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Interaction of manganese and copper with wood pulp fibres

The metal ions of Mn and Cu interact with the lignocellulosic matrix of pulp fibres and their presence is detrimental in pulp processing. The design of suitable strategies for effective management of metals requires understanding the behaviour of these NPEs in kraft pulp suspensions. The present study is focused on the characterization of kraft pulps and provides experimental data for chemical interactions of metals with wood fibres. Kraft pulps from two different geographic locations were studied. Also, the pulps were collected from different processing stages (chemical environment). Metal profile of pulps was obtained and their removal efficiency using an acid assisted chelation process was studied. Fibre properties like Fibre Saturation Point (FSP) and fibre charge essential for understanding metal partitioning behavior were determined. Metal partitioning between fibre and bulk solution (pH ) was carried out by introducing various concentrations of Mn and Cu and the effect of pH on partitioning behaviour was studied. S E M and E D X of the pulp samples were conducted to investigate the precipitation of metals in the pulp suspension. The Mn and Cu concentration in the pulp was maximum in the BSW stage pulp and gradually decreased down the processing stages. Maximum of 59 ppm and 71 ppm of Mn was found in coastal and interior mills respectively. Cu was found in very small quantity of 2 ppm and 1 ppm in coastal and interior mill samples. The removal efficiency using acid assisted chelation was studied. It is seen that Mn removal is about 90 % with both the pulps. However, the low concentrations of Cu limited the analytical precision of removal efficiency. The FSP of the pulps increased with the increase in pH and reached a plateau at about pH=9. Potentiometric titration data was used for charge determination and it was seen that at any given pH the charge on the unbleached pulp was higher when compared to bleached pulp. Also fibre charge tends to increase with increasing pH. Partitioning experiments showed that the fibres do not get saturated with Mn even at high concentrations (600 ppm). However there is a gradual increase in Mn in the bulk phase while the change is limited in the fibre phase. Fibres did not get saturated with high concentration of Cu (20 ppm). Finally, the SEM micrographs and E D X analysis employed helped in detecting Mn and Cu precipitates in the pulp at pH=7. The experimental data and values from partitioning prediction model agreed. However; the model may not hold good for pH>7 due to precipitation of Mn and Cu.