Recirculating Water Treatment in Closed Hydroponic Systems: Assessment of Granular Activated Carbon and Soft Templated Mesoporous Carbon for Adsorptive Removal of Root Exudates

Abstract

The present study deals with the characterization of reused nutrient solution (RNS) from a hydroponic farm (lettuce). Chemical analysis with GC-MS revealed the presence of eleven individual organic acids as a part of root exudates (acetic, acrylic, maleic, succinic, benzoic, phthalic, sebacic, myristic, palmitic, oleic and stearic acid). Based on these results, first batch tests were developed to determine the breakthrough time and the best kinetic model for adsorption of benzoic acid (as an indicator of these 11 organic acids) on two different types of carbon based adsorbents (granular activated carbon-GAC and soft templated mesoporous carbon-SMC). In order to evaluate the adsorption kinetics, four different kinetic models were fit to the experimental data. The pseudo-second-order best represented the experimental data for both adsorbents at all dosages, exhibiting high linear correlation coefficients (R2 ≥ 0.98). Second, to gain a better understanding of the adsorption efficiency during the batch experiments, several parameters were measured (i.e., COD, UV254, pH and EC) and the effect of adsorbent dosages on these parameters was studied. Third, the effect of different flow rates and bed heights on the breakthrough curve was studied during column tests. These curves were fitted to four well-known fixed-bed adsorption models, namely Thomas, Bohart−Adams, Yoon–Nelson and BDST models. The latter model was found to be most suitable (R2 ≥ 0.88) to predict the breakthrough curve for benzoic acid uptake on GAC in fixed bed column. Scale up analysis revealed that approximately 72 kg GAC is required to treat RNS during 27 days for a typical flow rate of 360 L h−1 (3200 kg L−1).