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Novel pH-sensitive physically crosslinked hydrogels were synthesized by grafting D,L-lactic acid (LA) onto the amino groups in chitosan (CS) without a catalyst. The structures of these graft copolymers were analyzed by FTIR and X-ray measurements. Degree of substitution, side-chain length, and yield of copolymers were evaluated from 1 H-NMR, salicylaldehyde, and elemental analysis. The crystallinity of chitosan gradually decreased after grafting, since the side chains substitute the ONH 2 groups of chitosan randomly along the chain and destroy the regularity of packing between chitosan chains. Water uptake of the hydrogels was investigated as a function of side-chain length and degree of substitution. The influence of pH and salt concentration on the swelling behavior of the hydrogels was determined and interpreted.

The role of sediment-bound organic phosphorus (Po) on lake eutrophication was studied using sequential extraction and enzymatic hydrolysis by collecting sediments from Dianchi Lake, China. Bioavailable Po species including labile monoester P, diester P, and phytate-like P were identified in the sequential extractions by H2O, NaHCO3, and NaOH. For the H2O-Po, 36.7% (average) was labile monoester P, 14.8% was diester P, and 69.9% was phytate-like P. In NaHCO3-Po, 19.9% was labile monoester P, 17.5% was diester P, and 58.8% was phytate-like P. For NaOH-Po, 25.6% was labile monoester P, 7.9% was diester P, and 35.9% was phytate-like P. Labile monoester P was active to support growth of algae to form blooms. Diester P mainly distributed in labile H2O and NaHCO3 fractions was readily available to cyanobacteria. Phytate-like P represents a major portion of the Po in the NaOH fractions, also in the more labile H2O and NaHCO3 fractions. Based on results of sequential extraction of Po and enzymatic hydrolysis, lability and bioavailability was in decreasing order as follows: H2O-Po > NaHCO3-Po > NaOH-Po, and bioavailable Po accounted for only 12.1-27.2% of total Po in sediments. These results suggest that the biogeochemical cycle of bioavailable Po might play an important role in maintaining the eutrophic status of lakes.

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