Ca2+ sorption on regenerated cellulose fibres

High calcium content in cellulose materials can cause considerable problems in pulp processing, textile chemical treatment and consumer use, e.g. dyeing operations or household laundry. The Ca2+ binding capacity of cellulose also is of relevance in food and medical applications. Through their carboxyl group content regenerated cellulose fibres can act as weak anion exchangers, thus all types of regenerated cellulose fibres such as lyocell, viscose and modal fibres, show a distinct ability to bind Ca2+ ions. The binding capacity is limited by the carboxyl group content, which was determined with 15 mmol/kg for lyocell fibres and 20 mmol/kg for viscose fibres, using the Methylene Blue sorption method. The presence of bound Ca2+ also was demonstrated by complex formation with alizarin. The molar ratio between carboxylic group content and bound Ca2+ ions was one Ca2+ ion for a single carboxyl group. As a result of Ca2+ sorption a positive net charge of the cellulose results and another anion has to be bound as counter ion for reasons of charge neutralisation. Results of potentiometric titrations indicate HCO3− to be present as counter ion in the Ca2+ cellulose system. Thus under the experimental conditions studied, bound Ca2+ is proposed to be present in the form COO−Ca2+HCO3−.