Biodeinking of old newspaper pulp using a cellulase-free xylanase preparation of Aspergillus niger DX-23

Old newspapers (ONP) are a significant source of raw materials for recycled paper industry. To obtain bright pulp for recycling, the ink from the ONP pulp is removed by chemical deinking process which is highly polluting and expensive. Due to high xylan content in ONP, the ink particles adhering on ONP pulp surface can be removed through detachment of ink/fiber bond by action of xylanases. In the present study, cellulase-free xylanases from fungi were specifically screened for deinking of ONP pulp. Among 16 cellulase-free xylanase producing isolates, strain DX-23 (identified as Aspergillus niger) produced maximum xylanase (48.9±0.02 U ml−1). The xylanase of A. niger DX-23 (50 U g−1 pulp), efficiently deinked the ONP pulp which exhibited 34.5% ISO brightness (22% higher than the untreated pulp). Improvement up to 78.8% ISO brightness in ONP pulp compared to untreated pulp was achieved after optimization of conditions for deinking by response surface methodology and subsequent mild H2O2 treatment of deinked pulp. ATR-FTIR spectra, X-ray diffraction analysis and scanning electron micrographs of pulps confirmed the removal of surface ink particles from ONP pulp. The xylanase of A. niger DX-23 possessed molecular weight of 15.0 kDa, showed maximum activity at 50 °C and pH 5.0. Additives such as Mn+2, Fe+2, and sodium dodecyl sulfate enhanced the activity of xylanase, whereas, Zn+2, Mg+2, Ca+2 and Hg+2 and EDTA completely inhibited the activity. The Km and Vmax for the xylanase of A. niger DX-23 for hydrolysis of Birchwood xylan was found to be 2.38 mg/ml and be 230.8 µM/min/mg, respectively.