Study On Characteristics Of Cr (Ⅵ) Adsorption In Red Soil-aqueous Solutions

Following the rapid development of galvanization, printing and dyeing industries, Chromium-containing wastewater pollution has become a severe problem that gives potential hazard to ecosystems as well as the public health. Development of new techniques for removal of Cr (VI) 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 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 sciences. Removal of harmful pollutants using adsorbent as CW fillers plays an important role in the CW process for wastewater treatment. 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 concentration1 to a stipulated discharge standard. This involves mechanisms of ion adsorption in liquid/solid systems.All classical 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 existing kinetic adsorption models deal with only the relationship between adsorption density q and contacting time t. The main problem associated with classical adsorption isotherms and kinetic models is the instability of their constant parameters. Furthermore, limited by their defined functions, classical models cannot be directly used for prediction of ion adsorption for a given adsorption system with known initial ion concentration Co and adsorbent concentration Wo. Establishment of a quantitative relationship with adsorption as a function of Co and Wois thus of high values for use of CW technology in wastewater treatment engineering practices.The theoretical ground of this study is the recently developed ion adsorption component model. According to this model, the equilibrium state of a liquid/solid ion adsorption system should be 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. Different from that defined by classical adsorption isotherms, the equilibrium ion adsorption density qe (the ratio of the equilibrium ion adsorption Qe to adsorbent concentration Wo) in the adsorption component model is defined as a single function of Ce/Wo (the ratio of the equilibrium ion concentration in liquid phase Ce to Wo) or Co/Wo (the ratio of initial ion concentration Co to Wo) in the following forms, k=ce(qm-qe)/qe2 ce=CeWo qe={co+qm-[(co+qm)2-4coqm(1-k)]1/2}/[2(1-k)] Co=rCo/WoThe main objective of this study is to test the fitness of the developed model to anion adsorptions in liquid/solid systems with focus on examining the basic relationship among the ion/adsorbent ratios, qe, ce and co. In addition a kinetic adsorption model was further developed for prediction of anion adsorption as a function of contacting time t.Using red soil as the adsorbent, both equilibrium and kinetic adsorption experiments were therefore conducted to investigate the adsorption characteristic of Cr (â…¥) in red soil-aqueous solution systems in the range of initial ion concentration 5-375 mg/L and adsorbent concentration 50-250 g/L under different conditions. The experimental results are summarized as follows:(1) Red soil contains high amount of iron and aluminum oxides and has thus relatively high capacity to adsorb Cr (â…¥). As a natural resource widely distributed in south China, red soil can be easily obtained with very low cost and has thus also economic values when being used as a CW filler for treatment of Cr(â…¥)-containing wastewater.(2) The rate of Cr (â…¥) adsorption on red soil was found to be very high in the first 2h. After that period the adsorption rate gradually descended and finally approached to zero at approximately 10 h. The time needed for adsorption to reach its equilibrium was related to both the initial Cr (â…¥) concentration (Co) and the red soil concentration (Wo) in the adsorption system. The general trend was that the time needed was longer at higher initial Cr (â…¥) concentration levels but shorter at higher adsorbent concentration levels. (3) Two major problems were observed when applying the classical adsorption model to describe the Cr (VI) adsorption on red soil:one was a decline of the adsorption isotherm with increasing adsorbent concentration, interpreted as adsorbent effect, and the other was the inconstancy of the equation parameters, namely, the defined constant parameters varied significantly in the sample series with great variation in adsorbent concentration. The observed results suggests that unlike that defined by classical adsorption isotherms, the equilibrium Cr (VI) adsorption density qe is not a single function of its equilibrium concentration in bulk solution Ce.(4) In agreement with the theory of thermodynamics that the sate function of a system is only determined by its difference between states, the obtained results showed that the equilibrium Cr (VI) adsorption density qe was uniquely determined by the difference between Co (the ratio of initial Cr (VI) concentration Co to adsorbent concentration Wo) and ce (the ratio of the equilibrium Cr (VI) concentration in liquid phase Ce to Wo) independent of Co, Wo and the process history. This is an indication that the intensity factor in a liquid/solid ion'adsorption system is not the volume-based ion concentration but rather the ratio of ion quantity to adsorbent quantity as it is the relative level of ion quantity to adsorbent quantity that determines both the direction and rate of the ion adsorption. Ion adsorption arrives at equilibrium only when the difference in sum of ion and adsorbent chemical potentials between liquid and solid phases is zero.(5) Both the concentration and volume tests conducted at different Co and Wo levels proved that giving the Co/Wo ratio co, the CJWo ratio ce and QJWo ratio qe remained nearly unchanged, confirming that qe can be expressed as a single function of either ce or Co. As qe corresponds to a unique ce rather than to a unique Ce, the widely observed adsorbent concentration effect on traditionally defined adsorption isotherm is an expected result.(6) Results from repeated Cr (VI) adsorption tests indicated that the proposed model fit well the combined experimental data with satisfactory prediction accuracy. The adsorbent concentration effect was eliminated in both the qe-ce and qe-co plots.(7) Subject to the qe-co relation, the following kinetic equation was further proposed q=qe{1-[bl(b+t)a), a=1/2, b= qe/(coqm)1/2 Results from the kinetic experiment indicated that the above defined parameters remain nearly constant in the tested range, showing that given C0 and W0, the proposed model can be used to describe the kinetic Cr (VI) adsorption process in red soil-aqueous solutions.The result from the present study confirms that the adsorption component model can be applied to describe the anion adsorption in aqueous solution.