The voltammetric behavior of paraquat and diquat was investigated at Natural phosphate (NP) modified carbon paste electrode NP-CPE in Na2SO4. A method was developed for the detection of the trace of these herbicides, based on their redox reaction. The reduction peaks of paraquat were observed around -1.2 V and 0.8 V (vs. SCE), and at about -0.8 V and 0.9 V for diquat in cyclic voltammetry. Experimental conditions were optimized by varying the accumulation time, apatite loading and measuring solution pH. The use of Moringa oleifera as natural chelating agent gave a significant improvement in the depollution of contaminated Water sample.