Knowing the sorptive behavior of anionic polyacrylamide (APAM) by soils is useful in predicting the appropriate application rate, depth of effective treatment, and its mobility in soils. Sorption isotherms of PAM by soil materials, six natural soils, and their subsamples with partial organic matter (OM) removed by H2O2 oxidization under different dissolved salt concentrations were examined. The PAM sorption isotherms can be well described by the Langmuir equation. Soil texture, OM content, and dissolved salts (a combined contribution of soil salinity and irrigation water quality) influenced the extent of PAM sorption. Soils with high clay or silt content and low OM content had a high sorptive affinity for anionic PAM. The amount of PAM saturation sorption increased significantly as the total dissolved salts (TDS) increased. Divalent cations Ca2+ and Mg2+ were about 28 times more effective in enhancing PAM sorption than monovalent cations Na+ and K+, mainly because of their stronger charge screening ability. The effectiveness of cation enhancement on PAM sorption varied with soil texture and was greater in fine soils than in sandy soils. Organic matter had a negative effect on PAM sorption. Soil samples after the removal of partial OM adsorbed more PAM than natural soils. The negative effect of OM on PAM sorption was attributed to the reduction of accessible sorption sites by cementing inorganic soil components to form aggregates and to the enhancement of electrostatic repulsion between PAM and soil surface by its negatively charged functional groups.
Water-soluble PAMs have been used as soil amendments for various agricultural purposes, such as minimization of surface water run-off, soil erosion and crusting, stabilization of soil structure, and enhancing infiltration (Barvenik, 1994; Sojka and Lentz, 1996). High molecular weight (12–15 Mgm mol−1) and moderately anionic charged (a substitution of NH2 by OH at ∼10–20%) PAMs are the most effective types in soil application (Barvenik, 1994; Anonymous, 1995).
Addition of PAM to soil will affect soil dispersion, flocculation, and aggregation (Ben-Hur et al., 1992). Knowledge of the sorptive behavior of PAM is useful in predicting appropriate dose of application, depth of effective treatment, its mobility in soil, and changes in soil physical conditions. With good knowledge of polymer–soil interactions, the optimal amount of polymer application can be potentially prescribed.
Polyacrylamide is one of the most widely used polymers. Its sorption by sand, silica, alumina, clay minerals, latex, cellulose, and other materials has been the topic of previous publications. Greenland (1972) and Theng (1982) presented comprehensive reviews on the sorption of polymers in clay suspensions. Pefferkorn (1999) discussed interfacial processes involving PAM sorption. Nevertheless, little information was found in the literature regarding the sorption reaction between soil and PAM. In addition, the molecular weight of PAM used in irrigation is much higher but the concentration is much lower compared with those earlier studies. Information on PAM sorption at concentrations as low as 10 mg L−1 is sparse, possibly because of the difficulty in determining PAM concentrations in soil solutions (Lu and Wu, 2001).
Nadler and Letey (1989) and Malik and Letey (1991) determined sorption isotherms of several types of polyanion by an Arlington sandy loam (coarse-loamy, mixed, thermic Haplic Durixeralfs) through the use of tritium labeled polymers. Their results suggested that polymer sorption by soil was mostly limited to external (outer) surface area and was considerably influenced by water quality (Aly and Letey, 1988). Nadler et al. (1992) also found that there was little desorption after the polymers were adsorbed onto soil.