Enzymes are protein molecules with complex structure that accelerate biochemical reactions. The activity of these chemical compounds is limited to a specific range of pH and temperature. The effect of pH variations on deinking efficiency of old newspaper (ONP) by pectinase was investigated in this study. Old newspaper was repulped in a laboratory disintegrator for 10 minutes at 5% consistency and 26500 rpm. Enzymatic treatment of recycled ONP was carried out for 20 minutes at 0.1% enzyme concentration and 10% consistency, and pH levels of 3.5, 4, 4.5, 5, 5.5 and 6. The results showed that more efficient pectinase deinking of old newspaper can be achieved at pH level of 4-4.5, as indicated by the improved optical and mechanical properties of standard handsheets obtained by enzymatic deinking at pH levels of 4 to 5.
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