Liquid purification or separation – Processes – Ion exchange or selective sorption
Patent
1999-01-14
2000-10-31
Hruskoci, Peter A.
Liquid purification or separation
Processes
Ion exchange or selective sorption
210688, 210730, 210912, 405128, 536121, 536122, 5361231, C02F 162
Patent
active
061397512
DESCRIPTION:
BRIEF SUMMARY
The invention relates to environmentally friendly complex-forming agents for heavy metals, which agents are based on carbohydrates.
Heavy metals are a major environmental problem. Many heavy metals, such as copper, zinc, cadmium and the like, occur in particular in sludge and manure (pigs). The concentration of heavy metal ions in these materials often exceed the limits set by legislation, for reuse as fertiliser or building soil. Consequently these materials have to be treated as waste materials and burned in incinerators or dumped under special conditions in large depots at considerable cost. Thus there is a need for processes which remove heavy metals from these materials in such a way that reuse is possible.
One way of removing heavy metals is a treatment with compounds which are capable of complexing heavy metal ions from aqueous solutions. Far and away the most important complexing agent is EDTA (ethylenediaminetetraacetic acid). The sequestering capacity (SC), which is expressed as the number of mmol metal bound per gram of complexing agent, is a measure of the complexing power, The SC value of EDTA for cadmium is 4.1. EDTA displays a broad preference for (heavy) metal ions.
The disadvantage of this broad preference is that calcium in particular is captured, as a result of which a large amount of EDTA is needed to be able to complex other, and less environmentally friendly, metal ions in addition to calcium. A second disadvantage of EDTA is that this compound is not biodegradable, as a result of which accumulation of EDTA takes place in water treatment plants. This accumulation has adverse consequences for the effectiveness of the water treatment. A third disadvantage of EDTA is that a flocculating agent is always needed to precipitate the complex of EDTA with the metal ion, so that the contamination can be effectively separated off from the liquid phase.
Consequently, there is a need for new complexing agents which do not have these disadvantages. In the past a great deal of research was carried out on starch derivatives, xanthates (the salts/esters of dithiocarbonic acid) coming to the fore as being suitable (see, for example, U.S. Pat. No. 4,238,329, U.S. Pat. No. 4,083,783, U.S. Pat. No. 4,051,316 and U.S. Pat. No. 3,947,354). The SC value of these starch derivatives is about 0.8 for cadmium. The principle of the use of starch xanthates is to remove the heavy metal ions from aqueous solutions by immobilisation of these ions in an insoluble (cross-linked) starch-xanthate matrix.
It has now been found that complexing derivatives can be prepared from fructans, including inulin, which derivatives are found to have a high complexing capacity for heavy metals, and which are capable of selectively extracting heavy metals from insoluble mixtures containing them.
The process and derivatives according to the invention are defined in the appending claims. The derivatives preferably have a degree of substitution (that is to say a content of complexing groups per monosaccharide unit) (DS) of 0.1-2.5, in particular of 0.2-1.5.
Fructans are understood to comprise all oligo- and polysaccharides which have a majority of anhydrofructose units. The fructans can have a polydisperse chain length distribution and can be straight-chain or branched. They may be linked by .beta.-2,1 bonds as in inulin or by .beta.-2,6 bonds as in levan. The fructans comprise both products obtained directly from a vegetable or other source and products in which the average chain length has been modified (increased or reduced) by fractionation, enzymatic synthesis or hydrolysis. The fructans have an average chain length (=degree of polymerisation, DP) of at least 3, rising to about 1000. Preferably, the average chain length is 3-40, in particular 5-30 monosaccharide units. In particular, the fructan is inulin (.beta.-2,1-fructan) or a modified inulin.
Modified fructans which according to the invention can be converted to complexing derivatives are, for example, hydrolysis products, that is to say fructan derivatives having a shortened cha
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Bogaert Piet Marc Paul
Raaijmakers Henricus Wilhelmus Carolina
Slaghek Theodoor Maximiliaan
Cooperatie Cosun U.A.
Hruskoci Peter A.
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