Application of sodium hydroxide coating on steel slag for phosphorus removal

Abstract

Phosphorus is an essential element

however, an excess amount of phosphorus in natural waterbodies can cause eutrophication which can be detrimental to water quality and causes various environmental problems. Many technologies to remove phosphorus from wastewater have been developed in the last several decades. The use of slag is one of the novel approaches to remove phosphorus. Slag that contains calcium, aluminum and free lime has the potential to be applied in the phosphorus removal process using a precipitation-coagulation mechanism. A sodium hydroxide coating is expected to improve phosphorus removal with a precipitation-coagulation mechanism by providing a high pH and favorable conditions for coagulation. A batch test was conducted with distilled water to clarify the main factor affecting slag exhaustion. Afterwards, a column test was conducted with wastewater to demonstrate phosphorus removal by the coated slag. Results of the batch test indicate that the ability of the slag to increase pH (10.5) decreased with time while the cations that help the precipitation process were consistently released. When the slag was coated with sodium hydroxide, the volume of distilled water, whose pH value can be raised to 10.5 by slag, increased by 42%. In a column test using fine-grained slag, a high phosphorus removal efficiency was obtained. However, the removal efficiency drastically decreased as the pH decreased down to 8. When coarse-grained coated slag was used pH values remained high. Accordingly, the coarse-grained coated slag showed better phosphorus removal efficiency from 16 bed volumes than the fine-grained uncoated slag. In addition, aluminum and sodium, released from the coating materials, promoted the coagulation of the precipitated phosphorus. A surface analysis using XRD and SEM-EDS showed the existence of calcium phosphate and aluminum hydroxide on the surface of the exhausted slag. However, the wastewater treated by the coated slag contains a high concentration of aluminum and sodium which might cause further environmental problems. To solve this problem, a return process to recycle the excess amount of aluminum and sodium was applied. The effluent in the initial stage that contains excess aluminum and sodium will be returned to the influent before the column. As a result, the concentrations of aluminum and sodium were decreased, and the phosphorus removal efficiency was more than 50% till the end of the test, which implies that the lifespan of slag was extended. In conclusion, this study demonstrates sodium hydroxide coating of slag is an efficient method for phosphorus removal.