Scope and Goal
Humic substances (HSs) are the product of microbial degradation, chemical polymerisation and oxidation of organic matter. HSs can be divided into different fractions: humic acid (HA), hymatomelanic acid (HMA), fulvic acid (FA) and humin. HSs play a fundamental role in accumulation and exchange processes of chemical compounds (metals and organic pollutants) in the environment. HSs can form soluble complexes that migrate long distances or precipitate, carrying bound cations with them. The migration/precipitation abilities depend on the metal ion, the ion charge, the degree of ionisation of the organic molecule, the ionic strength of the media, and the location of the metal ion.
Estonia is rich in the Baltic Sea and lake sediments. Historically, mud has been used in human treatment as a curative mud. High-molecular HSs are an important part of curative mud and they have a strong effect on its properties. The curative mud which is used in human therapy may not be polluted with different organic and inorganic contaminants. The aim of this work is to characterise and compare HSs isolated from the Baltic Sea mud (Haapsalu Bay) and from the sediments of Lake Ermistu (Estonia).
Results and Conclusions
We determined the yield of basic extraction of different HSs components from mud in the course of separation. We found that acid pre-treatment of mud increased the amount of extracted HSs more in the sea mud than in the lake mud. These results show that HSs are bound to the inorganic/organic structure of mud and are released during prolonged treatment with an acid.
We performed elemental analysis of the different fractions of HS extracts. HMA fractions had the highest carbon content and the lowest nitrogen content. HMA contains more polysaccharides than amino acid residues. These subunits may cause a better solubility of HMA in water as compared to HA. Acid pre-treatment of the natural sea and the lake mud diminished the content of carbon in most of the HS fractions. The content of nitrogen in the sea and in lake FA diminished by about two times.
We determined the metal content in the mud and its HS fractions. We found that the concentration of heavy metals Pb and Cr is lower than <0.08 mg/kg. As expected, the total metal concentration is considerably lower in the lake than in the sea mud. Acid pre-treatment of mud shows that the sea HA forms more stable Fe and Mg complexes, while the sea HMA contains more stable Zn and Cu complexes. The lake HSs result in more stable Mg complexes with HA and HMA fractions. Sea FA binds Cu and Mg better, but lake FA is more effective in binding Zn. To compare the amount of metals extracted from HA, HMA, FA (with that, remains in HS fractions), the metal concentration in the alkaline full extract of the sea mud was determined. Alkaline treatment removed 0.8% Mg, 7.9% Cu, 5.2% Zn and 3.8% Fe together with HSs from the sea mud, the rest remained in humin and in the mineral part of the mud. The following work-up was additionally carried out for most of the metals. So, we found that there was only 1.7% of Fe, 2.1 % of Mg, 23.2% of Zn and 45.2% of Cu left in HA, HMA and the FA fraction (as a sum) from the total amount of those metals in HSs. This means that Cu is the metal most strongly bound to HSs. On the bases of HS separation data we found that the order of stability of the metal-humate complexes is Cu>Zn>Mg>Fe.
The ultraviolet spectra of HA, HMA and FA revealed that HMA had the biggest molar absorption and calculated aromaticity. Recommendations and Outlook. The characteristics of HSs, isolated from the Baltic Sea and Lake Ermistu mud reveal the difference of HSs from the sea and the lake mud. Also, different properties of HS fractions are observed. Metals are concentrated variously in different HS fractions. In all cases the content of Pb and Cr was low, meaning that the mud preparations are nontoxic in respect to these metals. The order of stability of metal-humate complexes is Cu>Zn>Mg>Fe. The order of affinity of metals to HS fractions obtained have to be extended to other metals of environmental interest. The ability of HSs to bind metals may make them a candidate for natural, environmentally safe substances to concentrate hazardous metals and to remove them from natural water reservoirs.