A Study On Polyacrylamide Application In Soil And Water Conservation

As the world population increases and people’s living standard improves, mankind’s demand of and pressure on natural resources increase as well. To meet this increasing demand, new lands need to be reclaimed, or more intensive cropping must be practiced in the existing croplands to boost grain yield per unit area. If not properly managed, both land reclamation and cropping intensification would lead to severe soil and water losses and degradation of eco-environments, consequently affecting the sustainability of agricultural production. Therefore, conserving eco-environments and natural resources, reducing soil and water losses, and improving land productivity are essential for sustainably developing our national economy and improving our people’s living standards. The goal of this thesis is to investigate the mechanisms and processes of using a new kind of organic polymer (polyacrylamide, PAM in short) for reducing surface water turbidity, ameliorating soil, and controlling soil and water loss, and to explore the complex interactions between different types of PAMs and soils. Since the effectiveness of PAM in controlling soil and water loss and in reducing surface water turbidity are strongly affected by physiochemical properties of both PAM and soils, this study selected various types of PAM (with different molecular weights and charge types and densities) and clay specimens and soil types having distinct properties, and studied the ability of two clay specimens (kaolinite and smectite) in adsorbing and desorbing PAM as well as PAM’s ability to flocculate the two clay specimens and mixed clay minerals in four different soils.The experiment results with the two clay specimens showed that in the absence of cation bridges the PAM’s flocculation power followed the order of cation>neutral>anion. Cationic PAM was the best flocculent, while the anionic PAM was the best dispersant. This experiment demonstrates that (1) charge type and density of PAM molecules are the uttermost important factors affecting clay behavior (i.e., flocculation or dispersion), the presence of cation bridges is the sufficient and necessary conditions for using anionic PAM as a flocculent,(2) the magnitude of PAM adsorption by clay particles is strongly affected not only by PAM’s charge properties but also by clay’s charge property and surface areas,(3) PAM’s adsorption is a irreversible processes (i.e., once adsorbed by clay particles, desorption of the PAM molecules is almost impossible),(4) van der Waals’ force and hydrogen bond between clay particles and PAM molecules are the root causes for the irreversible adsorption, and (5) the interactions between clay and PAM are very complex, and such complexity serves as the theoretical basis for ameliorating different soils with different PAMs at different concentrations.The soil experiment showed that six different PAMs having different charge properties and molecular weights were very effective in flocculating clay and silt soils having high kaolinite and silt contents, and the optimal flocculation ranged from one to three ppm, indicating a minuscule amount of PAM can cause rapid flocculation and reduction of surface water turbidity. The studied PAMs were not effective in flocculating sandy soils having low smectite and silt contents, implying that addition of divalent salts would speed up the flocculation. It should be pointed out that when PAM concentration is higher than the optimal flocculation concentration, the PAM’s flocculation power tends to decrease. This is because PAM’s adsorption increases as PAM concentration increases, and the increased adsorption of anionic PAM would increase the overall negative charge of the clay-PAM complex, resulting in increased Columbic repulsion among complexes. Thus, when using PAM to reduce water turbidity, it is critical not to use the concentration greater than the optimum.Taken together, the use of PAM to ameliorate soil and to reduce water turbidity has great potential with an enormous prospect. However, due to the existences of enormous number of PAMs, various kinds of clay minerals, and strong interactions between PAM and clay, it is imperative to conduct a large number of experiments to match up PAM characteristics with soil characteristics based on the flocculation effectiveness and to further document optimal concentrations according to PAM and soil properties.