Effect of hydroxypropyl methyl cellulose as soil modifier on solute migration in soil

Abstract: Hydroxypropyl methyl cellulose (HPMC) is a kind of soil modifier. Effect of HPMC addition on solute migration characteristics of soil was studied in this study. The test soil was silty loam. HPMC was added into soils based on different contents of 0, 0.1, 0.2, 0.3, 0.4 and 0.5 g/kg. The column-shaped plexiglass soil column with a height of 50 cm and a diameter of 5 cm was prepared. The soil loading height was 20 cm and the bulk density was 1.31 g/cm3. Water was supplied in a Markov bottle. After the soil column was fully infiltrated, water was supplied continuously at the fixed water head of 4.5 cm. The effluent was collected to calculate the saturated water conductivity of the soil. Then, water supply stopped and the surface water of column was removed. The water was replaced with 0.2 mol/LCaCl2 solution for the solute migration experiment. Tubes (10 mL) were used to collect the fluid at the lower end of the soil column once every 10 mL. The concentration of chlorine ions was determined by potassium chromate-silver nitrate method until the concentration of Cl( in effluent almost equaled to that in markov bottle. At the end of the experiment, the saturated water conductivity, penetration curve characteristics and penetration time of soil under different HPMC additions were compared and analyzed. The main parameters of the convection-dispersion equation (CDE) and the two-region model (TRM) were fitted by the software CXTIFIT2.1. The migration characteristics of soil solute were further studied by analyzing the change of parameters. The results showed that: 1) When the HPMC content was within the range of 0-0.5 g/kg, and the saturated conductivity decreased with the increase of HPMC content. 2) Applying a certain amount of HPMC in the soil could significantly reduce the migration velocity of solute. With the increase of HPMC content, the relative concentration of solute decreased with the same pore volume. 3) Increasing HPMC addition content could delay the initial penetration time and complete penetration time, and thus the total penetration time. 4) Both the CDE and TRM models could well simulate the migration of solute in soils with different contents of HPMC. The fitting curves of both models could be in good agreement with the measured curves, but the simulation accuracy of the TRM model was higher. 5) Based on the parameter fitting results of the TRM, with the increase of HPMC, the mean pore water velocity was smaller, and the hydrodynamic dispersion coefficient, dispersion degree and mass exchange coefficient were increased. Since HPMC had good water holding capacity, the application of HPMC also increased the water content ratio of immovable water in soil. Therefore, HPMC has great potential in adjusting soil pore structure and slowing soil nutrient loss. This study provides new ideas and methods for improving soil and water nutrient loss.