Size-exclusion chromatographic study of ECF and TCF softwood kraft pulp bleaching liquors.

BACKGROUND, AIMS, AND SCOPE: Currently, elemental chlorine-free (ECF) and totally chlorine-free (TCF) bleaching systems are widely used for pulp production. Low and medium molecular weight lignin break-down products are known to have harmful effects on the environment. According to some recent results, also high molecular weight (HMW) material consisting mainly of lignin and carbohydrates may cause toxic effects to the environment. For these reasons, toxicity and structure studies of HMW materials are of great importance. This investigation is a part of a larger project to obtain more structure information of HMW materials and toxicity of ECF and TCF bleaching effluents. Size-exclusion chromatography (SEC) has been commonly used for the characterization of organic macromolecules such as lignin, but to our knowledge, no reports have appeared dealing with the comparison of SEC of ECF and TCF bleaching liquors. The aim of the present study was to get more information about the molecular weight distribution (MWD) of HMW fractions of waste liquors from ECF and TCF bleaching sequences by SEC. METHODS: The MWDs of organic materials dissolved during different stages of ECF bleaching (O-D-EOP-D-ED) and TCF bleaching (O-Z-Q-P-Z-Q-P-P) of softwood (Pinus sylvestris) kraft pulp were determined and compared by SEC. All effluent samples from the above bleaching stages were ultrafiltrated using a membrane with a cutoff value of 1,000 Da. SEC was performed on high and also low molecular weight fractions and non-fractionated effluents. In the SEC experiments, a Superdex 75 column was used with 0.1 M NaOH solution as the eluent. Standards used for calibrating the SEC system were albumin, carboanhydrase, cytochrome C, tannic acid, dehydrodiacetovanillone, and vanillin. RESULTS AND DISCUSSION: The chromatograms of liquors from TCF bleaching stages vary more than those from ECF bleaching. Peroxide and chelating stages contained mostly high molecular weight (HMW) matter whereas chlorine dioxide and ozone stages had more low molecular weight compounds. The lignin content in HMW matter was higher than in stages that consisted of low molecular matter. Bleaching effluents contained the highest amounts of HMW material, mainly lignin, in the beginning of the sequences; the amounts decreased towards the end of the bleaching sequence. CONCLUSION: Determinations of MWD by the SEC method showed that effluents from the TCF sequence contained more HMW material than those from the ECF stage. This might be due to peroxide stages (P) that dissolve HMW lignin effectively. However, the molecular weights of ozone stages (Z) were very low compared to other stages. Chlorine dioxide stages also dissolved mostly low molecular weight lignin. Ultrafiltration of bleaching liquors showed that high molecular weight fraction also included some low molecular weight compounds and vice versa. High polydispersity and high lignin content correlated with the amount of HMW material in ECF and TCF bleaching stages. RECOMMENDATION AND OUTLOOK: Our liquor samples were studied by using a UV detector commonly used for lignin preparations; in upcoming investigations, it will be interesting to determine carbohydrates such as hemicelluloses. The results are applicable in papermaking in order to improve commonly used bleaching procedures, to test new potential bleaching systems, and to study chemical behavior of HMW materials in various bleaching liquors. The present results also form a good basis for toxicity measurements of ECF and TCF bleaching effluents and for more comprehensive spectroscopic and chromatographic experiments with samples taken from various bleaching stages. From the behavior of liquors studied, it appears that our other structure investigations by spectroscopic and chromatographic (NMR, Py-GC/MS, etc.) methods mostly correlate well with the present results.

Biological effects of high molecular weight lignin derivatives.

A number of high molecular weight (HMW) lignin derivatives possessing varied chemical properties were screened for their biological effects in order to obtain more information on the possible structural features of HMW lignin-related effects. The studied compounds were both commercial and in-house extracted lignin derivatives. Bioassays used include reverse electron transport (RET), Vibrio fischeri, Daphnia magna, and juvenile rainbow trout (Oncorhynchus mykiss) hepatocytes. The studied lignin derivatives inhibited the in vitro systems and luminescence of V. fischeri bacteria to some extent-daphnids were not affected. It seems that, at least in the RET assay, certain pH-dependent functional groups in lignin may be of importance regarding the biological effects.

Multivariate statistics of the pyrolysis products of high molecular components in pulping wastewaters to explain their toxicity.

BACKGROUND, AIM AND SCOPE: At present, large-scale paper manufacture involves delignification and bleaching by elemental chlorine free (ECF), or totally chlorine free (TCF) processes. The wastewater is purified by secondary treatment (mechanical, chemical and biological) which removes most of the toxic substances from the discharge. However, we found residual toxicity in the high molecular (> 1000 D) matter (HMWM) of the discharge by test of the RET (reverse electronic transfer) inhibition. This fraction consists mainly of polydispersed lignin (LIG) and carbohydrate (CH) macromolecules. Structural units in these molecules are studied by pyrolysis gas chromatography/mass spectrometry (Pyr-GC/MS). In the present work, our aim was to find out those structural units which could explain the RET toxicity of LIG or CH molecules. We compared statistically RET toxicity values of the HMWM samples from treated wastewaters of pilot pulping experiments and intensity variation of the pyrolysis product gas chromatograms of these samples. This application is a novel study procedure. METHODS: Pyrolysis products (Py-GC/MS results) and inhibition of RET (reverse electronic transport toxicity) as TU50 and TU20 of HMWM (High Molecular Weight Material; Mw > 1000 D) were compared by multivariate statistics. The samples were from laboratory pilot stages of TCF (Totally Chlorine Free) and ECF (Elemental Chlorine Free) manufacture of softwood pulp. Py-GC/MS was done without and with the addition of TMAH (Tetra Methyl Ammonium Hydroxide). The name and structure of each abundant fragment compound was identified from its retention time and mass spectrum compared to authentic reference compounds or literature. Four sets of Toxicity Units (TUs) and GC peak areas of the pyrolysis fragments were obtained. The data were normalized by division with LIG (lignin content of each sample). TU values were dependent and the fragment values independent (explanatory) variables in statistical treatments by SPSS system. Separate analyses of correlations, principal components (PCA) and stepwise multiple linear regression (SMLR) were performed from the four sample sets TCF and ECF with and without TMAH. RESULTS AND DISCUSSION: From the CH fragments, 2-furfural in TCF, and from the LIG fragments, styrene in ECF showed the highest probabilities of originating from source structures of toxicity. Other possible compounds in concern were indicated to be CH fragment 2-methyl-2-cyclopenten-1-one in ECF and LIG fragments 2-methoxy-4-methylphenol, 4,5-dimethoxy-2-methylphenol and 2-methylphenol in TCF.