procedure, small sample requirement, and relatively low detection limit (Siedlecka et al., 2008). Also, there are efforts to profiling fatty acids using plasma (Han et al., 2011; Yi et al., 2006). For quantification of VFAs using GC, quantifying VFAs by their corresponding methyl esters via derivatization is favoured over their direct analysis as VFAs themselves because hydrogen bonds derived from their carboxyl groups ndez et al., 2016; cause low resolution and tailing of peaks (Ferna Orata, 2012). However, conventional derivatization methods are problematic. First, homogeneous acid catalyst is needed (Castro mez et al., 2014; Lepage and Roy, 1984; Morrison and Smith, Go 1964). Second, yield of methyl esters is low because of impurities such as contaminants and water in the samples (Jung et al., 2016; Khan et al., 2016). Third, a hazardous and potentially explosive solvent is required for esterification (i.e., diazomethane) (Orata, 2012; Siedlecka et al., 2008). Moreover, the conventional analytical methods requires extraction or isolation steps prior to analysis et al., 2009; Villar-Tajadura et al., (Luna et al., 2008; S anchez-Avila 2014). Thus, technical improvements for the analysis of VFAs need to be developed in an environmentally friendly way. To achieve this, discarding the solvent extraction procedure of VFAs is highly desirable, allowing a considerable reduction in the unwanted loss of VFAs from the sample. Our previous work showed that thermally-induced esterification of pure VFA into FAME in presence of porous materials (Jung et al., 2016). The process converted more than 90% of butyric acid into butyric acid methyl ester. In this context, this study focuses on an innovative way to quantify individual VFAs in the aqueous phase via thermally-induced esterification, without solvent extraction using a porous material. It suggests that this new derivatization method simplifies the procedure for VFA analysis and could exhibit an extraordinarily high tolerance against impurities in the analyte. Furthermore, shortening the time required for VFA analysis can be achieved by means of this novel procedure, leading to possible tracking of VFA traces during anaerobic digestion. Finally, all experimental findings in this study will be directly applicable to wastewater treatment and other applications such as in odour indication. 2. Materials and methods 2.1. Chemical reagents and porous material Methanol (99.9%), acetic acid methyl ester (99.5%), propionic acid methyl ester (99%), butyric acid methyl ester (99%), isobutyric
acid methyl ester (99%), valeric acid methyl ester (99%), and isovaleric acid methyl ester (98%) were purchased from SigmaAldrich (St. Louis, MO, USA). Acetic acid (99.7%), propionic acid (99%), and isobutyric acid (99%) were purchased from Junsei Chemical (Japan). n-Butyric acid (99.5%) and n-valeric acid (98%) were purchased from Kanto Chemical (Japan). Isovaleric acid (98%) and n-hexane (>98.5%) were purchased from Alfa Aesar (Haverhill, MA, USA) and Daejung Chemical (Korea), respectively. Silica (surface area: 480 m2 g 1; pore volume: 0.75 cm3 g 1; pore size: 6 nm; particle size: 150e250 mm) was purchased from Sigma-Aldrich (St. Louis, MO, USA) and was dried at 80 C for 72 h prior to experiments. 2.2. Thermally-induced rapid esterification of VFAs Mixtures of VFAs with different VFA concentrations were prepared by dilution with deionized water and stored at 4 C. It is known that an anaerobic digester with >1500 ppm VFAs has difficulty in producing biogas, and that
