ALUM COAGULATION FOR SIMULTANEOUS REMOVAL OF LDPE MICROPLASTICS AND TURBIDITY FROM HIGHLY TURBID WATER: EFFECTS OF PH, DOSE, AND SETTLING TIME

Abstract

Microplastics (MPs) reach drinking-water intakes together with high suspended-solids loads, yet most coagulation studies test MPs in clear or low-turbidity water. This study examined aluminium sulphate (alum) coagulation for the simultaneous removal of low-density polyethylene (LDPE) MPs and turbidity from kaolin-based synthetic water of 480–520 NTU containing 25 mg/L LDPE particles of 1–5 mm. Jar tests were run in three sequential one-factor-at-a-time batches, varying initial pH (4–12), alum dose (5–50 mg/L as Al₂(SO₄)₃·18H₂O; 0.40–4.05 mg Al/L) and settling time (5–60 min), each in triplicate. Turbidity was measured nephelometrically and residual LDPE gravimetrically after correction with kaolin-only and alum-only procedural blanks. Turbidity removal was highest at initial pH 7 (about 99%), whereas LDPE removal was highest at initial pH 8 (about 81%) and remained high at pH 10 (about 79%); LDPE removal was low at initial pH 4–7 (12–23%) and pH 12 (about 28%). Turbidity removal exceeded 98% from 20 mg/L alum, whereas LDPE removal needed 30 mg/L to reach about 81%, rose only to about 85% at 40 mg/L and fell to about 65% at 50 mg/L. LDPE removal rose steeply up to 30 min of settling (about 79%) and changed little thereafter, while turbidity removal continued to increase up to 60 min. Under the conditions tested, the operating window that maximised turbidity removal did not maximise LDPE removal: LDPE needed a higher dose and a slightly alkaline initial pH. Because the solution was unbuffered, the coagulation pH differed from the initial pH, and the mechanisms proposed here should be confirmed with floc and surface-charge measurements and with real river water.