Ashraf Aly Hassan
Date of this Version
To improve crop yield and control broadleaf weeds, many farmers apply nitrate-based fertilizers and chemical herbicides to their fields. Excessive use of these chemicals can cause contamination in waterbodies and impact on the public health. The physical and chemical methods have been suggested to remove these herbicides are usually costly. In this study, the capability of immobilized algal beads in removing nitrate and several herbicides of environmental concern from synthetic and actual groundwater and surface water was evaluated.
The experiments were performed in batch, sequencing batch and continuous flow modes and the effect of different operational conditions (light intensity, cultivation method, algal density, and temperature) on the performance of algal beads were assessed and the capability of the system in real working scenarios was evaluated.
The results from batch studies showed that Scenedesmus species cells, immobilized in sodium alginate are capable of removing 90% of nitrate from synthetic and actual groundwater with the beads/water ratio of 12.5%. The beads performance improved in heterotrophic conditions and the presence of light enhanced the nitrate removal. Immobilized algal beads were capable of uptaking nitrate for 100 consecutive days, and the equal volumetric ratio of alga: alginate led to the highest nitrate uptake in sequencing batch mode.
Immobilized beads showed higher nitrate and atrazine uptake at room temperature (20 °C) in continuous flow reactor. When tested with the actual groundwater and surface water samples, atrazine, oxadiazon, triallate, cycloate, alachlor, simetryn and ametryn were partially removed through adsorption, bioaccumulation, and biodegradation. The presence of herbicides and toxins in the water lowered the efficiency of the immobilized beads.
Embedding immobilized algal beads with nanoclay was found to be an effective method to accelerate nitrate and atrazine uptake rate. Nearly 100% of 10 mg/L nitrate and 90% of 0.1 mg/L atrazine were removed in 3 days at the concentration of 0.3 mg Nanoclay per bead. The nitrate uptake capacity of the nanoclay-embedded algal beads was found to be through three mechanisms of adsorption, biological assimilation, and extracellular enzyme activity resulting from toxicity.
Advisor: Mohamed Dahab