Preparation, characterization and performance evaluation of lanthanum orthoferrites for humic acid removal via photocatalysis

Abstract

Humic acid (HA) is categorised as a natural organic matter (NOM). Excessive concentrations of HA present in water treatment system may lead to adverse effects such as undesirable taste, effluent coloration towards and production of carcinogenic by-products such as trihalomethanes. One promising way of HA elimination is by photocatalysis. Lanthanum orthoferrites (LaFeO3) has been regarded as an efficient visible-light driven photocatalyst due to its narrow band gap. In order to synthesize LaFeO3 nanoparticles, chelation is one of the important chemical processes to form the interaction between metals that directly affects the physicochemical properties of the nanoparticles. One of the common chelating agents used is glucose. However, synthesized LaFeO3 face issues in their physicochemical properties such as low surface area and poor morphology when glucose is used as the main chelating agent. Therefore, the effect of citric acid addition on glucose as a secondary chelating agent was investigated in this study. Interestingly, after the addition of citric acid (LFO2), the BET surface area dramatically increased from 15.68 m2/g to 40.77 m2/g. The field emission scanning electron microscopic (FESEM) images showed that LFO2 possesses a better spherical-shaped like growth and less agglomeration. More importantly, results revealed that LFO2 degraded 80% of HA within 120 minutes, which is a 1.3-fold increment compared to LFO1 (glucose only). Furthermore, the effects of different calcination temperatures (400 °C, 500 °C and 600 °C) were also investigated using glucose and citric acid as a dual chelating agent. From the study, LaFeO3 nanoparticles calcined at 400 °C were selected as the most promising photocatalyst due to its amorphous nature which benefits from the presence of surface defect. In addition, the amorphous LaFeO3 also recorded the highest surface area with a value of 70.02 m2/g which contributed to the enhancement of photocatalytic activity for the degradation of HA. Besides that, effect of operational parameters such as photocatalyst loading (0.6-1.20 g/L), initial concentration of HA (10-40 mg/L) and aeration (presence of oxygen) for HA degradation under visible light irradiation were studied using the amorphous LaFeO3. Overall, the optimal values for degradation of HA were observed to be at a catalyst loading of 1.0 g/L and initial concentration of 10 mg/L. The result also showed that the presence of oxygen as electron acceptor from aerated samples preventing recombination of electrons and holes, thus enhancing the photocatalytic degradation. In a nutshell, the perovskite basedphotocatalyst, LaFeO3 was successfully synthesized using glucose and citric acid as a dual chelating agent assisted by low temperature calcination.