Experimental investigation of the effect of henna extract on the swelling of sodium bentonite in aqueous solution

Abstract This paper reports for the first time the effect of Henna extract as a new, naturally occurring, and ecofriendly additive on swelling of sodium bentonite in aqueous solution. This is performed via a number of methods including dynamic linear swelling test in two distinct temperatures of 28 °C and 82 °C, sodium bentonite inhibition and sodium bentonite sedimentation tests. The results indicated that inhibition properties of Henna extract are a function of its concentration and are comparable with potassium chloride and polyamine as the two most common clay stabilizers. Inhibition mechanism was assessed using adsorption measurements obtained by conductivity technique, contact angle measurements via the sessile drop method, and scanning electron microscopy (SEM) analysis. Results of adsorption measurements indicated that Henna extract has a lower adsorption isotherm in alkaline medium (pH = 9) compared to natural medium (acidotic). Adsorption of Henna extract increases hydrophobicity of sodium bentonite particle surface; nevertheless, this adsorption mechanism is slightly weakened in alkaline medium due to the effect of caustic soda as a pH adjustment agent. SEM analysis demonstrated that sodium bentonite particles have an extended area when exposed to Henna extract solution instead of distilled water indicating sodium bentonite particle stability. Henna extract has deflocculating properties at low concentrations (especially up to 0.2 mass%); yet it indicates good inhibition properties to sodium bentonite swelling at a concentration several times higher than that of deflocculating concentration, about 3 mass%. Inhibition properties could be attributed to the hydrogen bonding between the hydroxyl group of Henna extract constituents and oxygen atoms available on silica groups of sodium bentonite, especially to the lawsone (2-hydroxy-1,4 napthaquinone) constituent.