Absorbance spectroscopy based evaluation of humic acid under non-oxidative and oxidative conditions

Abstract

The aim of this study was to examine the mathematical relationships between humic acid concentration and UV-vis spectroscopic parameters under oxidative and non-oxidative conditions. Humic acid concentration was represented by dissolved organic carbon contents (DOC). UV-vis spectroscopic parameters were presented by absorbance measurements at wavelength λ = 436 nm as Color436, λ= 365 nm as UV365, λ=280 nm as UV280, and λ= 254 nm as UV254. Humic acids were selected as representing terrestrial humic acids (FHA, AHA and RHA) and aquatic humic acid (NHA). As a lot of different humic molecules in very diverse physical associations are mixed together in natural environments, it is difficult to determine their exact concentrations (units of ppm, mg L-1 or mol L-1) and allocate them to a certain class of organic molecules. Since direct determination of humic acid concentraton is not possible to the unspecified chemical structure,various methods have been developed to express the humic acid contents of the aqueous humic solutions. Due to the fact that humic acid does not have a well identified structure, the researchers applied some methods to represent the humic acid concentration as a function of DOC concentration and/or UV-vis parameters (UV254, UV280, UV365 and Color436).This study includes evaluation of the experimental work performed on the UV-vis parameters in relation to the humic acid “prepared” concentration and DOC contents. The mathematical relationships between i. DOC concentration and HA “prepared” concentration, ii. DOC concentration and UV-vis parameters (UV254, UV280, UV365 and Color436) and iii. HA “prepared” concentration and UV-vis parameters (UV254, UV280, UV365 and Color436) were investigated for NHA, FHA, AHA and RHA under the non-oxidative conditions. DOC concentrations, corresponding to HA concentration (NHA, FHA, AHA, and RHA), were correlated with UV-vis parameters (UV254, UV280, UV365 and Color436) under the non-treatment condition and HA concentrations (NHA, FHA, AHA, and RHA) were correlated with UV-vis parameters (UV254, UV280, UV365 and Color436) with high regression coefficient under the non-treatment condition. Furthermore, the overall humic acids (NHA, FHA, AHA, and RHA) and also DOC concentrations of the overall humic acids were correlated with UV-vis parameters. The same steps were done under the oxidative treatment conditions where photocatalytic treatment was applied. In general, photocatalytic oxidation can be considered as an example of innovative technologies collectively known as ‘‘Advanced Oxidation Processes’’ that rely on the generation of very reactive oxygen radicals. Those reactive species are subsequently used to degrade non selectively organic compounds. The known concentrations of NHA and AHA was treated by photocatalytic treatment. The removal of DOC concentration of NHA and AHA were determined by using TOC analyzer and the removal of UV-vis parameter (UV254, UV280, UV365 and Color436) was determined by using UV-vis spectrophotometer after the photocatalytic treatment.The mathematical relationships between DOC concentration of NHA and AHA, and UV-vis parameters (UV254, UV280, UV365 and Color436) were investigated after the photocatalytic treatment. DOC concentrations of NHA (in the presence of 0.25 mg mL-1 TiO2) and AHA (in the presence of 0.10, 0.25 and 1.00 mg mL-1 TiO2) were correlated with UV-vis parameters under the treatment condition (the photocatalytic treatment). Some researchers represented the removal of HA as a function of UV-vis parameter under the photocatalytic treatment. Moreover, DOC ‘DOCcalc’ was calculated as a function of UV254, UV280, UV365 and Color436 parameter of the removed HA concentration, by using the non-treatment Equations of NHA and AHA. The mathematical relationship between DOCobs and DOCcalc was evaluated..