Degradation of root exudates in closed hydroponic systems using UV/H2O2: Kinetic investigation, reaction pathways and cost analysis.

Qualitative screening of reused nutrient solution (RNS) displays the presence of several organic acids. In this study, the degradation by UV/H2O2 of three of those organic acids (benzoic (BA), phthalic acid (PhA) and succinic acid (SA)) present in RNS was investigated. The results indicated that (i) the degradation rate of BA was faster than that of PhA and SA and (ii) by increasing the contact time the degradation of all acids was improved. For example, the removal of BA increased from 83% and 91% when increasing the contact time from 90 min to 270 min in the presence of 50 mg L-1 and UV. A maximum COD (30%) and UV254 (68%) removal were obtained when 200 mg L-1 H2O2 was applied for 90 min. No significant change was observed in terms of parameters such as PO43- and NO3- while electrical conductivity (EC) and pH were slightly changed during the oxidation process. Pseudo- first -order represented well the experimental data for the degradation of the selected organic acids in RNS (particularly for BA and PhA), exhibiting high linear correlation coefficients (R2 ≥ 0.96). Moreover, the results showed that the decomposition of organic acids was significantly influenced in the presence of inorganic ions in RNS. GC-MS analysis revealed the presence of several intermediate products during the oxidation process and the primary reaction pathway of benzoic acid was accordingly proposed. Finally, a bench scale cost investigation showed that low concentration H2O2 (50 mg L-1) in longer time (270 min) is more cost effective than high concentration H2O2 (200 mg L-1) in a shorter time (90 min).