Low-cost heterogeneous composite photocatalyst consisting of TiO2, kaolinite and MMT with improved mechanical strength and photocatalytic activity for industrial wastewater treatment

Abstract

The industrially feasible TiO<sub>2</sub>-clay-based photocatalysts are essential to overcome practical barriers that are inherent to currently available TiO<sub>2</sub>-based photocatalysts. The current study demonstrates the fabrication of heterogeneous photocatalyst using TiO<sub>2</sub>, kaolinite, and montmorillonite (TKMCP), which has shown improved catalytic activity and mechanical strength, resulting in an industrially feasible photocatalyst. The TKMCP is prepared in a cost-effective manner using 60% TiO<sub>2</sub> and 40% clay with different kaolinite to MMT ratios (1:3 TKMCP1, 1:1 TKMCP2, and 3:1 TKMCP3) by employing mechanical compression and dehydroxylation. The clay ratio predominantly determines the TKMCP mechanical strength and photocatalytic efficiency, where the lowest MMT percentage results in a uniform matrix, in which TiO<sub>2</sub> particles are embedded on clay-sheets. The TKMCP surface became uniform when the MMT percentage is low, whereas a high MMT fraction results in a disordered catalytic surface due to large clay fragments and agglomerates. All three composites accounted for more than 85% of the degradation rate, exhibiting pseudo first order kinetics, resulting in high-rate constants, with the highest observed for TKMCP3, which is 1.55 h<sup>–1</sup>. The TKMCP3 accounts for the highest mechanical strength, which is 5.83 MPa, while the lowest is observed with TKMCP1, indicating that the TKMCP strength decreases significantly with high MMT fraction. TKMCP has several advantages, including easy fabrication, low cost, free of hazardous chemicals, high production capacity with minimal machinery/supervision, non-self-degradability, easy disposal, easy installation in pilot-scale reactors, compatibility with both batch and flow reactors, environmental, and user-friendliness. TKMCP can also be obtained in variable sizes and shapes that ensure dynamic wastewater treatment applications.

Low-cost ternary composite photocatalysts consisting of TiO2, kaolinite and cement for an efficient organic waste decontamination in water

“…Such phase transitions are prevented in binary composites that are fabricated using clay. This is because clay eventually acts as a phase transformation barrier and predominantly undergoes dehydroxylation, preventing anatase from rutile transformation [26,27]. In the present study, kaolinite also acts as a barrier for anatase to rutile conversion, preventing the phase transformation, which results in a marginal change in rutile percentage (Table 2).…”

“…These binary composites have shown a less than 95% degradation rate in comparison to TKCe, which exhibits a DR > 98% (Table 1). TKCe 1-1 rate constant, which is 1.57 h −1 , is quite similar to that of TKMCP (1.55 h −1 ) and is greater than that of TKCP (0.58 h −1 ) [26,27]. The mechanical strength of TKCe follows a similar trend as photocatalytic activity, with the highest observed with TKCe 1-1 (6.97 MPa), implying that the optimal cement percentage is critical for a better composite matrix (Table 1).…”

“…In contrast, the composite with a high clay fraction results in better photocatalytic activity than the one with a low clay fraction, which is TKCe 1-3 (Table 1). TKCe 1-1 mechanical strength is greater than that of binary composites, including TKCP (3.55 MPa) and TKMCP (5.83 MPa), that were fabricated previously [26,27]. Therefore, an optimum clay-cement combination simultaneously results in high photocatalytic activity and mechanical strength, which is observed in TKCe 1-1, revealing that proper raw material combinations can have a significant impact on composite properties.…”

“…TKCe 1-1 accounts for the highest photocatalytic activity and mechanical strength, revealing that the 1:1 kaolinite to cement ratio resulted in an effective composite matrix (Table 1). Over 98% degradation rate (DR) is observed with TKCe 1-1, which is the highest among all binary composite photocatalysts that are fabricated previously and raw TiO2 [26,27]. The authors' previous study observed a very low DR (72.5%) and pseudo-first-order rate constant (k = 0.28 h −1 ) related to raw TiO2 when used in powder form, which is mainly due to the ultra-fine particle nature of TiO2, which results in a colloidal suspension where the tiny particles are dispersed throughout the entire solution [26].…”

“…The authors of the present study recently developed a novel composite preparation method based on mechanical compression and heat treatment, resulting in a stable binder-free graphite and clay composite electrode [21][22][23][24][25]. Following a similar technique, authors recently developed efficient binary photocatalysts that are composed of raw materials, including TiO2 and clay (kaolinite/MMT), resulting in improved photocatalytic activity and mechanical strength [26,27]. However, authors also found that ternary composites that are related to graphite-clay composite electrodes outperform the binary electrodes, revealing that such ternary composites result in matrix enhancements [23,24].…”