| Along with the accelerated industrialization course, a mass of heavy metal containing waste waters was discharged into rivers and lakes, resulting in sever pollutions of heavy metals. Not only was the shortage of useable water resource directly affected by heavy metal pollution, but also the security of drinking water, food and crop production, and the health of human being were endangered. As a new technology developed in recent years with characters of low energy consumption and operation and maintenance cost, constructed wetlands (CW) used for treatment of wastewaters containing heavy metal pollutants have received great attention in fields of environmental science and ecology. Compared with that of conventional treatment processes, however, the application of CW techniques has been limited to certain areas mainly due to their relatively low treatment efficiency, large land use area,low capacity to resist hydraulic, organic and heavy metal pollutant loads and particularly instability to seasonal changes. As an effective solution to above mentioned problems, introduction of an adsorption buffer unit using materials with high adsorption capacity into a CW system can improve its treatment efficiency as well as enhance its sustainability.All traditional adsorption isotherms, when being applied to describe the ion adsorption in liquid/soild systems, define the equilibrium ion adsorption density qe as a single function of the ion concentration in bulk solution Ce.Results from a large number of studies,however, show that there were obviously effects of adsorbent concentration on the traditional adsorption isotherms(i.e.,a phenomenon of decline of the traditionally defined adsorption isotherms with increasing adsorbent concentration).Many scholars think that changes in system ionic strength and pH value are the main factors responsible for the adsorbent effect and thus in order to eliminate this impact they usually adopt a buffer system for control of solution pH value and ionic strength when conducting their adsorption tests. Later studies, however, have demonstrated that the direction and the rate of ion adsorption reactions is dependent on the relative level of ion quantity to adsorbent quantity, which suggests that the intensity factor in liquid/solid ion adsorption systems is not Ce but Ce/Wo in liquid phase and Qe/W0 in the solid phase, and therefore there will be an adsorbent effect when qe is plottedto against Ce.Based on presence of four essential adsorption components(ion in liquid phase Ce,ion on solid surface Qe,covered adsorption site wc and uncovered adsorption site wu) in the adsorption system, a qe-Ce/W0 adsorption isotherm model has been established. The new model, however,has been tested so far only for single ionic species systems and whether or not it can be used for mixed ion system needs further investigations.Therefore, it is of great significance to test the basic relationship among ion adsorption components in mixed ionic species systems so as to improve the liquid/soild ion adsorption theory.Using vermiculite(0.2-0.4mm, dried after rinsed with distrilled water) as the adsorbent and distrilled water as the solvent, designed experiments were carried out to investigate the adsorption characteristic and mechanisms of Zn2+,Cd2+ and NH4+ in both buffer solutions and mixed ion species systems in the range of initial ion concentration 0.2-2.0 mmol/L and adsorbent concentration 50～150 g/L.The results obtained from the tests are summarized as follows:(1) Accounted for by its nontoxic nature, low cost renewable and high cation adsorption capacity, the natural vermiculite was proved to be a fine wetland filler wetland for removal of Zn2+,Cd2+ and NH4+ from wastewaters. Ion exchange was found to be the main mechanism for Zn2+ and Cd2+ adsorption in the tested system.Because of adsorption competition, coexisting cation can inhibit the adsorption of Zn2+ and Cd2+.(2) Results from buffer experiments show that ionic strength and pH are not the fundamental cause of W0 effect; introduction of buffer solution has not eliminated the adsorbent effect, but instead, increased the experimental error. Strong adsorption competition exists in buffer system and the existence of large number of other cationic spcies has greatly inhibited adsorption of Zn2+ and Cd2+.Due to adsorption of other types of ions,the potential adsorption capacity available for Zn2+ and Cd2+ in buffer system is below that in the non-buffer system.(3) In the mixed system, repeated tests indicated that the three ion/adsorbent ratios (the adsorption density qe, the ratio of initial ion concentration to adsorbent concentrationC0/W0 and the ratio of equilibrium ion concentration in liquid phase to adsorbent concentration Ce/Wo)are closely related with unique values in the tested ranges. The observed phenomenon indicates that the proposed intensity factor theory holds basically in mixed ionic species adsorption systems.(4) Based on the four adsorption components model proposed for single ionic species adsorption systems, qe={co+qm-[(co+qm) 2-4coqm (1-k)]1/2}/[2(1-k)]co=C0/W0a new model extend for mix ionic species systems has been developed by assuming that in the mixed system adsorption competence of all ions abide by the rule of equivalent concentration,i.e.,the total equivalent adsorption of the system equals to the sum of equivalent amount of different ionic species adsorbed by the system:qeT=∑qei, and thus the equivalent adsorption density of ion A(qeA) equals to the sumof equivalent adsorption density of all ions presence in the system(qeT) multiplying the ratio initial equivalent concentration of ion A(CoA) to the sum of initial equivalent concentration of ionic species present in the system (C0T), i.e.,qeA=(C0A/C0A)*qeT.(5) The model extended for mixed ionic spscies systems fit well the experimental data obtained from the examined sample in the tested rangle with satisfactory prediction accurary.The model can also be used to predict equilibrium liquid ion concentration Ce in buffer systems though its accurary level is lower than that in non-buffer systems.
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