| Ca is an important metal element in natural waters, whose ecological function remains to be researched. In water column, calcium is liable to combine with inorganic carbon, causing the precipitation of calcite. During calcite crystal growth, inorganic phosphorus, regarded as one of the key factors responsible for eutrophication in water bodies, could be partly removed in the process of adsorption to, or co-precipitation with calcite. The process can enhance the removal of inorganic phosphorus and improve water quality. As nutrition of Potamogeton crispus, bio-available calcium in sediment could drive the formation of coprecipitation of calcite with phosphorus, which occurs in foliar of P. crispus. Since bio-available calcium plays a vital role in the restore function of submerged plants it is necessary to examine the different forms of calcium in sediment. However, the accuracy and validity of sequential extraction methods for the purpose of metal speciation have been questioned and different element. Most of sequential extraction methods were universal applied to different elements but their utilities were questionable, and nearly scarce effort has been made for the improvement of calcium extraction.In our study, sediments with different calcium content were used to select optimum extraction conditions of Tessier sequential extraction procedure to develop a specific sequential extraction procedure for Ca (Modified Tessier procedure). And then it was established that Inorganic Calcium Sequential Extraction (ICSE) method. In order to select a suitable calcium extraction method, the efficiency and suitability of modified Tessier method, ICSE method and BCR method were tested by application of the methods to lake sediment samples. By modeling environment of lakes the system of P. crispus-sediment-overlying water was established to investigate the effect of calcium on phosphorus in overlying water. The main results are summarized as follows:(1) After modification of sequential extraction conditions of Tessier method (extractant species, extracting time, solid-to-solution ratio, concentration and pH of extractant), selected settings for exchangeable Ca, carbonate bound Ca and Fe/Mn oxide bound Ca of sequential extraction were:25ml1mol L-1NH4AC (pH7.0),3h;25ml1mol L-1NH4AC-HAC (pH5.0),5h;30ml0.5mol L-1NH2OH-HC1(pH1.5),50℃,6h.(2) A higher robustness and selectivity of results is obtained by applying the modified Tessier procedure, which promotes the recoveries of Ca in sediments. The major portion of Ca in Tangxunhu Lake sediment (T-Sed), Donghu Lake sediment (D-Sed), Yuehu Lake sediment (Y-Sed) and Nanhu Lake sediment (N-Sed) was extracted in exchangeable, carbonate-bound and Fe-Mn oxide-bound fractions and the proportional calcium distributions of all the sediment samples is in the order of exchangeable> carbonate bound> Fe/Mn oxide bound. Organic bound calcium and residual calcium constituted the smallest fractions which could be almost ignored. Furthermore, by considering availability of calcium and simplification of extraction operations the extraction procedure with only the first three steps was recommended.(3) In the scheme of ICSE fractionation, calcium was divided into exchangeable, carbonate bound, phosphate bound, sulfate bound and residual fraction. Calcium distribution of the four lake sediment samples is in the order of exchangeable calcium> carbonate bound calcium> sulfate bound calcium≥phosphate bound calcium. Percentages of Ca extracted in residual fraction for four lake sediment samples were scare and almost negligible. Extractants for extraction of phosphate bound and sulfate bound fraction were lack of selectivity and specificity, however, ICSE method was still recommended for the stable fractionation results from this method. The reagents used in modified Tessier procedure were more selective for the desired target phases.(4) Compared with ICSE method and BCR procedure, modified Tessier procedure had highest recoveries of Ca. Discharge of washing water between each step in these sequential extraction methods might lead to the low recoveries of calcium. Lower calcium extracted in ICSE method and BCR procedure might be attributed to the inability of some of the reagents and extracting time or other extraction conditions. Whereas the reagents used in modified Tessier procedure were more selective for the desired target phases. The correlation of Ca forms with some sediment properties showed that sediment pH and total calcium in sediment are the most important factors controlling the distribution of Ca in sediment while total Fe and Mn had no correlation with calcium distribution.(5) Change of phosphorus in overlying water was consistent with index equations y=axb and y=A1exp(-x/t1)+y0. Phosphorus in overlying water had not been reduced by the uptake of P. crispus. Sediment, regarded as storage of nutrition, might influenced cycle of phosphorus and lead to reduction of phosphorus in overlying water. Change of calcium with time in water column was consistent with linear equation or linear-quadratic equation. Calcium concentration decreased with time. P. crispus absorbed calcium from water column and causing a decrease of Ca. At the same time the form of calcium would be affected by the root of P. crispus, and part of liable calcium might be absorbed by P. crispus and then released into water column.(6) In water column, calcium had a significant correlation with SRP. Calcium reacted with phosphate forming precipitation and settled on sediment, which would decrease the concentration of calcium and phosphorus. It might be influenced by the co-precipitation of calcite with phosphorus around the foliar of P. crispus also. |
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