Determination and removal of selected heavy metals in treated wastewater from Ruai sewage treatment plant for possible agricultural applications

Abstract

Fresh water has become scarce and many arid and semi-arid regions in the world suffers from water shortage. Wastewater reuse remains the only reliable and potential source of water. One of the major challenges in recycling of wastewater is the presence of toxic heavy metals which are persistent and non-biodegradable and are known to affect human health. This study sought to synthesize and characterize soda lime and borosilicate waste glass and a composite of Multi-walled Carbon Nanotubes (MWCNTs)/soda lime waste glass adsorbents and utilize them for removal of Pb2+ from wastewater. Glass wastes were collected within Chuka University and MWCNTs purchased from Hongwu International Group Ltd. The adsorbents were washed, dried, functionalized with nitric-sulfuric acid mixture and characterized using FTIR. Wastewater samples were collected in Ruai wastewater treatment plant, Kenya using grab method, transferred to 250 ml plastic bottles and were transported to Chuka university for analysis in a cooler box at 4oC. Standard laboratory procedures of determining the physicochemical parameters were employed. Batch adsorption experiments were conducted to study the effect of contact time, pH, temperature, shaking speed, initial adsorbate concentration and adsorbents dosage on removal of lead (II) ions. Residual Pb2+ concentration was determined using AAS. The findings were: pH 5.5-7.9, Temperature 22.70C-26.10C, Conductivity 526.7- 1209.7µS, turbidity 73- 1000 NTU, nitrates 6.66-25.1 mg/l, Phosphorus 1.16-10.30 mg/l, BOD5 ranged from 10-480 mg/L, COD 90-980 mg/L, TSS 14-422 mg/L, and TDS 244-967 mg/L. pH, temperature, NO3-, BOD5(wet season) results met the WHO and NEMA standards for wastewater reuse in irrigation while EC, turbidity, P, COD, BOD5(dry season), TDS and TSS did not. The results of heavy metals were Ni 0.02-0.22 mg/L, Zn 0.03-1.67 mg/L, Cu 0.01-0.23 mg/L, Cd 0.01-0.05 mg/L, Fe 0.05-7.24 mg/L, Mn 0.14-2.26 mg/L, and Pb 0.04-0.78 mg/L. The levels of Zn, Cu, and Ni were within WHO and NEMA standards while Cd, Mn, Pb and Fe did not meet the threshold at some sampling points. All the metals studied met the FAO guidelines for reuse of wastewater in irrigation. Characterization of the adsorbents was done using FTIR which displayed the dominant functional groups to be silanols, hydroxyls, carboxylic and carbonyl groups. Adsorption of lead (II) ions was conducted using a composite of soda lime waste glass and multiwalled carbon nanotubes, borosilicate and soda lime waste glass. The composite and borosilicate adsorbents reported 100% adsorption of lead (II) ions while soda lime was average. Adsorption of Pb (II) ions followed Freundlich isotherms for borosilicate and soda lime adsorbents with r2 of 0.8665 and 0.9257 while Composite had a better fit in Langmuir isotherm with r2 of 0.9446. Cd (II) and Ni (II) ions did not interfere with adsorption of lead (II), but a stiff competition for the adsorption sites was observed for the case of Mn (II) ions. Regeneration efficiencies of 99.61%, 97.45%, and 99.82% were observed for borosilicate, soda lime, and composite adsorbents. The findings of this study clearly showed that soda lime waste, borosilicate waste glass and composite of soda lime waste glass/MWCNTs are effective for the removal of lead (II) ions from waste water.