Waste materials like sewage sludge and biosolids are becoming more common with global population growth, and they present both problems and opportunities. On the one hand, they contain significant quantities of dangerous toxins, like heavy metals, but they are also full of organic matter and important chemicals that can be used to fertilize crops.

Biosolids, a type of processed and dewatered sewage sludge, has great potential to be used as a soil amendment, but they often concentrate toxins like cadmium and lead, which makes farmers justifiably distrustful of its use. Phytoextraction, which consists of absorbing a matrix’s contaminants into the above-ground organs of plants for subsequent removal, is widely used for heavy metal removal, often with great success. A little-explored factor that could influence plants’ ability to tolerate and accumulate heavy metals is phosphatase activity.

Sewage sludge contains heavy metals which must be removed to avoid bioaccumulation in the food chain, and phosphorus that must be extracted to avoid eutrophication in waterways. Among the methods that have been applied to achieve the removal of phosphorus, electrocoagulation is becoming popular due to recent technological advances. In this technique, an electrical current pass between two iron or aluminium electrodes placed in a batch containing the sludge, releasing metal cations that react with phosphate to form an insoluble salt. It may be possible to accelerate electrocoagulation by degrading organic phosphorus at some point before or during the process, as these molecules take more time and more intense currents to degrade.

This project was divided in two parts:

(1) Two fast-growing, high-biomass crop plants, maize (Zea mays) and sunflower (Helianthus annuus), were grown under different cadmium concentrations to determine the impact the metal has on their early growth. Several parameters were measured, most importantly the activity of three classes of phosphatases in root exudates, root tissue, and shoots.

(2) Electrocoagulation was conducted with filtered suspended sludge either pre-treated with phosphatase or with the enzyme added at the start of the process, and outcomes were compared with chemical precipitation of phosphorus with iron salts.

Cadmium had a significant effect on all growth parameters, reducing shoot height, chlorophyll content, and root length and weight, while increasing root hair area in both species. Activity levels for all phosphatases in root exudates was increased, except for monoesterase at the highest cadmium concentration. Most of the increase in total root diesterase and phytase activities in the presence of cadmium was due to exudate activity, and not root tissue activity. Maize stems showed decreased activity of monoesterase and phytase, and no change in diesterase activity. Sunflower stems had no change in monoesterase activity and increases in diesterase and phytase, but at the highest cadmium concentration, all enzymes had decreased activity. Both root and shoot tissue accumulates cadmium, with a translocation rate from root to shoot higher than expected for young plants: shoot cadmium reached 39% of total plant cadmium in maize and 33% in sunflower. Phosphatase activity, unlike the other parameters, did not necessarily follow the same pattern under cadmium stress as in phosphorus deprivation.

Electrocoagulation combined with phytase was the only treatment that enabled the complete recovery of phosphorus dissolved in sludge. Pre-treatment with phytase did not have great influence on the result of the process compared to phytase added at the beginning of the process, but it did lead to more rapid precipitation for the first 20-30 minutes of electrocoagulation; phytase also lead to greater phosphorus recovery from chemical precipitation with two iron salts, but not complete recovery. Alkaline monoesterase did not lead to complete phosphorus recovery under any treatment, but it contributed to greater precipitation when combined with iron(II) sulphate.

This project provides the first comprehensive profile of phosphatase activity under heavy metal stress in two major crop plants, and the results are relevant for engineering lines that can tolerate and extract Cd from the soil. They also lend credence to the status of maize and sunflower as important phytoextractors. The electrocoagulation experiments provide the first evidence that enzymes can improve the rate of phosphate recovery from sewage sludge; future research should focus on recovery efficiency with combinations of enzymes and with the conjugation of phosphatases and other promising phosphorus precipitation methods, such as ultrasonic cavitation.