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我国每年皮革产量折合标准皮约为7000万张,制革过程中原皮质量的30%变成铬屑[1],铬屑中有3%—5%的铬含量[2],90%以上为胶原纤维,目前,主要通过酸法、碱法和酶法等方法[3—5]将铬屑水解转化为工业蛋白或工业明胶,并已实现产业化[6—7]。但由于铬屑水解过程中溶解性胶原分子量分布的不可控[8],导致工业明胶和蛋白中残余铬达不到产品质量要求[9],成为革屑资源化的技术瓶颈。目前有关铬屑脱铬过程中铬残留问题,人们都通过水解方式及工艺参数控制加以解决[10—11],对水解脱铬过程中铬的形态变化与结合方式没有给予关注。
鞣制化学理论表明,胶原分子内由3条肽链以平行,右手螺旋形式缠绕形成三螺旋结构,胶原分子在三螺旋结构基础上通过共价键交联形成稳定的胶原微纤维,并逐步聚集成束形成胶原亚纤维、胶原原纤维最终组成胶原纤维[12]。制革鞣制过程中,铬通过渗透达到不同层级的胶原纤维表面,再与胶原纤维表面相邻分子链上离解羧基配合形成铬鞣革。胶原分子肽链中每3个氨基酸残基就有一个要经过此三螺旋结构中央区,残基沿轴向距离为0.29 nm[13],分布于胶原纤维相邻分子链上离解羧基之间间距约1.4 nm,铬一般通过多核配位来实现架桥络合[14]。从渗透阻力和胶原分子结构等方面来说,鞣制过程中铬分布在胶原分子内部的可能性极小,甚至进入胶原微纤维层次的比例也较少。但是在水解过程中,由于水解工艺对铬屑胶原结构的破坏作用,使铬在胶原纤维内部和水解液中铬的分布发生显著变化,由此导致铬从固相脱除至水解液中的脱除过程并非单纯的铬鞣的逆过程。目前水解过程中胶原与铬分离不彻底与铬在胶原间分布有关关系目前并不明确,有机酸等强络合剂浸提后仍无法从胶原上彻底脱除铬的原因也不清晰,在当前含铬危废资源化利用要求越来越严格的背景下,相关方面的研究也越来越迫切。
本实验以碱水解协同酸淋洗对铬屑脱铬的实际生产过程进行模拟,探讨了不同水解程度下铬屑的脱铬效果和水解液中铬与水解胶原的分布规律,以Ⅰ型胶原为铬屑皮胶原模拟物采用荧光光谱法解释了柠檬酸及柠檬酸-铬络合物与胶原的相互作用,为铬屑资源化提供参考依据。
铬屑碱解过程中铬与胶原分子量分布特征
Dechromation and molecular weight distribution of hydrolyzed collagen from chromium—containing leather during alkaline
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摘要: 酸碱法对含铬革脱铬时,由于胶原水解过程中胶原分子分布的不可控性,导致资源化利用生产的工业明胶和蛋白中残余铬达不到产品要求。通过实际生产过程模拟,探讨了不同碱解程度下铬屑水解液中胶原分子量分布与铬的分布规律。研究结果表明,随着碱浓度增加,铬屑脱铬率逐渐上升。碱水解24 h协同硫酸淋洗1 h的脱铬率为91.60%,加入柠檬酸可促进了铬屑上铬的溶解和分离,脱铬率达到97.96%。不同碱解液中溶解性胶原分子量分布表现出显著差异,而铬与水解胶原分子量则存在着明显的非均匀性分布,其中55.63%—72.66%的总铬分布在>1000 kDA区间;0—26.53%的总铬与1000—250 kDA区间的水解胶原结合;7.92%—18.53%的总铬与胶原分子和明胶混合物结合;0—4.60%的总铬与工业明胶结合;<50 kDA的多肽混合物上未结合铬。上述残存的铬除大量[Cr(H2O)6]3+外,部分以络合态与水解胶原结合。结合的铬含量占比最高达到20%,水解过程中从铬屑中脱除的铬与小分子胶原片段发生了再络合。追踪水解液中>250 kDA区间总铬占比约55.63%—88.89%,可以认为制革鞣制过程中铬经渗透后主要结合在胶原微纤维或亚纤维层次结构上。加入的柠檬酸在促进了铬从固相中分离出铬的同时,柠檬酸及柠檬酸-铬络合物可与皮胶原形成复合物,铬脱除的过程中以上两种结合皮胶原的方式,是铬与皮胶原无法彻底分离的主要原因。Abstract: The uncontrollable distribution of collagen molecules during collagen hydrolysis in the acid-base method for chromium-containing leather results in the production of gelatin and protein with residual chromium that does not meet industrial product requirements. In this study, the distribution patterns of collagen molecular weight and chromium in chromium scrap hydrolysate under different alkali concentrations were investigated by simulating the actual production process. The results showed that the rate of chromium removal from chromium chips gradually increased with the increase of alkali concentration. The chrome removal rate of alkali hydrolysis for 24 h in cooperation with sulfuric acid drenching for 1 h was 91.60%, while the addition of citric acid promoted the dissolution and separation of chromium on the chips, and the chrome removal rate reached 97.96%. The chromium and hydrolyzed collagen molecular weight showed a significant non-uniform distribution, where 55.63%—72.66% of total chromium was distributed in the >1000 kDA interval; 0—26.53% of total chromium was bound to 1000—250 kDA interval; 7.92%—18.53% of the total chromium was bound to the collagen molecules and gelatin mixture; 0—4.60% of the total chromium was bound to the industrial gelatin; and no chromium was bound to the peptide mixture of <50 kDA. The above residual chromium was partially bound to hydrolyzed collagen as complex, except for a significant amount of [Cr(H2O)6]3+. The bound chromium content accounted for up to 20%, and the chromium removed from the chromium chips during hydrolysis underwent re-complexation with small molecule collagen fragments. The experimental data confirmed that the percentage of total chromium in the >250 kDA interval in the hydrolysate was about 55.63%—88.89%, which can be assumed that chromium was mainly bound to the collagen microfibrils or sub-fibrillar hierarchy after penetration during the tanning process. While the added citric acid facilitated the separation of chromium from the solid phase, citric acid and citric acid-chromium complexes can reform complexes with collagen, and the above two ways of binding collagen in the process of chromium removal are the main reasons why chromium and collagen cannot be completely separated by acid-base method.
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图 6 胶原多层结构与铬分布图(a)胶原多层结构图[12](b)不同浓度碱水解液中总铬占比图(c)铬屑SEM图
Figure 6. Collagen multilayer structure and chromium distribution map (a) collagen multilayer structure (b) proportion of total chromium in different concentrations of alkaline hydrolysis solution (c) SEM images of chromium-containing leather shavings
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