| The tanning process is the crosslink and denaturation of the collagen in the rawhide.The substance that cross-links and denatures rawhide collagen is called tanning agent.In the current tanning industry,the tanning agents mainly include chromium salts,tannin extracts,acids,aluminum salts and polymer resins.The compounds of heavy metal trivalent chromium account for 85% of the total tanning agents.A large amount of solid waste will be produced during the shaving and trimming processes in tanning,the main components of which are chromium and collagen.Removal of chromium salts and extraction of collagen are of great interest in current research.In this paper,the chrome shavings were firstly hydrolyzed under the combination of acid and alkali to prepare collagen powder.And then the collagen powder was further hydrolyzed by the enzymatic method to increase the degree of hydrolysis,reduce the molecular weight of the hydrolysate(peptide and amino acid),release more free amino groups,and increase its reactivity.Finally,the obtained product was modified and used as a scale inhibitor to inhibit the formation of calcium carbonate scale in circulating water,which supplied a new way for the utilization of chrome shavings.the chrome leather shavings were pre-soaked at room temperature for 24 h by using of 15% concentrated sulfuric acid.And then the pre-treated chrome leather shavings were hydrolyzed by 5% CaO and 3% NaOH at 90 °C with a liquid-solid ratio of 7 for 8 h.Collagen powder products were prepared by spray drying with crude protein content of 90.19%,ash content of 4.59%,chromium content of 16 mg/kg,and free amino content of 1.02%.The collagen powder was hydrolyzed again by protease in order to break more peptide bonds and obtain small molecular weight products.The optimal enzymatic hydrolysis process with the degree of hydrolysis(DH)as the index is as follows.First,the alkaline protease was used to hydrolyze the collagen powder.And the conditions are the liquid-solid ratio of 25,the hydrolysis temperature of 50 °C,the pH value of 10,the hydrolysis time of 6 h,and the enzyme dosage of 5.5%.After the temperature is increased to inactivate the enzyme,and the pH of the system is adjusted to 6,the neutral protease is used to continue the hydrolysis for 6 h.The conditions are the hydrolysis temperature of 50 °C and the enzyme dosage is 4%.The final hydrolysis degree of the collagen peptide powder product was 32.93%,and the content of free amino group was increased by 3.59%.The molecular weight was 514 Da detected by gel permeation chromatography(GPC),which is a small molecular weight peptide.The peptide chain of enzymatically hydrolyzed collagen is cross-linked using glycerol triglycidyl ether,and then acrylic acid is used as a graft monomer to graft copolymerize with the cross-linked collagen to obtain a modified product rich in carboxyl groups.Infrared spectrometer(FT-IR)and differential scanning calorimeter(DSC)are used to detect the molecular structure and thermal stability before and after modification,the results show that the crosslinking and grafting reactions occur successfully,and the thermal stability of the modified molecule is significantly improved.The modified product was used as the scale inhibitor for circulating water and its inhibitory effect on calcium carbonate scale was studied through static scale inhibition experiments.It is found that the scale inhibitor under the conditions of 15 g acrylic acid monomer,8% initiator ammonium persulfate(based on monomer mass)and the dosage of 20 mg/L are added to the system,the scale inhibition rate can reach 89.20%.Compared with the unmodified raw material,the scale inhibition rate increased by 51.89%.Scanning electron microscopy(SEM)was used to observe the morphology of calcium carbonate crystals.It was found that the addition of scale inhibitor could completely destroy the morphology of calcium carbonate crystals,which further confirmed that the modified product had a strong calcium carbonate scale inhibition effect. |
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