Water lettuce (WL), a common aquatic plant, was studied as a potential adsorbent for the removal of toxic malachite green (MG) in a batch adsorption system. The batch adsorption studies included the effects of pH, ionic strength, dye concentration, thermodynamics and kinetics studies, estimation of activation energy, and regeneration experiments. Elemental analysis of the adsorbent was determined by X-ray fluorescence, while the functional groups were characterised by Fourier transform infrared spectrophotometer. Experiments involving the effects of pH and ionic strength indicated that hydrophobic-hydrophobic interactions might be the dominant force of attraction for the MG-WL adsorption system. The Freundlich isotherm model best represented the adsorption systems, while the Langmuir model predicted maximum adsorption capacity (qm) of 99.0 mg g-1 at 25°C. Weber-Morris model suggested intraparticle diffusion was not rate-limiting, while the Boyd model suggested film diffusion may be the rate-limiting step. Thermodynamics studies indicated spontaneity and endothermic reaction, while the Arrhenius studies indicated a physical adsorption dominant process with the activation energy estimated to be at 12.6 kJ mol-1. Spent adsorbent was regenerated with 0.1 mol L-1 NaOH and it was effective even after five cycles.
The current chlorine decay models involve complex calculations, and they are impractical for rapid chlorine dose/decay product calculations. This problem limits their applicability to treatment systems. Previously, less attention was paid to developing empirically based bulk chlorine decay models for predicting chlorine decay pathways. In order to bridge this gap, we developed a chlorine decay model based on kinetic data. The model was calibrated using well-characterized humic substances to simulate the natural organic matter content in water. Subsequently, the decay model was utilized to make chlorine decay predictions in the water of the Greater Kandy Water Treatment Facility, Sri Lanka. The chlorine decay predictions are in accordance with model calculations on a short-time scale (less than 5 h). Accordingly, it was noted that the chlorine dose of 1.6 mg/L was found to be inadequate for maintaining the mandatory residual chlorine balance at 0.2 mg/L at distribution. Furthermore, the reported average total of trihalomethanes (TTHMs) below 20 µg/L leaves room to increase the initial chlorine dose.