The effect of various metal-salts on the sedimentation of soil in a water-based suspension

Abstract

Soil particles and bound nutrients that erode from agricultural land may end up in surface waters and cause undesirable changes to the environment. Various measures, among them constructed wetlands have been proposed as mitigation, but their efficiency varies greatly. This work was motivated by the assumption that the induced coagulation of particles may accelerate sedimentation in such wetlands and by that help reduce the amount of material that is lost from the vicinity of the diffuse source. Our specific aim was to laboratory-test the effectiveness of various salt-based coagulants in accelerating the process of sedimentation. We tested the effect of Na+, Mg2+, Ca2+, Fe3+ and Al3+ cations in 10, 20, 40 and 80 mg L-1 doses added to a soil solution in select, soluble forms of their chlorides, sulphates and hydroxides. We mixed such salts with 1 gram of physically dispersed, clay and silt rich (>85% in total) soil material in 500 mL of solution and used time-lapse photography and image analysis to evaluate the progress of sedimentation over 3 hours. We found that 20-40 mg L-1 doses of Mg2+, Ca2+ in their chloride or sulphate forms appeared to provide the best consensus in terms of efficiently accelerating sedimentation using environmentally present and acceptable salts but keeping their dosage to a minimum. Comprehensive in-field efficiency and environmental acceptability testing is warranted prior to any practical implementation, as well as an assessment of small scale economic and large-scale environmental benefits by retaining soil and nutrients at/near the farm.

Fig 1. Size distribution of the sedimented soil material, determined by laser diffractometry.

Note: The soil was not submitted to chemical pre-treatment for disaggregation.

Fig 2

Top: Concept of differences in turbidity being an indicator of progress in sedimentation. Bottom: glass cylinders with 4 ion application rates of Ca in the form of CaCl₂ and an ion-less control, after 3 hours.

Fig 3. Sedimentation results of four concentrations of MgCl₂ solutions, and a control using only distilled water.

Exponential curves were fitted using the Origin 6.0 software; curve parameters are presented in Table 2.

Fig 4. pH of the suspension upon start of the experiment (i.e. after mixing the soil and applied salts) vs. the change in pH after 3 hours.

Fig 5. Overall efficiency of sedimentation after 3 hours, using the cation-anion pairs introduced in Table 1.

Fig 6. Half-times of overall sedimentation–defined as the time required to achieve 50% of the expected final sedimentation efficiency, using an exponential model fitted to the data—using the cation-anion pairs introduced in Table 1 as coagulants.