A separation-free and pizza-structure PAM/GCN/PAA composite hydrogel (PCH) in wastewater treatment at visible light...

During wastewater treatment, the separation of powder nano-photocatalyst from treated water is a limiting factor for the commercial application of the powder photocatalysts. In this study, a photocatalyst, i.e. graphitic carbon nitride (GCN) was immobilized in network of polyacrylic acid (PAA) hydrogel to solve this issue. To further immobilizing GCN and strengthening hydrogel, polyacrylamide (PAM) was introduced to form interpenetrating network with PAA. In this structure, PAM acted the role of cheese in a pizza, tightly covering and immobilizing GCN into the interpenetrating network. During the cyclic tests, PCH with 20 mg (PCH₂₀) can be successively reused 5 times compared 3 times of GCN/PAA with the RhB photodegradation efficiency over 95% each time. Meanwhile, GCN retention rates of GCN/PAA and PCH₂₀ are 71.2 ± 5.2% and 97.2 ± 1.9% respectively. Besides, GCN played initiator role in the polymerization of PAM. PCH₂₀ was more stable comparing with GCN/PAA hydrogel at both mechanical and thermal characterization. Furthermore, PCH₂₀ showed excellent photocatalysis capability to RhB dyed wastewater at both visible and solar light. During 5 time's continuously cyclic tests, the photodegradation efficiency of PCH₂₀ over RhB solution (10 mgL^-1) was over 95% within 2 h under visible light (100 mWcm^-2) each time. By changing pH values of solution from 3 to 9, the degree of swelling ratio (D_sw) of PCH₂₀ could achieve from 307 ± 39% to 2361 ± 135%. PCH₂₀ was feasible to obtain large surface area through swelling and it was beneficial for GCN to harvest the light. Hence, the photodegradation performance of PCH₂₀ in RhB solution of pH 7, 9 was better than its in the original (pH 5.7) or the pH 3 RhB solution. The preparation of PCH₂₀ was environmental friendly and cost-efficient without using any photoinitiators and crosslinking agents except GCN.