摘要
壳聚糖是自然界中迄今为止发现的唯一的碱性多糖,其大分子链上含有很多活泼的氨基和羟基,能与金属离子结合形成螯合沉淀物。通过对壳聚糖进行化学改性,可以大大改善其对金属离子的螯合能力和稳定性,被广泛应用于处理含重金属离子的废水。与此同时,随着养殖业规模化、集约化发展,大量的饲料被应用到其中,而饲料中含有大量重金属微量元素,有些未被动物吸收的重金属元素会随粪便排入水体,对水体环境造成威胁。
本文就是利用壳聚糖和二硫代氨基甲酸(DTC)基团的结构和特点,将壳聚糖进行化学改性,通过黄原酸化反应,在壳聚糖分子链上引入二硫代氨基甲酸(DTC)基团,得到一种DTC类改性壳聚糖。通过一系列单因素试验和正交试验得到制备DTC类壳聚糖的最优反应条件为：甲醇体积为150mL,溶胀时间为1.5 h,CS2与乙醇混合溶液的滴加时间为1.5 h,NaOH溶液浓度为50%,CTS(g)/CS2(mL)为1：6,水浴温度为45℃,水浴时间为14h,且四个主要试验因素对试验结果影响的主次顺序为：CTS(g)/CS2(mL)>NaOH浓度>水浴时间>水浴温度。
分别从介质酸度、改性壳聚糖剂量、反应温度、吸附等温线、振荡时间及离子共存等6个方面,研究了改性壳聚糖对Cu2+、Zn2+的吸附性能。试验结果表明：改性后的壳聚糖适用的pH值范围广,随着pH值的增大,捕集剂对Cu2+、Zn2+的去除率有明显的增加,pH值控制在3.0～5.0范围内较好。改性壳聚糖剂量对吸附性能有较大影响,初始Cu2+、Zn2+浓度为100mg/L时,最佳剂量为0.15g。DTC类改性壳聚糖对Cu2+、Zn2+的捕集能力受温度的影响不大,所以试验选择在室温条件下进行。通过对重金属离子的吸附类型分析发现,改性壳聚糖对Cu2+Zn2+的吸附属于单层吸附,与Langmuir公式较为吻合。改性壳聚糖对Cu2+、Zn2+有较快的吸附速度,基本在20min内就达到了吸附平衡。在初始浓度均为100mg/L的Cu2+、Zn2+共存的溶液中,Cu2+、Zn2+平衡吸附容量分别为33.801、31.182mg/g,表明在共存条件下DTC改性壳聚糖对Cu2+有更好的选择性。
利用改性壳聚糖处理含重金属离子的畜禽废水,结果表明,改性壳聚糖对复杂体系下的实际废水有很好的处理效果,对废水中Cu2+的吸附效率优于Zn2+,且出水中的Cu2+、Zn2+的残余浓度能达到国家排放标准。
So far, chitosan is found as the only alkaline polysaccharide in nature, there are lots of lively amino and hydroxyl groups which can chelate with metal ions to form precipitates connected in molecules chain. The ability of chelating metal ions as well as stability of chitosan can be greatly improved by chemical modification, so that to be widely used in treatment of wastewater containing heavy metal ions.
We made use of the structure and characteristics of chitosan and dithiocarbamate (DTC) groups, proceeded chemical modification on chitosan, and introduced dithiocarbamate (DTC) groups into molecular chain of chitosan through xanthation to form a class of DTC-modified chitosan. A series of single factor experiments and orthogonal test were conducted to found out the optimal preparation of DTC-modified chitosan were:methanol volume of 150mL, swelling time of 1.5 h, dropping time of CS2 and ethanol mixed solution of 1.5 h, NaOH concentration at 50%, ratio of CTS (g) /CS2 (mL) at 1:6, water bath temperature at 45℃, bath time of 14h. The influence order of four main experimental factors was:CTS (g)/CS2 (mL)> NaOH concentrations> bath time> bath temperature.Meanwhile, with the scaled and intensive development at the breeding business, large quantity of feed be used, but there are many heavy metal ions in the feed, the ion which unabsorbed by animals may into water through excrement, which may bring about intimidate to water environment.
The adsorption of modified chitosan for Cu2+ and Zn2+ was discussed form the medium acidity, modified chitosan dose, reaction temperature, adsorption isotherm, shaking time and ions coexistence. The results showed that:the modification of chitosan widened the range of pH value, the removal rate of trapping agent for Cu2+. Zn2+ significantly increased with the pH value increase, the pH value should controlled in the range of 3.0～5.0 for better results. The dose of modified chitosan had a greater impact on adsorption properties, the optimal dose was 0.15g when the initial concentration of Cu2+ and Zn2+ at 100mg/L. The capture ability of DTC-modified chitosan was not affected too much by temperature, so the experiments were carried out at room temperature. Through analyzing the adsorption type of heavy metal ions, we found the adsorption of modified chitosan for Cu2+ and Zn2+ was monolayer, and this was identical with Langmuir equation. Modified chitosan had a faster absorption rate on Cu2+ and Zn2+, the equilibrium was reached basically in 20minutes. The Cu2+ and Zn2+ equilibrium adsorption capacity were separately 33.801 mg/g and 31.182 mg/g in co-exist solution with initial Cu2+ and Zn2+ concentrations both at 100mg/L, this indicated that DTC-modified chitosan had a better selectivity on Cu2+
Through the treatment of livestock wastewater containing heavy metal ions with modified chitosan, we could found that the modified chitosan have a good treatment effect on actual wastewater under complex systems. The adsorption efficient for Cu2+ was higher than Zn2+, and the residual concentrations of Cu2+ and Zn2+ in effluent both reached the national emission standards.
引文
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