H. Yu, G.H. Covey and A.J. O’Connor
Wastes containing ellagic acid, such as those from the neutral sulfite pulping of Eucalypts, have been identified as a potentially valuable resource material. After simple chemical treatment such wastes have been demonstrated to be efficient adsorbents of heavy metals from dilute aqueous solution. A pulp mill waste rich in ellagic acid was treated with methanol and hydrochloric acid to remove bound metal ions and tested as a metal adsorbent. The adsorption of copper(II), zinc and chromium (III) were found to fit the Langmuir isotherm model, with maximum adsorption capacities competitive with other biosorbents and commercial ion exchange resins. Selective adsorption from binary metal ion solutions can be achieved by adjusting the solution pH in the range 3.0 – 6.0. Separation coefficients of up to 19 have been determined for the preferential removal of copper (II) ions over chromium (III) ions from a binary solution. The new adsorbent was regenerated using hydrochloric acid and reused over seven adsorption cycles with minimal loss in capacity. The application of a waste material from the pulping industry to treat heavy metal bearing wastewaters is representative of the potential economic and environmental benefits to be gained by utilising wastes as resources.
The increased use of heavy metals in process industries such as the electroplating, tanning and textile industries often results in the generation of large quantities of dilute effluents containing mixtures of heavy metals, posing environmental disposal problems. In addition, mining, mineral processing and extractive metallurgical operations produce large amounts of metal bearing liquid wastes requiring treatment [1, 2]. Unlike most other toxic pollutants, metals can accumulate throughout the food chain due to their nonbiodegradability and thus have potentially detrimental effects on all living species. Thus there is a continuing interest in the development of more cost-effective and environmentally-benign processes of selective removal of heavy metals from dilute wastewater streams.
Biosorption is a form of adsorption process involving the removal of metal ions from a liquid phase onto a solid biomass material. Some biosorbents are expected to be competitive with existing non-biomass based treatments . This will be true particularly if the biomass is produced as a waste product from another industrial process and requires minimal pretreatment. Wastes from the pulp and paper industry containing ellagic acid have been identified as promising candidates for application as biosorbents [4-6]. Ellagic acid (Figure 1) is one of the polyphenolic compounds found in many plants (dicotyledonous, Eucalyptus, etc.) and fruits (walnut, strawberry, grape, etc.); it occurs widely in nature either in its free form or in the form of ellagitannins or glycosides . The heartwood of Eucalyptus species, commonly used in the pulp and paper industry in Australia, is known to contain large amounts of ellagitannins which can be converted to ellagic acid . Ellagic acid forms complexes with many metal ions including magnesium and calcium, that have very low solubility in water [9, 10]. It has therefore been a nuisance to the pulp and paper industry using Eucalyptus species in the neutral sulphite process due to the formation of metal deposits inside the processing equipment. However this property also makes it potentially valuable as a metal scavenger for wastewater treatment.
A waste pipe deposit from a neutral sulphite pulp mill processing Eucalyptus has been demonstrated to adsorb copper (II), chromium (III) and zinc (II) ions from dilute single metal solutions after pretreatment of the waste with methanol and hydrochloric acid to remove complexed metal ions . The adsorption kinetics were found to be comparable to those of other biosorbents, with 95% of the equilibrium adsorption of chromium (III) achieved within two hours’ contact. The adsorbent capacity for copper (II) ions (up to 6.2 meq⋅g-1) was higher than many other biosorbents and commercial ion exchange resins described in the literature [11-17]. The primary mechanism of metal uptake was found to be ion exchange of protons from the adsorbent with metal ions from solution. Thus, the extent of adsorption onto the treated waste was pH dependent with different optimum pH conditions found for each metal tested. This result indicated the promise of this material for selective metal ion separations from multicomponent solutions.
Figure1. Ellagic acid
In order to be successfully applied for metal removal from wastewater, adsorbents must also perform well in multicomponent solutions which are typical of industrial wastes. Therefore, this paper compares the performance of an ellagic acid adsorbent obtained from a pulp mill waste in removing metal ions from single and binary metal solutions. The regeneration and reuse of the adsorbent have also been studied, as this will be a further prerequisite for its effective application to industrial wastewater treatment.