Study On Characteristics Of Cu(â…¡) Adsorption In Vermiculite-aqueous Solutions

Along with the rapid development of industry, heavy metal pollution has become a severe problem that gives potential hazard to ecosystems and ultimately the public health. Development of new techniques for removal of heavy metal ions from wastewaters has thus become an important topic in water environmental protection. As a new technology developed in recent years with characters of low energy consumption and low operation and maintenance cost, constructed wetlands (CW) used for treatment of wastewaters containing heavy metal pollutants have received great attention in the field of environmental science. The ability to remove heavy metal ions from wastewaters in the CW system depends highly on the adsorption capacity of the fillers or adsorbents. For selection of proper adsorbents with low cost and high adsorption capacity it needs to know the quantitative relationship between the amount of wastewater to be treated, the metal ion concentration in the wastewater, the adsorption capacity of the adsorbent and the adsorbent quantity to be used for reducing the metal ion concentration to a stipulated discharge standard. This involves mechanisms of ion adsorption in liquid/solid systems.All traditional adsorption isotherms, when being applied to describe the ion adsorption in liquid/solid systems, define the equilibrium ion adsorption density qe as a single function of the ion concentration in bulk solution Ce while all classic kinetic adsorption models deal with only the relationship between adsorption density q and contacting time t. The main problem associated with classic adsorption isotherms and kinetic models is the instability of their constant parameters. Furthermore, limited by their defined functions, classic models cannot be directly used for prediction of ion adsorption for a given adsorption system with known initial ion concentration Co and adsorbent concentration W0. Establishment of a quantitative relationship with adsorption as a function of Co and Wo is thus of high values for use of CW technology in wastewater treatment engineering practices.Experiments were thus carried out to investigate the adsorption characteristic of Cu2+ in vermiculite-aqueous solution systems in the range of initial ion concentration 20-750 mg/L and adsorbent concentration 20-150 g/L under different conditions. Main results obtained from this study are summarized as follows:1. The adsorption capacity(qm, namely, the maximum adsorption density) of vermiculite mineral was determined for Cu2+as 4.5 mg/g. Dynamic adsorption tests showed that the Cu2+adsorption rate was very high in the initial period and the adsorption density q reached 65% of its equilibrium value within 10 minutes. Thus the vermiculite mineral can be well used as an adsorbent to remove Cu2+from wastewaters.2. Adsorbent concentration effect was observed in the tested Cu2+adsorption systems. The qe-Ce curves declined apparently with increasing Wo and the traditionally defined equilibrium constants varied significantly at different W0 levels, showing clearly that qe is not a single function of Ce but a function of Ce and W0. Both Langmuir and Freundlich equations did not fit the combined experimental data obtained from the sample series with significant variation in adsorbent concentrations.3. The equilibrium adsorption density qe is the difference between C0/Wo (the ratio of initial ion concentration Co to adsorbent concentration Wo) and C0/Wo (the ratio of equilibrium ion concentration in liquid phase to adsorbent concentration). Repeated tests indicate that these three ion/adsorbent ratios are closely related with unique values in the tested range. The observed phenomenon indicates that the intensity factor in liquid/solid ion adsorption systems is not Ce but Ce/Wo in the liquid phase and Qe/Wo in the solid phase. The argument to support this intensity factor concept is that it is the relative level of ion quantity to adsorbent quantity that determines the direction and the rate of ion adsorption reactions.4. Different from the classic adsorption theories, the adsorption component model assumes that the equilibrium state of a liquid/solid ion adsorption system is determined by four mutually related components:ions in liquid phase C, ions in solid phase Q, uncovered adsorption sites Wu and covered adsorption sites Wc, with a reaction formula: C+Wu ====== Q+Wc and an equilibrium coefficient k=([C]e[Wu]e)/([Q]e[Wc]e)=(Ce/Wo) (qm-qe)/qe2 In accordance with the reaction model, the condition for ion adsorption to reach its equilibrium defined by the chemical potentials of the components isμC.e+μWu,e=μQ,e+μWc,e.5.Derived from the four adsorption components model, the following equation is applied for prediction of equilibrium Cu2+adsorption, qe={c0+qm-[(co+qm)2-4coqm(1-k)]1/2}/[2(1-k)], C0= C0/Wo Results from the tests indicate that the proposed model fit well the combined experimental data obtained from the examined samples with satisfactory prediction accuracy.6. Based on the four adsorption components model, the following kinetic equation is further tested: q=qe{1-[b/(b+t)]α), a=1/2, b= qe/(c0qm)1/2 Results from the kinetic experiment indicate that the above defined parameters remain nearly constant in the tested range, showing that given Co and Wo, the presented equation can be used to describe the kinetic ion adsorption process for Cu2+ adsorption in vermiculite-aqueous solutions.