Decolorization of Amino Acids by Modified Activated Carbon
Due to its superior adsorption properties, activated carbon has been used in water purification, food purification and decolorization, gas adsorption, and many other fields. Regarding surface functional groups containing oxygen, the higher the number of acidic groups, the higher the adsorption efficiency for polar compounds; whereas the higher the number of alkaline groups, the higher the efficiency for low-polar and non-polar substances.
Activated carbon plays an important role in the decolorization of amino acid hydrolysate, but its adsorption of some amino acids is excessively strong, resulting in low yields. In some studies, high yields are achieved by using resin to decolorize amino acids but this also greatly increases the production cost. There are also studies that use reagents such as HNO3, HClO, H2O2, (NH4)2S2O8, KOH to modify and improve the structure, types, and content of functional groups of activated carbon. The tests below are to find the optimal conditions regarding both the decolorization rate and yield of amino acids.
Materials for testing
The tested amino acids: tryptophan (Trp), aspartic acid (Asp), alanine (Ala), methionine (Met), lysine (Lys), phenylalanine (Phe), glutamic acid (Glu), glycine (Gly), histidine (His), arginine (Arg), isoleucine (Ile), leucine (Leu), proline (Pro), serine (Ser), tyrosine (Tyr), threonine (Thr), valine (Val).
The modification of activated carbon: deionized water, 3% sulfuric acid by volume, 0.25% potassium hydroxide by mass, 5% acetic acid by volume are used.
Under the test conditions (pH 7, decolorization temperature 40 °C, decolorization time 40 min, activated carbon dosage 1.5%), among the unmodified activated carbon sample, and samples treated by water, H2SO4, KOH, acetic acid, that treated by water has the highest decolorization performance, but relatively lower yield of amino acids. Whereas that treated by 0.25% KOH has the highest yield and relatively good decolorization performance. So KOH modified activated carbon is chosen for the following steps in the test.
KOH modified activated carbon samples with pH from 2 to 10 are tested under the conditions of temperature 40 °C, time 40 min, carbon dosage 1.5%, the decolorization rate of amino acids gradually increases with the increase of pH, and the yield reaches the maximum at pH 8, and decreases slightly at 10. So with pH 10, the carbon sample has the best decolorization performance and relatively high yield.
Under the conditions of pH 10, the same temperature and time as above, the decolorization rate of activated carbon increases with the increase of the dosage of activated carbon in the range of 0.5% to 2.5%, while the yield of amino acids gradually decreases in the range of 0.5% to 1.5%, sharply decreases in 1.5% to 2% and slightly increases at 2.5%. The results show that the 1.5% dosage rate has the best overall performance.
Under the conditions of dosage rate 1.5%, pH 10, time 40 min, the change of temperature (20 – 90 °C) has no significant effect on the decolorization rate of activated carbon. While the yield of amino acids greatly increases from 40 to 60 °C, and remains high at 80 °C. So 80 °C is selected as the best decolorization temperature as its decolorization rate is higher than 60 °C.
Under the above-mentioned optimal conditions, decolorization times from 10 to 50 min are tested. And both the decolorization rate and yield of amino acids reach the highest at 40 min.
On the basis of the above tests, and the influence of each of the five factors, the optimal production conditions are 0.25% KOH modified activated carbon, pH 10, an activated carbon dosage of 1.5%, decolorization temperature 20 °C, time 20 min, whose amino acid yield is 79.14%, and decolorization rate 83.89%.
Because amino acids contain an amino group (−NH2), an acidic carboxyl group (−COOH), the activated carbon samples have different degrees of adsorption to various amino acids, among which, tyrosine (Tyr) and phenylalanine (Phe) are most easily adsorbed due to the aromatic ring in their structure. And 0.25% KOH modification greatly reduces the adsorption rate of activated carbon to Tyr, as well as the four amino acids, histidine (His), lysine (Lys), isoleucine (Ile), and leucine (Leu).
Adsorption rates of amino acids
|Amino acids||Unmodified activated carbon||KOH modified activated carbon|