Introduction
Co-disposal in cement kilns is a disposal method to achieve harmless and resourceful reuse of solid waste. However, cement kiln co-disposal technology is constrained by some specific components in solid waste. For solid wastes with a high content of heavy metals, such as waste battery slag, fly ash and electroplating sludge, heavy metals are involved in the clinker firing process through solid-phase and liquid-phase reactions during co-disposal in cement kilns, which may ultimately increase the content of heavy metals in the cement clinker, which can be re-released in the course of use, posing a threat to the environment and human safety. Also the presence of heavy metal ions can have a hindering effect on cement hydration.In recent years, fiber adsorbent materials have been widely used to treat heavy metal ions in wastewater, and researchers have prepared related materials with excellent performance. Chitosan is one of them, which is a natural biopolymer similar in structure to cellulose fibers commonly used in construction projects. Chitosan has its unique advantages: renewable resource, wide range resource, relatively low cost, having functional groups such as hydroxyl and amino, which, makes it an excellent adsorbent for metal ions. This work prepared modified chitosan material and its application on heavy metal-containing cement was investigated.
Mthods
Chitosan, disodium EDTA, 1-ethyl-(3-dimethylaminopropyl)carbodiimide hydrochloride, N-hydroxysuccinimide, formaldehyde, aminothiourea, copper nitrate, lead nitrate, zinc nitrate, anhydrous ethanol and other compounds at analytically pure grade were supplied by Beijing Jiashiteng Trade Limited Liability Company. The cement P·I 42.5 and standard sand used in this study were produced from Fushun Ausair Technology Limited Liability Company.In this study, an aminothiourea combined with ethylenediaminetetraacetic acid modified chitosan material (TS-EDTA-C) was prepared to increase its adsorption and stability of heavy metal ions by modification. TS-EDTA-C was added to the cement containing heavy metal ions to test the leaching concentration of heavy metal ions in the cement specimens at 7, 14 d and 28 d as well as the compressive and flexural strengths of the cement specimens at 3, 7, 14 d and 28 d. In addition, the heat of hydration of the cement specimens at 7 d was tested. Cement specimens at various ages were tested by XRD. The presence of heavy metals in the cementitious materials was tested by Tessier's five-step sequential extraction method.
Results and discussion
The adsorption of modified chitosan (TS-EDTA-C) for Zn2+, Cu2+ and Pb2+ in solution was greatly improved and far exceeded that of unmodified chitosan (CS). The higher adsorption is mainly due to the introduction of a large number of groups such as —OH, —COOH, —NH2 and S on chitosan that can react with metal ions, while the modified chitosan surface is rougher, providing a larger adsorption surface. By comparing the cement specimens, it was found that the strength of the cement specimens containing heavy metals with the addition of TS-EDTA-C was higher than that of the specimens containing heavy metals without the addition of TS-EDTA-C at all ages, and the strength of the cement specimens without the addition of heavy metal ions was the highest. Because the hydration of cement occurs in an alkaline environment, heavy metal ions preferentially calcium ions react with OH– to generate some insoluble products, and the precipitation adsorbed on the surface of the cement particles impedes the further dissolution of cement mineral ions, leading to a decrease in the hydration products contributing to the cementitious properties and causing a significant decrease in the early strength of the cement mortar. The additional TS-EDTA-C will react with the heavy metal ions in the cement and adsorb them on the modified chitosan material, reducing the presence of free heavy metal ions and insoluble hydroxyl compounds, weakening their influence on the hydration reaction of the cement as well as lowering the leaching concentration of heavy metal ions.From the heat of hydration and XRD plots of the cement specimens at 7 d, it can be seen that Zn2+, Cu2+ and Pb2+ played an obvious inhibitory effect on the early hydration of cement and prolonged the hydration induction period of cement. The addition of TS-EDTA-C attenuated the effect of heavy metal ions on the early hydration of cement. Because when TS-EDTA-C was added to the heavy metal-containing cement, TS-EDTA-C adsorbed the heavy metal ions in the cement, which led to an increase in the heavy metal ions in the Q4 form and a decrease in the heavy metal ions in the Q1+Q2+Q3 form, reduced the presence of free unstable heavy metal ions, and lowered the unfavorable effects of the heavy metal ions. To a certain extent, it enabled Ca2+ to react with OH-, which promoted the hydration reaction. Meanwhile, the adsorbed heavy metal ions existed in a more stable form, reducing the leaching toxicity of heavy metal ions in cement.
Conclusions
In this paper, modified chitosan TS-EDTA-C (aminothiourea combined with ethylenediaminetetraacetic acid modified chitosan), which can effectively adsorb heavy metal ions, was prepared, and TS-EDTA-C had a good adsorption effect on Zn2+, Cu2+ and Pb2+, with the maximal adsorption amounts of 21.92, 68.08 mg/g, and 36.27 mg/g, respectively. TS-EDTA-C can effectively mitigate the adverse effects of heavy metals on cement properties. The 3, 7, 14 d, and 28 d compressive strengths of cement specimens with TS-EDTA-C were increased by 41.6%, 7.87%, 14.46%, and 2.51%, respectively, compared with those of heavy metal-containing cement specimens without modified chitosan. The heat of hydration and XRD tests proved that the addition of TS-EDTA-C attenuated the adverse effects of heavy metals on cement hydration. Modified chitosan is beneficial for cement stabilization and curing of heavy metals. The leaching concentrations of heavy metal ions were lower than those of the heavy metal-containing cement specimens without the addition of TS-EDTA-C. This is understood like that the addition of TS-EDTA-C stables the organic bound state (Q4) of heavy metals in cement, leading to a more stable form of heavy metal inos in cement.