The interfacial interaction between typical microplastics and Pb2+ and their combined toxicity to Chlorella pyrenoidosa.

Microplastics (MPs), a new type of pollutant, have attracted much attention worldwide. MPs are often complexed with other pollutants such as heavy metals, resulting in combined toxicity to organisms in the environment. Studies on the combined toxicity of MPs and heavy metals have usually focused on the marine, while on the freshwater are lacking. In order to understand the combined toxic effects of MPs and heavy metals in the freshwater, five typical MPs (PVC, PE, PP, PS, PET) were selected to investigate the adsorption characteristics of MPs to Pb2+ before and after the MPs aging by ultraviolet (UV) irradiation through static adsorption tests. The results showed that UV aging enhanced adsorption of Pb2+ by MPs. It is noteworthy that MPs-PET had the highest adsorption capacity for Pb2+, and the interaction between MPs-PET and Pb2+ was the strongest. We specifically selected MPs-PET to study its combined toxicity with Pb2+ to Chlorella pyrenoidosa. In the combined toxicity test, MPs-PET and Pb2+ had significant toxic effects on Chlorella pyrenoidosa in the individual exposure, and the toxicity of individual Pb2+ exposure was greater than that of individual MPs-PET exposure. In the combined exposure, when MPs-PET and Pb2+ without adsorption (MPs-PET/Pb2+), MPs-PET and Pb2+ had a synergistic effect, which would produce strong physical and chemical stress on Chlorella pyrenoidosa simultaneously, and the toxic effect was the most significant. After the adsorption of MPs-PET and Pb2+ (MPs-PET@Pb2+), the concentration and activity of Pb2+ decreased due to the adsorption and fixation of MPs-PET, and the chemical stress on Chlorella pyrenoidosa was reduced, but the physical stress of MPs-PET still existed and posed a serious threat to the survival of Chlorella pyrenoidosa. This study has provided a theoretical basis for further assessment of the potential environmental risks of MPs in combination with other pollutants such as heavy metals.

[Differential Effect and Mechanism of in situ Immobilization of Cadmium Contamination in Soil Using Diatomite Produced from Different Areas].

In order to understand the difference of in situ immobilization effect and mechanism of Cd contamination in soil using diatomite produced from different areas, the test was conducted using diatomite produced from Yunnan Tengchong, Jilin Linjiang, Zhejiang Shengzhou and Henan Xinyang of China as modifiers to immobilize cadmium contamination in simulated soil. The results indicated that the diatomite from all the four producing areas could effectively immobilize available Cd in soil, decreasing the available Cd content in soil by 27.7%, 28.5%, 30.1% and 57.2%, respectively when the adding concentration was 30 g x kg(-1). Their ability for immobilizing available Cd in soil followed the sequence of Henan Xinyang > Zhejiang Shengzhou > Jilin Linjiang > Yunnan Tengchong. It was also found that the physical and chemical properties of diatomite played a main role in soil cadmium immobilization, lower bulk density, larger specific surface area, more micro pores and wider distribution range of aperture were more favorable for available Cd immobilization. The results also showed that, the diatomite could control Cd contamination by changing soil physical and chemical properties, among these properties, pH and organic matter content were the key factors, increasing soil pH value and organic matter content was favorable for available cadmium immobilization, while the soil water content had little effect on available cadmium immobilization. The control of soil cadmium contamination by using diatomite to change cation exchange capacity was limited by time in some degree. The diatomite produced from Henan Xinyang, Zhejiang Shengzhou and Yunnan Tengchong increased the soil pH value and organic matter content, and was favorable for available Cd immobilization, while the diatomite from Jilin Linjiang showed converse effect.

[Preparation of Poly-hydroxy-aluminum Pillared Diatomite and Characteristics of Cu2+, Zn2+ Adsorption on the Pillar in Aqueous Solutions].

As an alternative solution to the limitations in adsorption properties of natural diatomite caused by its physicochemical structure defects and modification effects to diatomite using conventional physical/chemical methods, the diatomite was pillared using poly-hydroxy-aluminum to improve its adsorption properties. The change in physicochemical characteristics of the diatomite before and after pillaring were analyzed by Scanning Electron Microscope (SEM), Fourier Transform Infrared (FTIR) and X-ray Diffraction (XRD). The difference in surface properties of diatomite and its adsorption characteristics of Cu2+, Zn2+ before and after pillaring were analyzed at the same time. The obtained results showed that the optimal conditions for diatomite pillaring were 2.2 of the [OH-]/[Al3+] molar ratio, 1.8 mol·L-1 of potassium chloride, 10 mmol·g-1 of the Al/diatomite ratio, 60℃ of the pillaring temperature, 24 h of the pillaring time, 200℃ of the aging temperature, and 0.5 h of the aging time. After pillaring, the poly-hydroxy-aluminum was successfully exchanged into the diatom shell, forming available pillars, which increased channel spacing, expanded adsorption reaction interface, increased the number of microporous surface hydroxy groups and enhanced the microporous surface activity. The Cu2+, Zn2+ adsorption data obtained both before and after pillaring well fitted the Langmuir isotherm model and the pseudo-second-order kinetics model. After pillaring, the adsorption capacity of Cu2+, Zn2+ onto diatomite reached 7.491, 11.312 mg·g-1, with an increasing percentage of 32.9%, 33.3% respectively, the diatomite adsorption capacity got great improvement.

[Application of classical isothermal adsorption models in heavy metal ions/ diatomite system and related problems].

In order to fully understand adsorption nature of Cu2+, Zn2+, Pb2+, Cd2+, Mn2+, Fe3+ onto natural diatomite, and to find problems of classical isothermal adsorption models' application in liquid/solid system, a series of isothermal adsorption tests were conducted. As results indicate, the most suitable isotherm models for describing adsorption of Pb2+, Cd2+, Cu2+, Zn2+, Mn2+, Fe3+ onto natural diatomite are Tenkin, Tenkin, Langmuir, Tenkin, Freundlich and Freundlich, respectively, the adsorption of each ion onto natural diatomite is mainly a physical process, and the adsorption reaction is favorable. It also can be found that, when using classical isothermal adsorption models to fit the experimental data in liquid/solid system, the equilibrium adsorption amount q(e) is not a single function of ion equilibrium concentration c(e), while is a function of two variables, namely c(e) and the adsorbent concentration W0, q(e) only depends on c(e)/W(0). Results also show that the classical isothermal adsorption models have a significant adsorbent effect, and their parameter values are unstable, the simulation values of parameter differ greatly from the measured values, which is unhelpful for practical use. The tests prove that four-adsorption-components model can be used for describing adsorption behavior of single ion in nature diatomite-liquid system, its parameters k and q(m) have constant values, which is favorable for practical quantitative calculation in a given system.