Chimie  |  Biochimie  |  Médecine

 

Anjalika Malhotra, 2007 | Genève , GE

 

A total of 36% of Switzerland’s area is agricultural land from which water flows into lakes and rivers via surface
runoff. Dissolved fertilizers containing phosphorus are harmful for aquatic life, leading to lake eutrophication
and serious degradation in dissolved oxygen levels. While phosphorus levels in Lake Léman have been brought
down significantly, dissolved oxygen is still below target. The preferred method to lower phosphorus is
chemical precipitation using alums. Therefore, this study was undertaken to determine optimal reaction
conditions for phosphorus removal. An experiment was designed to study influence of alum concentration, pH,
and temperature. It was concluded that a pH of 6.0 and temperature around 20°C would be optimal. While this
is a promising finding, a large-scale study would be required to determine optimal conditions for practical
application.

Introduction

The research question explores optimal reaction conditions for phosphorus removal during chemical
precipitation using potassium alums. The aim is to determine optimal levels of alum-to-phosphorus
ratio, temperature, and pH for maximum phosphorus removal efficacy from agricultural runoff before it enters
Lake Léman.

Methods

A multi-factorial experiment was carried out. First, potassium alum was synthesized. Second, a calibration
curve was generated using colorimetry, to measure phosphorus concentration before and after reaction.
Diammonium phosphate was used to generate a range of phosphorus concentrations. Since phosphorus level
in agricultural runoff was too low to be replicated in the laboratory, phosphorus was scaled up by 100x to
0.1g/L. The yellow vanadate-molybdate method was used to generate a calibration curve, wherein phosphate
absorbance at 470nm was measured using a spectrophotometer. Third, it was essential to find a usable range
of alum-to-phosphorus ratios required for phosphorus precipitation. Scientific papers indicated a range of 80-
100x. However, no precipitation was seen at these ratios, despite phosphorus at 0.1g/L. Phosphorus was
therefore scaled up much higher, and that resulted in precipitation, at much lower alum ratios, compared to
literature. A new calibration curve was generated with a new maximum phosphorus concentration 4g/L.
Eighteen experimental runs were conducted with varying alum-to-phosphorus ratios (1:1, 1:2), temperatures
(20°C, 30°C, 40°C), and pH levels (5, 6, 7). Final phosphorus concentration for each sample was determined by
measuring absorbance after precipitation and 30 minutes of phase separation.

Results

Alum-to-phosphorus ratio had very little effect on phosphorus removal. Alum-to-phosphorus ratio 1:1 behaved
almost identical to 1:2. A pH of 6.0 was found to be most favorable for phosphorus removal, since it resulted in
lowest phosphorus concentration. This was especially true at 20 °C which yielded phosphorus level of 0.569g/L,
equivalent to removal efficacy of 85.775%. The higher pH of 7.0 had a negative impact on phosphorus removal,
with worst removal rate of 76.925%, at 20 °C. However, removal efficacy at pH of 7.0 was higher at 30 °C and
40 °C.

Discussion

The hypothesis “Higher alum ratio (1:1), highest temperature (40 °C and middle pH value tested, 6, should
result in highest removal efficacy,” was only partially validated. Alum-to-Phosphorus ratio had almost no effect
thus contradicting the hypothesis. pH level had most influence on phosphorus removal, thus validating the
hypothesis, with pH 6.0 showing highest removal efficacy. Temperature also affected phosphorus removal,
although to a lesser extent. At higher temperatures removal efficacy was lower, contradicting the hypothesis,
with best results at 20 °C. However, 20 °C was unfavorable at pH of 7, indicating that while temperature plays a
role, it is less critical than pH. A larger range of pH and temperatures would have resulted in better
optimization. Additionally, the high phosphorus scale up could have impacted results.

Conclusions

While eutrophication in Lake Léman is currently not a problem, the vast majority of aquatic life has already
been lost. The only way to restore aquatic life is to increase dissolved oxygen, achieved by lowering
phosphorus. The use of alums in agricultural catchment areas or special treatment zones is the preferred
method so far. Practically, in the context of runoff treatment, a pH of 6 and temperature of 20 °C seem highly
favorable and implementable. However, large-scale optimization would be required to determine ideal
conditions.

 

 

Appréciation de l’experte

Sarah Descloux

Anjalika’s project on alum-based phosphorus removal from agricultural runoff into Lake Léman stands out for its thoughtful approach and relevance. The complex experimental design reflects careful planning, and her perseverance in adjusting concentrations and methodology in response to real-world constraints is commendable. Her work showcases strong experimental rigour, adaptability, and a clear understanding of the challenges involved in translating laboratory findings to field-relevant conditions.

Mention:

bien

 

 

 

Institut Florimont, Grand-Lancy
Enseignant: Dr. Luke Handley