Research On The Adsorption Performance Of Graphene Reinforced Cement-Based Composites

With the rapid development of industrial civilization, the people’s livelihood has been improved and scientific and technological progress has been promoted, but it also has brought a serious water environmental problem of heavy metals. Water heavy metal pollution not only causes significant economic losses, but also causes the destruction of ecological balance, and furthermore may also cause serious casualties. Therefore, how to effectively remove heavy metal ions from wastewater has become one of the most pressing issues.Graphene, a new member of carbon family, has received widespread attention of research workers at home and abroad since it was first created. Research on graphene and its composites in the aspect of eliminating water pollution has been carried out in recent years, and the results showed that graphene-based materials possess a good prospect and significant value in the application of wastewater treatment. To improve the hydrophilicity and dispersibility, graphene used in all experiments was treated by nitric acid. This paper studied the the adsorption efficiency of heavy metals (Pb²⁺, Cu²⁺, Zn²⁺, Cd²⁺) by graphene reinforced cement-based composites. In addition, the strengthening and toughening mechanisms of graphene and adsorption mechanism of metal ions on graphene-cement composites was studied. The main work and achievements are as following:（1） Graphene has such characterizations as high adsorption speed and adsorption capacities and can effectively adsorp Pb²⁺ in water. At Pb²⁺ concentration of 10mg/L, pH of 7.0, time of 40min, temperature of 60℃, the purification effect by graphene was deal. Thermodynamics research showed that the adsorption behavior of Pb²⁺ by graphene fitted well with the Langmuir isotherm, and the adsorption process followed the pseudo-second order model.（2） The addition of graphene decreased the fluidity and improved the mechanical properties of cement mortars to some extent. The mechanical properties of prepared graphene-cement composites increased with the preservative age prolonged, and firstly increased and then decreased with the increase of the graphene quantity mixed. It reached the maximum compressive strength and flexural strength of mortars at 28 days when the content of graphene was 0.02wt.%, which increased by 31.63% and 20.28%, respectively. The mechanism of graphene on cement-based composites was studied by a variety of microstructure analysis method including SEM, XRD and TGA. The results showed that the effective improvement of mechanical properties could be attributed to the template effect and filling action of graphene.（3） The addition of graphene improved the adsorption capacities of cement-based composites for Pb²⁺, Cu²⁺, Zn²⁺ and Cd²⁺. Comparing with the blank samples, the removal efficiency of graphene cement composites for Pb²⁺, Cu²⁺, Zn²⁺and Cd²⁺ increased by 10.26%,10.49%,12.2% and 9.07%, respectively. The adsorption mechanism was analyzed by use of SEM, EDS, FT-IR and XRD, and it found that cement-based materials removed heavy metals from water by ion exchange, physical adsorption and metathesis precipitation.（4） The absorption efficiency of graphene-cement composites increased first and then decreased with the increase of pH value. When, the pH value increased to 7, cementitious materials obtained the best adsorptive effect. The removal efficiency of graphene cement composites for Pb²⁺, Cu²⁺, Zn²⁺, Cd²⁺ was 93.22%,91.38%,81.20% and 81.97%, respectively. Heavy metal ions removal from water was due to ionic-exchange under acidic conditions, with weakening of the competitive adsorption behavior between H+ and heavy metal cations, the electrostatic adsorption quantity increased with the increase of pH, then heavy metal ions began to hydrolysis and precipitation reaction to give the biggest share to removal efficiency with the solution pH increased further until neutral or weakly alkaline. Thus it can be seen that the main adsorption method for graphene cement-based materials varies by the change solution pH values.（5） In the four kinds of heavy metals coexistent system, the adsorption selectivity of heavy metals by graphene-cement composites at pH 7.0 was in the following order: Pb²⁺>Cu²⁺>Cd²⁺>Zn²⁺, which suggest that cement-based composites has selective adsorption for various heavy metal ions. The analysis revealed that the adsorption sequence of heavy metals on graphene-cement materials was determined by the characteristics of the ions.