Synthesis and characterization of walnut husk extract-silver nanocomposites for removal of heavy metals from petroleum wastewater and its consequences on pipework steel corrosion

A new nano-scale bio-based nanocomposite for treating petroleum wastewater has been synthesized using walnut husk (an agro industrial by-product). The walnut husk extract-silver nanoparticles (WHE-AgNPs) were round-shaped, monodispersed, none-aggregated and crystalline with nearly 46.2 nm size and zeta potential of –41.2 eV. On introduction into petroleum wastewater (PWW), WHE-AgNPs reduced the magnitude of some key water pollution indicators, which were initially very high, to values within World Health Organization (WHO) prescribed safe limits. Also, 0.75 g WHE-AgNPs removed Pb, Cr and Cd ions from PWW at efficiency of 72.6%, 81.3% and 88.1%, respectively, at 25 ^oCwithin 5 h, facilitated by physisorption of C–N, N–H, C[dbnd]O, C[dbnd]C, C–O and O–H sites on WHE-AgNPs surfaces with the metal ions. The extent of metal ions removal varies with temperature, concentration of WHE-AgNPs, dose of metal ions, contact time and pH. Unlike the crude extract feedstock, WHE-AgNPs remains efficient, stable and resistant to thermal and biochemical degradation even after 120 days of storage. Discharged untreated PWW produces higher corrosive effect on pipework steel than treated PWW. The extent and mechanism of anticorrosion effect of WHE-AgNPs on pipework steel is investigated using electrochemical, weight loss, FTIR and SEM/EDS analyses.