| The effective treatment of wastewater containing heavy metal ions is much important for the control and remediation of water heavy metal pollution. At present, the methods used for treatment of wastewater containing heay metal ions mainly includes ion exchange, colloid precipitation, electronic remediation, bioremediation and adsorption. Compared with other methods, the adsorption is used more widely due to its advantages of stable, reliable, simple, low power consumption, low repeated pollution. As an important method for wastewater treatment, the key issue of adsorption is adsorbent choice. For treatment of wastewater containing heavy metal ions, adsorbent should be in possession of potential for adsorbing and fixing heavy metal ions effectively, as well as rich source, low cost and easy regeneration. Diatomite is selected from series adsorbents due to it fit to all demands for adsorption application.Diatomite has micropore structure, large specific surface area and many active chemical groups. Hence, it is commonly used for adsorbent. However, diatomite has disadvantage of normal adsorption performance and poor maneuverability due to impurities and structure defects, these disadvantages limits application of diatomite in treatment of wastewater containing heavy metal ions. In order to improve the adsorption performance and maneuverability of diatomite, the studies on diatomite adsorption, modification, shaping and application have been conducted, a large number of scientific data have been acquired, and these data is very significant for the effective control of water heavy metal pollution and fully utility of diatomite store. Main results obtained from this study are summarized as follows.(1) Adsorption density (qe) of each heavy metal ions on natural diatomite increased swiftly at the begining and then slowly attained equilibrium with contact time prolonging, increased firstly and then decreased with increasing adsorbent concentration, increased constantly with increasing solution initial pH value, and increased at the beginning and then decreased with increasing ion initial concentration. However, the effect of solution temperature on ion adsorption was not obvious. The adsorption capacity of each heavy metal ions followed the sequence:Zn2+>Fe3+> Cd2+>Cu2+>Pb2+>Mn2+, and it was closely related with ions hydrated radius and hydration free energy.(2) The optimal conditions for Pb2+ions adsorption on natural diatomite is Wo8.0g/L, Co400mg/L, pH7.0, T2℃, t120min, that of Cd2+is Wo6.0g/L, Co200mg/L, pH5.0, T25℃,t120min, that of Cu2+is Wo6.0g/L, Co300mg/L, pH5.0, T25℃,/120min, that of Zn2+is Wo8.0g/L, Co500mg/L, pH6.0, T25℃, t120min, that of Mn2+is Wo8.0g/L, Co150mg/L, pH4.5, T25℃, t120min, that of Fe3+Wo4.0g/L, Co400mg/L, pH5.0,725℃, t120min.(3) The most suitable isotherm model for describing adsorption of Pb2+, Cd2+Cu2+, Zn2+, Mn2+, Fe3+on natural diatomite was Tenkin, Tenkin, Langmuir, Tenkin, Freundlich and Freundlich, respectively. The most suitable kinetic model for describing adsorption of each ion is Pseudo-second-order. The nature of each ion adsorption on natural diatomite was physical, favorable, and the control step of each ion adsorption process was adsorption reaction happened in diatomite micro-pores. The adsorption process of each ion on natural diatomite was spontaneous, endothermic, and disorder enhancing. Classical isotherm models have problems of obvious adsorption concentration effect and unstable parameters, equilibrium ion adsorption density qe is not single function of the equilibrium ion concentration Ce in bulk solution, while is the function of two variable viz. Ce and Wo, qe only depends on Ce/Wo.(4) The results of normal chemical modification indicated that, the optimal calcination temperature was400℃, the optimal acid solution (HC1) concentration (v/v) was10%, the optimal concentration of NaCl (n/v), BaCl1(n/v) and PAM (m/v) for diatomite modification was0.4mol/L,0.05mol/L and20g/L, respectively. Calcination and acid solution immersing only can remove the impurities from surface and pores of natural diatomite, modification by NaCl mainly increased diatomite adsorption capacity by enhancing surface negative charge, modification by BaCl2and H2SO4mainly improved diatomite pore structure by deposited crystal polishing, and PAM coated modification apparently increased adsorption capacity significantly (Adsorption density of Pb2+on diatomite modified by PAM is five times of natural diatomite). In fact, Pb2+ions removal mainly depended on PAM self flocculation.(5) The results of deep pillaring modification indicated that, compared with normal modification methods, polyhydroxyl-aluminum pillaring can improve diatomite pore structure obviously. Diatomite adsorption capacity attained greatly increase after pillaring. The adsorption capacity of Pb2+on pillared diatomite reached14.02mg/g, and it was higher than most other adsorbents. The experimental data showed that pillared diatomite yielding the optimal adsorption density (qe) of Pb2+was synthesized using the following parameters:addition of pillaring solution containing Al3+-oligomers with a concentration range of0.1-0.2mol/L to a suspension containing Na+-diatomite to obtain the required Al/diatomite ratio of10mmol/g; synthesis temperature of80℃for120min; aging at a temperature of105℃for16h.(6) The development of shaped diatomite sample indicated that, the optimal addition ratio of powder diatomite and superfine carbon was93.0%and7.0%, the optimal sintering temperature range was800-1000℃, the optimal calcination time was90min, the optimal diatomite particle size was2.40μm, while preparing pillared diatomite. The shaped diatomite sample was homogeneous brown oval particle with a size of5mm, and was not easy broken. The shaped diatomite samples remained original pore structure, its surface impurities was removed, its pore size was increased, and its pore structure became clear. The main phase of shaped diatomite samples was cristobalite.(7) The adsorption property of shaped diatomite samples was increased by7.58%compared with natural diatomite; it indicated that shaping didn’t destroy diatomite pore structure, instead improved. The maneuverability of shaped diatomite samples was better than natural diatomite. The aim of shaping attained, viz. increasing diatomite maneuverability significantly based on ensuring its adsorption property.(8) The results of diatomite application in treatment of wastewater containing Fe3+ions indicated that, industry wastewater containing Fe3+can be disposed effectively by the treatment process with core material of shaped diatomite samples, the effluent quality reached the requirement of GB/T18921-2002. it was feasible to dispose landfill leachate by Fenton reagent which prepared by H2O2and waste acid solution obtained from shaped diatomite sample regeneration. The successful use of waste acid solution achieved environmental protection idea "waste control by waste". |
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