Study On Cd²⁺ Accumulation On Algae In North Water

At present, the deterioration of water environment, such as the prevention of eutrophication and heavy metal pollution, becomes the focus of many researchers’ attention. Eutrophication phenomenon mainly presents on algal blooms, algae as the primary producer in the water, is the most vulnerable species to the toxic effects of heavy metals, affecting the aquatic plants, animals, and even human health by the enrichment of the food chain. Therefore, the enrichment behavior of algae and Cd2+ in water, which can accumulate data for the prevention of eutrophication phenomenon, and seek new ways to reduce the pollution of Cd2+, has the advantages of environmental protection and the necessity.At present, most scholars studied on adsorption of Cd2+ on non-active algae, but studies using activated algae to accumulate Cd2+ are little. This paper selected the common algae in water of China’s northern(Spirulina platensis, Chlamydomonas microsphaera, Microcystis aeruginosa, Scenedesmus quadricauda), simulating algae of natural water to accumulate Cd2+. The determinations of algae’s growth characteristics and influencing factors of Cd2+ accumulation on algae, effects of Cd2+ with different concentrations to the growth of algae, the accumulation of Cd2+, absorption of nitrogen and phosphorus, were performed, in order to further explore mechanism of Cd2+ on living algae and the interaction of them. The main research results are as follows:1. The growth characteristics of algae(1) The growth of Spirulina platensis, Scenedesmus quadricauda, Microcystis aeruginosa and Chlamydomonas microsphaera follows the regulation of general microorganism, which need to experience four stages: the adjustment period, the logarithmic growth period, stable period, the decline period. The whole period is about 21 days, and the logarithmic growth phase started on the eighth day, this stage is suitable for research on materials. The number of algae reached the peak after 16 days, that of Spirulina platensis, Scenedesmus quadricauda, Microcystis aeruginosa and Chlamydomonas microsphaera respectively was 2.1×108 cell/L, 2.3×107 cell/L, 1.1×1010 cell/L, 1.3×109 cell/L. Then the microscopic test showed that algal cells dissociated, even died, so the number gradually reduced.(2) Optical density value(OD) of algae liquid, such as Spirulina platensis, Microcystis aeruginosa, Scenedesmus quadricauda, Chlamydomonas microsphaera, and the cell dry weight concentration is proportional. The bigger OD determination value is, the bigger algal cell dry weight concentration is.2. Study on Cd2+ accumulation on algae(1) Cd2+ accumulation capacity on four kinds of algaes is higher than that of activated carbon and clay by 3~17 times. The accumulation saturated in 8 h later, and balance accumulation capacities of Spirulina platensis, Chlamydomonas microsphaera, Scenedesmus quadricauda and Microcystis aeruginosa were 8.92 mg/g(DW), 61.67 mg/g(DW), 9.09 mg/g(DW), 36.04 mg/g(DW) respectively. However, the adsorption capacity of activated carbon and clay is only about 3.5 mg/g.(2) Kinetics of Cd2+ accumulation on four kinds of algaes was studied. Results showed that fitting parameters and the correlation(r>0.99) of apparent adsorption two order kinetics model are better than the first-order kinetic equation(r=0.80~0.98). Therefore, the accumulation process mainly depend on chemical adsorption, especially ion exchange.(3) After determining accumulation efficiency of Cd2+ on algae, results showed that the best accumulation effect of Cd2+ presented on the algae concentration in about 0.1 g/L(DW); Cd2+ accumulation capacity on algae increased with the rise of pH(3.0~9.0); the accumulation capacity was largest at 25℃, and it is not conducive to the accumulation of Cd2+ on algae cells at low or high temperature; when there are cations Na+, Ca2+, Fe3+ in algae liquid, Cd2+ accumulation was restrained, and the inhibition of Fe3+, Na+, Ca2+ to Cd2+ accumulation on Spirulina platensis was enhanced, the inhibition of Fe3+, Ca2+, Na+ to Cd2+ accumulation on Chlamydomonas microsphaera, Scenedesmus quadricauda and Microcystis aeruginosa decreased in turn.(4) Mechanism of Cd2+ accumulation on algae were studied. The results showed that ion exchange function is dominant in the accumulation process of Cd2+ on algae, followed by active transport function, again is physical adsorption, and assimilation.3. The isothermal adsorption of Cd2+ on algaeUsing Langmuir, Freundlich, BET, D-R model can fit the adsorption of Cd2+ on Microcystis aeruginosa, Scenedesmus quadricauda, Chlamydomonas microsphaera very well. The fitting results of adsorption of Cd2+ on Spirulina platensis by Langmuir and BET, were better than that of Freundlich and D-R model. Through the calculation to adsorption free energy(E) of Cd2+ on 4 kinds of algae, it can be found that E was 8~16 KJ/mol, which further proved that chemical adsorption played a major role in the accumulation process of Cd2+ on algae.4. Effect of Cd2+ on the growth of algae(1) With the increase of Cd2+ concentration, dissolved oxygen content of 4 kinds of algae decreased by about 1~3 mg/L; pH value also reduced(Microcystis aeruginosa and Chlamydomonas microsphaera: ΔpH= 2.18~2.48, Spirulina platensis and Scenedesmus quadricauda: ΔpH=0.08~0.37); chlorophyll content reduced by more than 80%; optical density(OD) also decreased gradually.(2) Under different Cd2+ concentrations, absorption rate of total inorganic nitrogen(TIN) and nitrate nitrogen(NO3-N) on these four algaes in the water was as follows: high concentration area < low concentration region. Absorption rate of NH3-N was more than that of NO3-N for Microcystis aeruginosa. Cd2+ concentration can affect the absorption of nitrogen in cells of Spirulina platensis, Scenedesmus quadricauda and Chlamydomonas microsphaera. The absorption of NO3-N played a major role for them.(3) Among the low Cd2+ concentration of 0~5 mg/L, phosphorus uptake rate on algae is inversely proportional to Cd2+ concentration. At high Cd2+ concentration 10~100 mg/L, phosphate could chelate with Cd2+. The greater Cd2+ concentration was, the more amount of phosphate chelated, so phosphate residue in water reduced. P uptake rate on algal cells is proportional to the concentration of Cd2+.