Revealing the infiltration process and retention mechanisms of surface applied free DNA tracer through soil under flood irrigation.

It has been recently demonstrated that free DNA tracers have the potential in tracing water flow and contaminant transport through the vadose zone. However, whether the free DNA tracer can be used in flood irrigation area to track water flow and solute/contaminant transport is still unclear. To reveal the infiltration process and retention mechanisms of surface applied free DNA tracer through soil under flood irrigation, we tested the fate and transport behavior of surface applied free DNA tracers through packed saturated sandy soil columns with a 10 cm water head mimicking flood irrigation. From the experimental breakthrough curves and by fitting a two-site kinetic sorption model (R2 = 0.83-0.91 and NSE = 0.79-0.89), adsorption/desorption rates could be obtained and tracer retention profiles could be simulated. Together these results revealed that 1) the adsorption of free DNA was dominantly to clay particles in the soil, which took up 1.96 % by volume, but took up >97.5 % by surface area and densely cover the surface of sand particles; and 2) at a pore water pH of 8.0, excluding the 4.9 % passing through and 3.1 % degradation amount, the main retention mechanisms in the experimental soil were ligand exchange (42.0 %), Van der Waals interactions (mainly hydrogen bonds), electrostatic forces and straining (together 44.7 %), and cation bridge (5.3 %). To our knowledge, this study is the first to quantify the contribution of each of the main retention mechanisms of free synthetic DNA tracers passing through soil. Our findings could facilitate the application of free DNA tracer to trace vadose zone water flow and solute/contaminant transport under flood irrigation and other infiltration conditions.

[Effect of continuous flushing out of root canal on fluid replacement in root canal during mechanical preparation using Ni-Ti instrument].

PURPOSE: To evaluate the effect of continuous flushing out of the canal on fluid exchange in the root canal during mechanical preparation. METHODS: Sixty resin blocks with standardized root canals were divided into 5 experimental groups according to whether continuous flushing was performed during mechanical preparation. Injecting pure black ink into the root canals before each file preparation，the liquid exchange was calculated by measuring the absorbance value of the remaining liquid after performing different preparation and irrigation schemes. Meanwhile, computational fluid dynamics model was established which simulated the flow field in the canal when the file moved up-and-down. SPSS 19.0 software package was used for statistical analysis of the data. RESULTS: The absorbance value of the remaining fluid in the root canal of the three groups in which continuous flushing was performed during mechanical preparation differed significantly from the group without continuous flushing(P<0.05), but no significant difference among the three groups (P>0.05). Computer simulation confirmed that the "efficient regurgitation area" existed in the middle part of the root canal and fluid could be gradually transported to the apical area by the file's up-and-down motion. CONCLUSIONS: Continuous flushing out of the canal during mechanical preparation can replace the original solution in the canal partly, which is beneficial to conventional irrigation for cleaning of the root canal.