1 Introduction
There are several reports on microbial treatment of cyanidation wastewater with successful operational efficiency [1-4]. The impact of various factors such as temperature, pH, substrate and bioreactor configuration on the performance of organisms in cyanidation wastewater have been reported [5-7]. In addition, thermodynamic tools have been used to validate the microbial performance and the cyanide degraded [8-10]. However, the uptake of the microbial remediation by the industry have not been inspiring even though Homestake and LaRonde gold mine in Canada, including Gold Fields Limited have demonstrated the robustness and feasibility of the biological process [11-13]. Analysis of process performance should not only be based on microbial growth and toxicant degraded but also on the impact of the toxicant on the physical properties of the organism. This will provide further insight on the capability of the organism to manage their environment under stress.
Thermodynamic properties of a material are an essential tool for predicting the feasibility of any chemical and biological reaction including processes such as the microbial growth process and the biomass conversion of nutrient media to useful products. Among these thermodynamic properties, heat capacity of biological molecules such as starch, glucose, proteins and amino acids reportedly measured based on rudimentary heat capacity quantifications can be used to estimate entropy increments and/or changes at low temperatures (0 to 298.15 K); however, there is high uncertainty associated with this estimate [14]. Recently, some researchers have reported on the use of an adiabatic calorimeter for measuring heat capacity of biological materials at low temperature; based on the application of the third law of thermodynamics, for which incremental entropy and/or absolute entropy can be estimated [15, 16]. Nevertheless, the results were determined to be unreproducible because there was no reference material used and that each researcher had to fabricate their own calorimeter. This maybe the reason for Pyda’s [15] preference for Differential Scanning Calorimeter (DSC) measurements over adiabatic calorimeter measurements. Overall, there is only one report on heat capacity of microbial dried biomass thus far [17] which reported on the use of an adiabatic calorimeter for quantifying the heat capacity of lyophilised cells of Saccharomyces cerevisiae, subsequent to the estimation of entropy changes as a function of temperature based on the third law of thermodynamics.
From the second law of thermodynamics, the heat capacity of any material can be estimated/quantified using heat flow curves obtained from DSC generated profiles of the sample being studied. A DSC provides a more reliable, accurate and reproducible results because it is calibrated with a standard reference and/or material such as sapphire, which is used to ascertain and/or detect any error with the equipment, a parametric requirement not available with an adiabatic calorimeter. Nevertheless, it is often difficult to interpret the heat flow data from DSC experiments when multiple processes are involved, resulting in overlapping transitions. Besides, the heat capacity of a material cannot be determined directly from DSC data, it requires multiple experiments including data interpretation to ascertain or determine the heat capacity [18-21].
Furthermore, a modulated DSC (MDSCTM) overcomes these drawbacks and thus provide an insight into the thermal properties of materials being studied. MDSCTM uses a modulated temperature input signal to provide information on the heat capacity, both under isothermal and non-isothermal conditions. Further details on theory, principles, application and instrumentation requirements of the MDSCTM can be found in Vendonck [18] and Knopp [22].
Cells are known to be insoluble and using lyophilised cells, an approximate entropy per unit mass can be obtained if cellular integrity is not annihilated by lyophilisation [23]. Battley [24] in his report hypothesized that the constituent materials of formation do not affect the molecular weight and subsequently thermodynamic properties of lyophilised cell but Duboc et al [25] and Akinpelu et al [26] have proven otherwise with their report on several yeast, bacteria, algae and filamentous fungi. And this agrees with the basic concept that the specific heat is a function of the property of the substance. Therefore, the objective of this study was to determine the impact of different substrates (glucose, Beta vulgaris and cyanide) on the heat capacity of lyophilised biomass ofFusarium oxysporum in cyanidation wastewater using a modulated DSC (MDSCTM).