Alum sludge for adsorptive removal of copper and zinc from stromwater runoff

The investigation found that rainwater runoff, as a non-point source of pollution, carries various kinds of heavy metals that have threatened the diversity of aquatic life and human health. Therefore, in 2015, the UK Sustainable Drainage Systems promoting multi-functional, low-cost solutions to reduce and purify rainwater runoff.
Alum sludge (AS) is a byproduct from water purification plants, which is large output, low cost and easy to get. For most water purification plants currently the alum sludge is a waste product and the main fate is to direct AS to landfill, which wastes the residual value of AS. In order to reuse AS, the feasibility of recovering and the way to reuse air-dried AS to be an adsorbent was studied. The knowledge gap of this study is currently research on the recovery of AS were mainly focused on the adsorption of phosphorus and some high-concentration heavy metal wastewaters. The research on the use of AS to adsorb low-concentration runoff heavy metal pollution is still incomplete. This study systematically investigated the static and dynamic adsorption of Cu(II) and Zn(II) by AS. The results help us understand the adsorption mechanism of AS and provide suggestions for the recycling and reuse of AS. This study is a win-win strategy, which not only solve the AS disposal problem, but the use of low-cost AS also helped reduce the pollution load of heavy metals in runoff.
This thesis completed four tasks, including: 1) The composition and physicochemical properties of AS were studied by various characterization methods; 2) Batch experiments were carried out to determine the adsorption mechanism and capacity for Cu(II) and Zn(II); 3) By simulating the composition of stormwater runoff, the possible adsorption competition between multiple elements is studied; 4) The column experiment provides the completeness of AS adsorbent research from theoretical data to application performance.
AS was characterized by scanning electron microscopy, specific surface area measurement and Fourier transform infrared spectroscopy. The results show that AS is a porous material mainly composed of mesopores, with an average specific surface area of ​​75 m2/g for particle (1-2mm) and 58 m2/g for fine powder (< 0.15mm).
The results of batch experiments show that the adsorption of Cu(II) and Zn(II) conforms to the pseudo-second-order kinetic and the langmuir isotherm model, which indicates that the adsorption of Cu(II) and Zn(II) is a monolayer chemical adsorption. At pH = 6.5, the adsorption capacity for Cu(II) and Zn(II) was 22.9 mg/g and 8.7 mg/g, respectively.
The results of adsorption competition experiments show that the removal efficiency of AS for Cu(II) and Zn(II) in simulated stormwater runoff exceeded 95%, indicating that AS can be an effective adsorbent for Cu(II) and Zn(II) in stormwater runoff.
The results of the column experiment showed that when 160g AS is added, the adsorption saturation time for Cu(II) and Zn(II) is 600min and 240min, respectively. With the increase of AS dosage, the adsorption capacity of Cu(II) and Zn(II) increases. With the increase of flow rate, the adsorption capacity of Cu(II) and Zn(II) decreases.
In summary, this study explored the potential of AS for reduce Cu(II) and Zn(II) pollution levels in stormwater runoff, and also had a positive impact on waste utilization, economic conservation and environmental protection.